Lighthouse Construction and Illumination Thomas Svensson by thebombilla

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â&#x20AC;˘'

li''y'^

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ji-f^^-'

LIGHTHOUSE CONSTRUCTION Ain)

ILLUMINATION

^IGHTHOUSE CONSTKUCTION AND

ILLUMINATION

THOMAS STEVENSON, MEMBER OF THE INSTITUTION OF AUTHOR OF LIGHTHOUSE ILLUMINATION, '

F.R.S.E.,

CIVIL ENGINEERS '

THE DESIGN AND

CONSTRUCTION OF HARBOURS,' ETC.

LONDON AND NEAV YOEK E. & F. N. SPON MDCCCLXXXI

F.G.S. ;

Printed by R.

R. Clark, Edinburgh.

ยงir John gatokshato, F.R.SS. L.

E., F.G.S.,

ETC.

PAST PRESIDENT OF THE INSTITUTION OP CIVIL ENGINEERS.

MY DEAE HAWKSHAW, I

beg leave to dedicate to you the following

pages, not only on account of the distinguished position

which you justly hold

mark

in the profession, but as a

great personal regard and friendship.

Very truly

yours,

THOMAS STEVENSON.

Edinburgh, December 1880.

PEEFACE. In the following pages I have done condensed statement,

1st,

Of the

facts

best to give a

and principles which

regulate the design and construction of Lighthouse Towers

in exposed situations

;

and 2d, Of the practical application of

optics to Coast Illumination.

tion of the optical agents,

and

The

history of the introduc-

their combinations to

different requirements, have, so far as consistent

meet

with a clear

exposition and division of the subject, been arranged in the chronological order of their publication, or of their employ-

ment

id Lighthouses.

Though

there are

many

excellent publications on special

branches of this very important application of optical science, yet a systematic treatment of the whole subject, brought up to the present date,

from the

seems to be necessary.

fact that so

many new

This will appear

optical agents

and modes of

dealing with the light have been invented since the intro-

duction of the admirable dioptric system of Fresnel, which

PKEFACE.

VUl

consisted of only four agents, and tliree forms of apparatus in

which these agents were combined. All the later improvements have been fully described

and

illustrated, so

as to enable the engineer to select

from

the different designs, such plans as will best suit the special peculiarities of

any

line of coast

to illuminate.

Edinbtjegh, December 1880.

which he may be

called

CONTENTS. CHAPTER

PAGE

LIGHTHOUSE CONSTRUCTION

CHAPTER

II.

LIGHTHOUSE ILLUMINATION ADAPTED TO THE ILLUMINATION OF EVERY AZIMUTH, OR OF CERTAIN AZIMUTHS

ONLY, BY LIGHT OF

EQUAL

POWER, ACTING EITHER

CONSTANTLY OR PERIODICALLY

CHAPTER

.45

III.

AZIMUTHAL CONDENSING SYSTEM FOR DISTRIBUTING THE LIGHT

UNEQUALLY

different directions

EITHER CONSTANTLY OR PERIODICALLY

CHAPTER

IV.

DISTINCTIVE CHARACTERISTICS OF LIGHTS

CHAPTER

125

THE ILLUMINATION OF BEACONS AND BUOYS

152

CHAPTER VL THE ELECTRIC LIGHT

178

CONTENTS.

CHAPTER STATISTICS OF LIGHTHOUSE

VII.

APPARATUS

.192

CHAPTER Vm. 204

SOURCES OF ILLUMINATION

CHAPTER

IX.

MISCELLANEOUS SUBJECTS CONNECTED WITH LIGHTHOUSES

CHAPTER

214

247

FOG-SIGNALS

APPENDIX. 264

MATHEMATICAL FORMULA DESCRIPTION OF PLATES

INDEX

•

.276 283

CHAPTEE

LIGHTHOUSE CONSTRUCTION, 1.

Among

the

many works

of the saying that "

knowledge

man which prove the truth is power," we must not omit

those solitary towers, often half-buried in the surge, that

convert hidden dangers into sources of safety, so that the sailor

now

steers for those

dreaded and took so

nunc sahis would be a of the night.

hope

at the

when one

such beacons

true, as well as

a noble

of his privateers seized the

though he was at war with England,

war with mankind."

peculiarities of construction,

have

Olim periclum

Eddystone, and took them to France in

of reward, that "

he was not

care to avoid.

fitting inscription for all

was therefore a

saying of Louis XIV.,

workmen

much

very rocks which he formerly

It is to the history

and

and the general principles which

be kept in view in designing such towers, that we

have now to devote our attention. In considering

this subject, the first

matter requiring our attention,

have to

deal, its

destructive

and most important

the element with which we

power when excited by storms,

and the directions of the

forces it exerts against the

of a tower placed within

its

masonry

reach.

In his History of the Eddystone Lighthouse Smeaton says

â&#x20AC;&#x201D;

When we

control, those

have to do with, and to endeavour to

powers of nature that are subject B

no calcu-

LIGHTHOUSE CONSTKUCTION.

lation, I trust

such a

case,

and that would be likely

ment

deemed prudent not

will be

add

This state-

to the security."

of our greatest maritime engineer indicates the propriety

any

of carefully collecting

facts that

may

being "

Now,

no calculation."

suhjcct to

help us to a more

which he regarded as

accurate estimation of those forces

that

omit, in

anything that can without difficulty be applied,

must be

not an agent of constant power that

it is

with here,

for the towers

recollected

we have

to deal

which are built on rocks in the sea

The

are not all assailed equally.

forces

that act against

buildings confronting the open ocean are never found in seas of smaller area, where, however, they

may yet prove sufficiently

powerful to destroy our works, unless w^e have correctly estimated the amount of exposure.

mately

ought,

waves in relation

2 from

have a safe approxi-

heights of the waves which are due to

at least, the

different lengths of " fetch."

therefore,

In order

we must know,

guide to direct us in each case,

Before making

possible, to

know

to the distance

any

design,

the law of generation

from the windward shore.

Laio of the liatio of the Sciuare Roots of the Distances

the

Winchvard Shore.

â&#x20AC;&#x201D;In

the Edinburgh Philosophical

Journal for 1852 I stated, as the result of a series of observations begun in 1850, that the height of waves increased most nearly " in the ratio of the square roots of their

windward

distances from the of

waves in

A= 1-5

\/f^,

and

feet,

shore."

from which the storm curve was

be found sufficiently accurate for A^liere

Wliere h

the fetch

formula

annexed

table,

I'S

less

\/^+

the height

the fetch in miles, the formula

all

laid

down, will

but very short fetches.

than 39 miles, the most accurate (2*5

â&#x20AC;&#x201D;

Vd).

In calculating the

both formulcC were employed.

The ordinates representing the height

waves

heavy

GENEEATION OF WAVES. gales

(Fig. 1)

were

all

that had been observed

diagram was made, but since then obtained,

which

show the lengths

also

corroborate

when

many more have been

the law.

The

abscissae

of fetch,

Vertleal Scal/e. Fig.

Table shovnng apjproximate heights of Waves due Miles.

Heights.

the

lengths of Fetch.

LIGHTHOUSE CONSTRUCTION.

continue to increase in height, however long

The maximum height

line of exposure.

carefully

measured by the

gave 559

also

late Dr. Scoresby,

as the

feet

noticed

â&#x20AC;&#x201D;At

heights were ascertained

4. Force of Waves.

the

â&#x20AC;&#x201D;As

Cialdi

we do

regards the greatest heights at

Bishop Eock Lighthouse

100

the level of

Mag.,

vol.

land,

a wall

xxxi.

262).

feet

]\Ir,

cliff

hung

bell its

attach-

At Unst

Lighthouse, in Shet-

high and 2 feet thick was thrown

mass of stone from 2

the top of the

may be

above high water (Naut.

down and a door was broken open at above the sea. At the Fastnet Light Clear, a

but by

;

not know.

their force, the following facts

from a bracket on the balcony was broken from

ments

crest,

Greater waves than

32-^ miles per hour as their velocity.^

which waves exert

feet.

between each wave, and

those have been recorded, and are given theii'

the

was 43

width from crest to

1 G seconds as the interval of time

what means

may be

in the Atlantic, as

to 3 -I-

a height of

feet

Cape

(Plate VI.), off

tons was thrown off

at the level of

195

above the

feet

ConoUy, inspector of the works, states that the sea

sea.

rises

over the top of the rock in what he describes as " a solid

dark mass," at a height of 36 to 40 feet above the base of the lighthouse tower, being

122 is

feet

above the

sea,

and

an elevation of from 118 to

this

sheet of water and spray

of sufficient' density to shut out

w^indow on the third story. jet of

the daylight of the lee

one occasion,

rain-water (weighing about 6

Life of Dr.

p. 324.

a hea\'y

During another storm a 60 -gallon

Scoresby,

150

by Dr. Scoresby Jackson.

feet

Loiulon

off

cask,

from

cwt.), w^as burst

lashings on the balcony at a height of

its

when

water struck the tower, some cups were thrown

a small table. full of

above :

1861,

MARINE DYNAMOMETER, the

important to observe that in this case the

It is

sea.

which passes over the Fastnet and

"body of water

violently against the tower

chasm

of a formidable

in the rock,

and

6 2 1" feet above low water, there together,

tion of

some

which are

a block of

and there

situ,

was in

feet,

feet

in Shetland, at

a beach of blocks huddled

9-| tons weight.

At an

tons was' torn out of

its

eleva-

position

no doubt that another of 13^ tons

out of

above the

which has been known

Wick

5-|-

manner quarried

the level of

probably limited

upper surface.

Bound Skerry Lighthouse,

islet at

strikes

largely due to the existence

to a comparatively small portion of the

the rocky

tJie

by the waves,

rock

But the

sea.

to be exerted

greatest force

by the waves was

breakwater, where, on one occasion, a monolithic mass

1350

of concrete, weighing position in the work,

tons,

and on a

was moved bodily from later occasion a

its

mass of no

than 2600 tons was displaced and moved inwards in a

less

similar

manner

tions on

and in both of these cases the founda-

;

which the masses rested were not in the

least

disturbed. 5.

Numerical Values of the Force of In addition to the

Marine Dynamometer. been

stated,

means

Waves ly

facts

tlie

which have

numerical results have also been obtained by

of the marine dynamometer, an instrument

designed in 1842.

proverb "fas

est

there

ah hostc doceri"

no contest is

which I

which the old

more applicable than in

resisting the surge of the ocean, it

such

the

â&#x20AC;&#x201D;

may

be proper to give

a description of this simple, self-registering instrument

as will enable

DEFD

anyone is

have

made.

a cast-iron cylinder, which

firmly bolted

at the protecting flanges C to the rock where the observations are to be made.

This cylinder has a circidar flange

LIGHTHOUSE CONSTRUCTION

at D. is to

a door wliich

be read

off.

opened -when the observation

a circular disc on wliich the waves

Fastened to the disc are four guide rods B, which

impinge.

pass through a circidar plate

(which

screwed down to

the flange D), and also through holes in the bottom of the cylinder

Within the cylinder there

attached to the

12I=cir.

Fig. 2.

plate

a very strong steel spring, to the other or free end of

wliich

fastened the small circular plate K, which again

secured to the guide rods B. T,

which

slide

registering

how

on the guide far the rods

holes in the bottom

and serve

rods,

other words,

drawn out by The following formula

W = weight W.

stated in tons,

to produce a given

D = number a

amount

far the

will be found convenient

of the disc in

which

found by experi-

of yielding of the spring.

by the spring with weight square feet, d = the length in

of inches jdelded

= area

how

the action of the waves against

in the graduation of the instrument

ment

as indices for

have been pushed through the

E, or, in

spring has been

the disc A.

There are also rings of leather

JIAEIXE DYNAMOMETER.

inches on the proposed scale, corresponding to a force of one

ton per sqnare foot acting on the

The to,

employed in the

different discs

were from 3

disc.

to 9 inches diameter,

and the strength

of the springs varied

about 5

every

lbs. for

observ^ations referred

but generally 6 inches,

from about 10

lbs. to

inch of elongation, and the instru-

ments varied in length from 14 inches

to 3 feet.

Their re-

spective indications were afterwards reduced to a value per

square

foot.

The instrument was generally placed so as f ths tide, and in such situations

be immersed at about

would

to as

afford a considerable depth of water.

Forces indicated hy the Dynamometer.

of the kind

shown

â&#x20AC;&#x201D;With

instruments

in Fig. 2, the following series of observa-

commencing in 1843, was made on the Atlantic, at the Skerryvore, and neighbouring rocks, lying off the island tions,

of Tyi'ee,

Arg}'llshii-e

;

and in 1844 a

observations was begun on the

series

German Ocean,

similar

at the Bell

Eock. Eeferring for more full information to the tables of observations

which are given in the Edinburgh Transactions,

vol. xvi., it

wUI be

obtained

only premising that the values refer to areas of

;

sufficient here to state generally the results

limited extent, and are

not to be held as simultaneously

applicable to large surfaces of masonry. Relative Force of

Summer and Winter

Gales.

â&#x20AC;&#x201D; In

the

Atlantic Ocean, at the Skerryvore Eocks, and at the neigh-

bouring island of Tyree, the average of the results that were registered for five of the

1843 and 1844 was 611 The average

summer months during lbs.

the years

per square foot=0-27 ton.

results for six of the winter

months (1843 and

LIGHTHOUSE CONSTRUCTION.

1844) was 2086

lbs. =:

0-93 ton per square

summer

than three times as great as in the Forces in

Giratest recorded

Oceans.

â&#x20AC;&#x201D; The

result

greatest

Atlantic

the

obtained

and German

Skerryvore was

during the heavy westerly gale of 29th March 1845,

6083

a force of

lbs.,

when

or nearly three tons, per square foot,

on the surface exposed was registered.

was 5323

foot, or

montJis.

The next highest

lbs.

In the German Ocean, the greatest result obtained at the

3013

Bell Eock, on the surface exposed, was at the rate of lbs.

per square

tended

But subsequent and much more ex-

foot.

observations

Lothian, gave

Dunbar,

3|- tons

while,

;

county of East

the

in at

the

harbour works of

Buckie, on the coast of Banffshire, the highest result of observations extending

over a period of

several

years was

three tons per square foot.

In order to show the agreement between the indications of

dynamometers having springs of

different strength

discs of different area, the following results are given TJie

means of nine

observations,

different instruments, are

The means of three

1000

lbs.

which were made with two

and 415

lbs.

respectively.

made ivith 1183 lbs., and

eighteen observations, ivhich were

inst7'uments,

different lbs.

433

and

are

1247

lbs.,

respectively.

6, Force of the Waves at different Levels above the Sea, as by

ascertained tions

made

the

Marine Dynamometer.

Dunbar harbour with dynamometers

sunk in the masonry with of a curved sea wall,

was exerted

â&#x20AC;&#x201D; Erom

(Figs. 4, 5)

their discs flush with the surface

appeared that while the greatest force

at the level of high water, the forces quickly

decreased above and below that level, as shown by Fig. 3.

observa-

Owing

an uncertainty as

bb, cc,

dd,

the readings of

FOECE OF WAVES.

some

tlie

instruments, which

was not discovered

at the

Fis. 3.

time, the results

dynamometers

must be regarded

as only approximate.

Two

shown in

Fig. 2,

were

foot,

while

Fig. 3), similar to that

(a,

also fixed to a piece of

wood

over

projecting

the cope of the parapet of the wall, with their discs

down-

pointing

wards over the edge of the cope, so as to ascertain

the upward force

body

of the ascending

and

spray.

The maximum

vertical

water

force recorded

cope of feet

the

above the sea at

the time the tion

the wall (23

was

observa-

made)

was

upwards of one ton (2352

lbs.)

per superficial

LIGHTHOUSE CONSTRUCTION.

the greatest horizontal force wliich was at any time

re-

corded by the highest of the flush dynamometers, which

was 18 iuches lower, never exceeded 28

Fig.

lbs.

The

vertical

force tending to raise the cope of this sea-wall, if projecting,

was

therefore at least

tending

to thirst it

84 times greater than

the horizontal force

inwards.

7, Horizontal Force of the Back Draught of the Wave.

â&#x20AC;&#x201D;

Dynamometers having their discs pointed landwards were fixed to a pHe placed immediately outside of the Dunbar wall, while others were fixed to the same pile with their discs was found

pointed seawards.

The

ton per square

while the direct force of the waves before

foot,

they had reached the the force of

recoil

the direct force

force of recoH

waU was

only 7 cwts. per square

with this form

be one

foot, or

of wall was three times that of

â&#x20AC;&#x201D; a result which was doubtless due

to the con-

HEIGHT OF WAVES. of energy,

centration

and was produced by

resistance

tlie

of the masonry.

Law

8. Mr. Scott RusselVs

Waves

Depth of Water.

—Mr.

of the Eelation of Height of Scott Enssell states that

waves be propagated in a channel whose depth diminishes uniformly, the waves will break

when

equal to the depth of the water.

The depth

their height

round any rock on which a lighthouse therefore, the ruling element

for

becomes

some distance

placed becomes,

which determines the height of

the impinging waves on the masonry of the tower, whatever

may

But the law

be their height farther out in the ocean..

assigned by Mr. Eussell does not appear to hold true of

At Scarborough

waves. high,

which broke in 10

I measured

waves 5

feet 3 inches water,

all

feet 3 inches

and others 6

feet

6 inches high, which broke in 13 feet 8 inches water, so that

when h = height from hollow to

mean

waves of

in feet of

crest,

and

depth of water in

level

measured

this class, as

below the

feet

D — 2

At Wick breakwater waves were of the same

depth as their height.

seen to break in water

The height

of these

waves

above mean level was about f of their height, the hollow below mean level being about \.

The

late Dr.

Macquorn Eankine has shown, on

grounds, that the crest

mean

level is not equidistant

theoretical

between the

and the trough.

Let

H the height rolling circle =

be the length of a wave,

from trough

then, diameter of radius of o'l4l6 TT -— ; and elevation of middle level of wave orbit of particle

to crest

;

above

still

waters

31416H2 ^ — = '7854^rL 4L

;

LIGHTHOUSE CONSTRUCTION.

Consequently Crest above

still

Trough below

water

still

=—H +

-7854^

H— =—

-7854^

Tliese formulte are exact for water of considerable depth,

and only approximate

for

shallow water.

Lighthouse Towers. 9. Remarkable Lightlwuse Toivers. of

—In giving a description

some of the most remarkable towers we

the earlier works, the history of in obscurity,

and

therefore

which is more

more curious than

shall restrict ourselves principally to those

are exposed to

heavy

seas,

shall omit all

or less shrouded useful,

and

works only, which

and which furnish us with

from which practical lessons can be deduced.

facts

Plate No.

contains drawings of those towers on the same scale, and

placed at their relative levels above the sea. Wi7istanley's

Edclystone Light.

lying about 14 miles

— The

Eddystone rocks,

harbour of Plymouth, are fully

off the

exposed to the south-western

seas.

In 1696 Mr. Henry

Winstanley, of Littlebury, Essex, undertook to erect a tower

on the Eddystone.

The work was begun

first

and

Twelve large iron stanchions

completed in four seasons.

were fixed during the

in that year,

The

year.

erection of a solid

cylinder of masonry 12 feet high and 14 feet diameter occu-

pied the second year.

This column was in the third year

(1698) increased in diameter to 16 ing

to the height of

feet,

when

and finished by

rais-

the light was exhibited.

In the next year (1699), in consequence of some damage that

LIGHTHOUSE TOWERS.

occurred to the buildiug during the winter, the tower was encii^cled

and made the rock.

by an outer rmg solid

masonry of 4

from the foundation

The upper part

feet in thickness,

to nearly

of the tower

20 feet above

was taken down, and

besides being enlarged proportionally throughout,

feet higher,

making the

total height

120

feet,

was

raised

the whole

work being completed m the year 1700. On the 26th November 1703, durmg the gi-eatest British storm of which we have any record, the structure disappeared, and, as is weU known, Winstanley, who had gone perished with his workmen.

make some

repairs,

Budtjerd's Eddystone Light.— In 1706 a lease of 99 years

was given by the Trinity House of London to a Captain Lovet, who was to erect another tower on tliis rock. Captaia

—

Lovet made a strange selection of an engineer J. Eudyerd, a silk mercer in Ludgate Hill— who, however, had the good sense to engage experienced shipwrights to assist him. The

work was commenced in July 1706, and completed m 1709. The form which he adopted was the frustum of a cone, the height from the base to the ball on the top of the lantern

bemg 92

feet. The rock was cut out in level steps. The lower portion of the tower consisted of oak timber, carefully

bolted together and to the rock by an ingenious system of iron bars and jag bolts. Above this mass of carpentry were

placed courses of stone cramped

together,

with the timber- work and with the rock. tion of the cone consisted wholly of timber.

and connected

The upper porThe entrance-

door was 8 feet above the foundation, the storeroom floor 27 feet above it, and higher up were four rooms with lantern

and balcony. of 46 years,

1755.

This lighthouse stood for the long period

and was

at last destroyed

fire

December

LIGHTHOUSE CONSTRUCTION.

Smcaion's Eddystonc Tower.

â&#x20AC;&#x201D;This celebrated work

which was constructed entirely of

stone,

(Fig. 6),

was commenced in

August 1756 by cutting dovetailed holes in the

rock,

and in the

â&#x20AC;&#x201D;~\ Tfater

Loir

4-0

3 0Fc(l

Fig. 6.

second year (1757) the masonry was brought up to the top of the ninth course.

In 1758 the tower was nearly ready

lor

the light, but was stopped in consequence of some difficulties

connected wdth the Act of Parliament.

whole of the masonry was lighthouse engineer stones,

finished.

who adopted

In August 1759 the

Smeaton was the

first

dovetailed joints for the

which averaged one ton in weight.

In Plate

II. will

be found drawings of the ingenious method which he employed

LIGHTHOUSE TOWERS.

keeping tliem together during construction.

for

(Fig. 7) a vaulted

adopted

form for

the floors of the different

apartments, the voussoirs

\^^-^^mmmMimmim '^'

^^Â°-

of the arch being connected

together so as to form a continuous mass, in addition to which

he inserted metallic hoops (shown black in diagram) to prevent lateral thrust or tendency to spread outwards.^

damage which has recently led the Trinity House lution of removing the present tower will be afterwards referred Bell

Bock

â&#x20AC;&#x201D;The Inchcape

German Ocean.

to build a

new one

to.

the coast of Forfarshire, and the

and

The

to the reso-

Eock

or Bell

lies

12 miles off

fully exposed to the

The rock

waves of

of large dimensions, the

higher part being 427 feet in length and 230 feet in breadth,

and though the sandstone of which

what indurated

character,

dislodged by the sea

;

it lies

consists

in ledges

of a some-

which are

easily

and since the erection of the tower,

portions of the rock near

the base have been frequently

detached, leaving voids which have from time to time been

made up with cement tower not

erected

much above

or concrete.

The place on which the

16 feet below high- water spring

neap-tides hardly any part of this reef

is visible at

Captain Brodie proposed an iron pillar first

tides, or

the level of low-water springs, while during

light,

proposition of the kind, and in order to prove

he erected two structures of timber

its stability

at different dates,

they were both removed by the waves.

Mr. Eobert Stevenson, engineer

low water.

presumably the

to the

but

In 1799 the late

Northern Lighthouse

Board, also prepared, before visiting the rock, a plan for ^

Smeaton's Narrative of the Building of the Eddystone Lighthouse

London, 1791.

;

LIGHTHOUSE CONSTRUCTION.

an iron

In 1800 Mr. Stevenson made

pillar lighthouse.

on the

his first lauding

had no sooner

set

reef,

when, as he expressed

upon the rock than

foot

it,

" I

I laid aside

'3oFeet Fig. 8.

all idea

of a pillar-formed

structure,

fuUy convinced that

a building on similar prmciples with the Eddystone would

be found practicable." in

several

thickness

respects from

the

walls,

liis

design (Fig. 8) he deviated

Smeaton's

and

raising

tower; increasing the the

height

100

LIGHTHOUSE TOWERS, 68

feet instead of

feet,

and the level of the

solid to

alDove high water instead of 11 feet, while the base

feet instead of

Instead of employing vaulted

feet.

as in the Eddystone, he converted

which

them

the walls together (Fig.

tie

21 feet

was made floors,

into effective bonds,

The

9).

lintel stones of

the floors form parts of the outward walls, and are feathered

and grooved

as in carpentry, besides having dovetailed jog-

gles across the joints

where

they form part of the walls.

used

also

beacon or

barrack

and Plate

IV.),

engineer and his

in 1802, but

(Fig.

^ig- 9.

which was erected on the rock

workmen

The

of the tower.

E^^^^^^i^^^^^BBBBBWici-L.

temporary

Eock was

scarcely

Mr. Telford time they

dry at

loio

water, while the

the

ivater,

Commis-

Mr. Stevenson's views, consulted

and before going

;

also,

for

to financial difficulties.

Eddystone was scarcely covered cd high sioners, in order to fortify

the

to live in, during the building

Act of Parliament was applied

first

was not obtained owing

the Bell

for

to Parliament for the second

on Mr. Stevenson's suggestion, obtained the

support of Mr. Eennie to the scheme, with

whom

Mr. Steven-

son could afterwards advise in case of emergency during the progress of the work.

The second

bill

In 1808 the excavation

and the works were begun in 1807.

was

and the masonry brought up

finished,

rock.

In 1810 the whole tower was

the light was exhibited in 1811. is

2076

The

STcerryvore Lighthouse.

Account of

total

finished,

and

weight of the

tons.^

â&#x20AC;&#x201D;The

Skerryvore rocks in ArygU-

shire lie 12 miles off the island of Tyree, ^

to the level of the

In 1809 the work was carried up to 17 feet above

high water.

tower

was passed in 1806,

tlie

Bell

Rock Lighthouse, by

which

R. Stevenson.

the nearest Edin. 1824.

LIGHTHOUSE CONSTKUCTION.

The main rock

land.

and

carried out

by the

and being The works, which were designed Mr. Alan Stevenson, were com-

of considerable extent,

gneiss, is of great hardness. late

igMN^t^^e^>x^'?^-^<-^^$N^"^^s.^; 20

/â&#x20AC;˘ ^

Fig. 10.

menced

in 1838

by the

erection of the

framework of a bar-

rack similar to that used at the Bell Eock, but

away

November

of the

same

year, after

was carried

which another was

LIGHTHOUSE TOWERS. erected in a

more sheltered

The tower

finished until 1843. is

138

than

five

and 16

feet at

mass of stonework of 58,580 cubic

more than double that

less

(Fig.

The works were not 10), which is of granite,

feet diameter at the base

It contains a

the top. or

feet high,

position.

of the Bell Eock,

feet,

and not much

The curve

times that of the Eddystone.

adopted for the shaft of the tower was the hyperbolic.^ Fastnet Rock.

â&#x20AC;&#x201D;The

westward of Cape

to all oversea vessels

their landfall.

Fastnet

and

Clear,

The

about 4J miles to the

lies

a light of great importance

making the south which

rock,

coast of Ireland as

surrounded by deep water

in almost every direction, attains a

maximum

height of 87

feet above high water, the base of the tower being about

70 feet above that level

180

feet long

;

about 340 feet long and

broad at low water, and on the top about 120

feet

and 64

feet broad.

feet diameter at base

The lighthouse

and 63

feet high,

and

plates weighted with concrete at the bottom. also fixed to the rock is

bolts 2 feet long.

(Plate VI.)

consists of iron

The

structure is

The iron casing

lined with brick varying from 2 feet 9 inches thick at

bottom to 9 inches at

top.

The dwelling-houses, wliich

built close to the lighthouse, are

The

sea has excavated

made

some of the

are

of plates of cast iron. softer slates of

which

the rock consists, forming different chasms, one of which, facing the

feet,

extends from about 36 feet above low

west,

water to the

top,

and varies in breadth from about

and penetrates inward from the face of the

about 14

feet.

The top

10 feet of the tower. since its erection,

of this fissure

cliff

extends to within

This lighthouse has been strengthened

and the

up with brickwork ^

1 foot to

fissure in the rock has

been

filled

set in cement.

Account of the Skerryvore Lighthouse, by Alan Stevenson, LL.B.

Edin.

1 848.

LIGHTHOUSE CONSTRUCTIO^T.

Bishop BocL-^The Bisliop Eock

The

the Scilly Isles.

lies off

lighthouse was an iron pillar structure, which was

first

unfortunately destroyed before

was completed; and in

1852-8 the present stone structure was erected. designs were

by the

late

Both of these

Mr. James Walker, and the present

tower was carried out under the superintendence of Mr.

This lighthouse rises to the height of 100 feet above

Douglass.

high water,

feet in diameter at the base

The lowest part

top.

and 17

of foundation of tower

feet at

covered

The "solid "is 20

about 19 feet at high- water springs.

feet

above high water, where the walls are 9 feet in thickness, 2

decreasing to

The

feet at the top.

interior

was

laid

Douglass

July 1852, and the

states that "

last in

first

stone

August 1857.

Mr.

during a storm in the winter of 1874-75

completion of the

the heaviest seas experienced since the

lighthouse in 1858 were encountered.

this occasion the

tremor of the building was so great as to cause

leave the shelves.

his (Mr. Douglass')

and radiating

ties of

articles

recommendation,

by strong

the building has since been strengthened vertical

divided

The

into five stories, each 13 feet in diameter.

internal

wrought iron screwed

to the

walls and floors."

Wolf Bock.

â&#x20AC;&#x201D;

This rock

and the Lizard Point, and

170

feet long

and 114

depth of about 2 this

work was

was not

till

1862

lies is

about midway between Scilly

a hard porphyry.

feet broad,

feet, at

and

about

submerged to the

The

high water.

1823 by Mr. Eobert that a lighthouse

design for

first

Stevenson.

But

was commenced from a

design by the late Mr. James Walker, under the supermtend-

ence of Mr. James N. Douglass,

Its height is

116^

feet 8 inches diameter at base, decreasing to

the springing of the cavetto course.

The

" solid,"

feet,

feet at

excepting

LIGHTHOUSE TOWERS.

39^

a space for a water -tank, extends to about

feet in

height, the stones being laid with off- sets or scarcements in

up the

order to break is

smoothly

sea

but the surface above the solid

;

The walls

cut.

at entrance-door are

The

inches thick and 2 feet 3 inches at top.

tower

a concave elliptic frustum of granite, and contains

329 6 J

7 feet 9|-

shaft of the

It is

tons.^

Dim

founded 2 feet above low water.

14 nautic miles from The rock, the Island of Mull, which is which is a hard trap, is 240 feet long, 130 feet broad, with The a rounded top rising to 35 feet above high water. The

Heartacli Liglitliouse

the nearest shore.

curve which was found best suited for this rock was the parabolic.

was considered, on engineering groimds, that

the lines of the parabolic shaft should run continuously into

the cavetto without any belt course, which in this particular case,

owing

to the high level to

which the water

would have had to oppose a very considerable

The maximum diameter minimum 16 feet; the level

to shake the masonry. is

feet; the

was ultimately

amount

fixed at

masonry

tons, of

projected,

tendmg

of the tower of the solid

The

feet above the rock.

3115

tained in the solid part.

force,

total

which 1810 are con-

The barrack

for the

workmen

(Plate V.)

of malleable iron bars, with a malleable

iron

the top, in which the

was made drum placed on

men

lived.

The

height of lower floor of the barrack above the rock was 35 feet.

This tower, which occupied six years in erection, was

under the immediate Brebner.

Owing

superintendence

Mr. Alexander

to the great difficulty of landing, the

working

seasons were Hmited to only about 2i- months in the year.^ 1

The Wolf Rock Lighthouse. By J. N. Douglass. Min. Civ. Eng. vol. xxx. The Dhu Heartach Lighthouse. By D. A. Stevenson. Min. Civ. Eng. vol. xlvi.

LIGHTHOUSE CONSTRUCTION. The Chickens Boch

The tower

its

by a

and plain

belt course

123

feet

4 inches

feet at the base

submerged 5

and 16

feet at

surmounted

and parapet.

entering

3557

tides,

and

lies

open

George's Channel.

and contains 27,922 cubic

2050

tons, the

The

tons.

St.

The tower

feet at the plinth.

high water of spring

masonry, weighing

the tower being

axis,

of a

in height; the outside diameter

solid is 32-1- feet in height,

feet of

the Calf of Man.

cavetto, abacus

to the south-westerly seas

The

off

asymptote as a vertical

hyperbola about

one mile

lies

a frustum formed by the revolution

whole weight of

walls, as they rise

from

the solid, are 9 feet 3 inches in thickness, decreasing to 2 feet 3 inches

The work, which was

below the belt course.

1869, and completed in 1874. Great Basses Lighthouse.

â&#x20AC;&#x201D; The

Great Basses Eeef

lies

about 80 miles to the eastward of Point de Galle, and 6 miles from the nearest land.

The lighthouse was designed

by Mr. James N. Douglass, and

stone rock, measuring

and

rising 6 feet

range of the tide

tower

220

placed on a hard red sand-

above the mean sea is

about 3

feet,

feet in breadth,

The extreme

and the foundation of the

2 feet above high water.

level.

feet in length,

The tower

consists of

a cylindrical base 30 feet in height, 32 feet in diameter,

surmounted by a tower 67 diameter at base, and

feet 5 inches in height,

feet in

7 feet at the springing of the cavetto,

the thickness of walls being 5 feet at the base and 2 feet at the top.

The

cylindrical base is solid for

and base contain 37,365 cubic ing about

2768

tons.

1870, and the Hght ^

The Great

Basses.

11^

feet of granite

feet.

The tower

masonry, weigh-

The work was begun on 6th March

w^as exhibited three years afterwards.^

By W.

Douglass.

Slin. Civ. Eng., vol. xxx^aii.

LIGHTHOUSE TOWERS. rig.

11 shows the Cordoiian Tower, in France, begun in

Fig. 11.

the time of Louis XIV., which

given more as a specimen

LIGHTHOUSE CONSTRUCTION.

of early liglitlioiise architecture than of a tower

much exposed

which

to the sea.

There are several other lighthouses in situations more or less exposed, such as the Horsburgh, near Singapore, de-

signed and executed by Mr. J. T.

Thomson

;

Alguada Eeef, which, though somewhat higher, respects, a replica

that on the in other

is,

the Skerryvore, and was successfully

by Captain Fraser; the Prongs in India, by Mr. Ormiston; the lights in the Eed Sea by Mr. Parkes; the South Eock and Haulbowline in Ireland; CraighiU carried out

Channel (Plate VII.) and Spectacle Eeef in America (Plate

which are exposed

VIII.),

to the

action of

Those which have been described

ice.

moving

floes of

in detail are the

most remarkable, and some of them will be referred illustration of the principles of construction,

to in

which wiU now

be explained.

General Principles which regulate the Design of Lighthouse Towers exposed to the Waves. 10. of

We cannot do better, as

a preliminary to the subject

Lighthouse construction, than quote the following remarks

by the "

late

]\Ir.

Alan Stevenson

primary inquiry as to towers in an exposed situation,

the question,

Whether

their stability should

strength or their weight or

their

^ :

inertia?

;

or,

depend upon their

in other words, on their cohesion

In preferring weight to strength

closely follow the course pointed out

by the analogy

Ave

of nature

;

must not he regarded as a mere notional advantage, for the more close the analogy between nature and our works, the less and

this

Account of Skerryvore Lighthouse.

burgh, 1848, page 49.

By Alan

Stevenson, LL. B.

Edin-

GENEKAL PRINCIPLES. difficulty

shall experience in passing

from nature to

upon the

artificial project.

for example,

If,

art,

and

phenomena bear

the more dii-ectly will our observations on natural

we make

a series of

observations on the force of the sea as exerted on masses of rock, to draw from those observations some conclusions amount and direction of that force, as exhibited by the masses of rock which resist it successfully, and the forms which

and endeavour as to the

those masses assume,

shall pass naturally to the determination

of the mass and form of a building which

ing similar forces, because

may be

conclude, with

capable of oppos-

some

reason, that the

mass and form of the natural rock are exponents of the amount

and

direction of the forces they have so long continued to resist.

It will readily

natural

phenomena

;

for

when we

are in a very different

less

solely concerned, to deduce

capable of resisting the same

artificial fabric

we must

and

attempt, from such observations of

in which weight

the strength of an forces

be perceived that

advantageous position

once pass from one category to another, and

endeavour to determine the strength of a comparatively light object

which

shall

we know, by may be resisted by a given weight. Another reason why we should prefer mass and weight to

be able to sustain the same shock, which

direct experience,

very obvious

strength, as a source of stability, is

is,

constant and unchangeable in

which

that the effect of mere inertia

its

nature

;

while the strength

even from the most judiciously-disposed and well-

results,

executed fixtures of a comparatively Hglit

fabric,

constantly

by the loosening of such fixtures, occasioned by the almost incessant tremor to which structures of this kind must be subject, from the beating of the waves. Mass, therefore,

subject to be impaired

seems to be a source of

stability,

tive principles

the effect of Avhich

at once

more in harmony with the conservaof nature, and unquestionably less hable to be

apprehended by the mind,

deteriorated than the strength which depends

upon the

careful

proportion and adjustment of parts.

Of any

* "

solid,

ultimate stability,

viewed

as monolithic, it

by which

may be

would understand

its

said that its resistance to

LIGHTHOUSE CONSTRUCTION.

the final effort which overturns

will greatly

it,

centre of gravity being placed as low as possible sectional

form which

its

and the general

this notion of stability indicates is that of

This figure

triangle.

depend upon ;

revolving

its

vertical

axis,

a of

must,

which has its centre of gravity its rounded contour would

course, generate a cone as the solid,

most advantageously placed, while oppose the least resistance Avhich

attainable in every direction.

Whether, therefore, we make strength or weight the source of stability,

the conic frustum seems, abstractly speaking, the most

advantageous form for a high tower.

which concur

siderations

modify

practice, to render the conical

be imagined. retically,

although,

as to

must

is,

and

first

confess, not

the most influential in

that the base of the cone is

must in many

to stand, in such

form an angular space in which the waves may

break with violence. in practice,

than might at

less eligible

meet the foimdation on which the tower

manner

are various con-

these considerations, the most prominent theo-

guiding our practice, cases

form

But there

this general conclusion, and, in

The second objection

more considerable

founded on the disadvantageous arrangement of

the materials, which would take place in a conic frustum carried to the great height which, in order to render

them useful Towards

marks, lighthouse towers must generally attain.

as sea its

top

the tower cannot be assaulted with so great a force as at the base, or rather, its top

entirely above the shock of heavy

intensity of the shock

which

waves

;

and

sohd should be proportioned to the

as the diameter of the conoidal

must

resist, it

follows that, if the

base be constructed as a frustum of a given cone, the top part

ought to be formed of successive frusta of other cones, gradually less slanting

than that of the base.

Eut

obvious that the

union of frusta of different cones, independently of the objection

which might be urged against the sudden change of direction their junction, as assault, tour,

affording the waves a point for advantageous

would form a

figure of

inharmonious and unpleasing con-

circumstances which necessarily lead to the adoption of a

curve osculating the outline of the successive frusta composing the

tower

;

and hence, we can hardly doubt, has

really arisen in the

GENERAL PRINCIPLES. mind

Smeaton the beautiful form which

his genius invented for

the hglithouse toAver of the Eddystone, and "which

subsequent

engineers have contented themselves to copy, as the general outline

which meets

And

solve.

by no means

all

the conditions of the problem which they have to

here I cannot help observing, as an interesting and uniisual psychological fact, that

to be conducted to a right

men sometimes appear

by an erroneous train of Narrative,' we are led to believe

conclusion

and such, from his must have been the case with Smeaton in his own conception of the form most suitable for his great work. In the Narrative imply that he seems to the trunk of an oak was the counter(ยง 81), reasoning

which his

part or antitype of that form

than calculations

him

led

between the case of the

tree

being assaulted at the top,

(ยง

246)

Now,

feelings, rather

is no analogy and that of the lighthouse, the tree and the lighthouse at the base ; and

to prefer.

there

although Smeaton goes on, in the course of the paragraph above alluded

suppose the branches to be cut

to, to

wash round the base of the oak, not thereby strengthened

;

forces can

it is

and water to

off,

to be feared the analogy is

as the materials

the tower are so different, that

same opposing

it is

composing the tree and

impossible to imagine that the

be resisted by similar properties in both.

Smeaton has unconsciously contrived to them with a fancied natural analogy between a tree which is shaken by the wind acting on its' bushy top, and which resists its enemy by It is obvious, indeed, that

own

obscure his

clear conceptions in his attempt to connect

its fibrous texture and wide-spreading ligamentous and a tower of masonry whose weight and friction alone enable it to meet the assault of the waves which wash round its

the strength of roots,

base

and

this subject,

of the tree

and the

"In

very singular that, throughout his reasonings on

he does not appear to have regarded those

which he has most

coherence of

fitly

its parts.'

a word, then, the

sum

contained in this proposition

characterised as

its

j)roperties

its elasticity,'

of our knowledge appears to be

That, as the ultimate stabiKty of a

sea tower, viewed as a monolithic mass, depends,

on the lowness of

cceteris

paribus,

centre of gravity, the general notion of

its

LIGHTHOUSE CONSTEUCTION.

28 form

that of a cone

but that, as the forces to which

;

horizontal sections are opposed decrease towards

the solid should be generated

ratio,

its

several

top in a rapid

its

by the revolution of some and gradually approach-

curve line convex to the axis of the tower, ing to parallelism with

And

it.

this

tion of the Eddystone tower devised

Thougli

views

tlie

been so well expressed,

liave

W'liicli

and the general conclusions

which Mr. Alan Stevenson

arrived, are unquestionably correct,

extensions of these

in fact, a general descrip-

is,

by Smeaton."

some modifications and

seem, nevertheless, to be required in

The following

order to give a complete view of the subject.

may

rules of

be regarded as of general application to the design

aU towers 1.

cient

plane,

masonry in exposed

situations

They should have a low centre

mass

to prevent their being upset

They should be throughout and

either straight

vertical plane, so as to present

which would check the

of gra\dty,

and

suffi-

by the waves.

circular in the horizontal

continuously curved in

no abrupt change

the

of outline

free ascent of the rising waves, or

the free descent of the falling waves, or the free vent of those

passmg round the tower.

external scarcements ia the

vertical plane, or polygonal outline in the horizontal plane,

are therefore objectionable. 3.

Their height,

by the distance the

sailor.

The

cceteris

paribus, should be

which the

rule for

determined

light requires to be seen

determining

tliis

height will be

afterwards given. 4.

Where

the rock

is soft,

or consists of ledges

easily torn up, the tower should spring

which are

from the foundation-

course at a low ancle with the surface of the rock, so as to

GENERAL PRINCIPLES. prevent

its

being broken np by reaction of the waves from

the building

curved

or,

;

in other words, the

shown

profile, as

But

in Fig. 12.

tower must have a especial care should

be taken to sink the foimdation-coui'ses below the surface as

the

superincumbent

weight decreases

-svith

the sine of the

the

rock,

angle of inclination of the wall.

If the

rock overhangs, owing to the wearing action of the waves, the tower should, if possible,

be built at a distance from

the place where this dangerous action is

in progress.

Where

ample

the rock

area, the

tower

hard and of

may

be of such a

curved form as will best suit the economic Fig. 12.

arrangement of the materials, so as to avoid an unnecessary thickness of the upper walls.

Where

the rock

hard, but of small dimensions, the

diameter above the base should not be suddenly reduced

by adopting a curved

profile,

adopted

(as in Fig. 13);

smaller

dimensions,

and

which

all

the rock be hard, but of yet

cylindric

to thicken the walls (Fig.

be raised

but a conical outline should be

form should be adopted

14), and

to increase the friction,

directly proportional to the weight, the tower should liigher

cases

than

the diameter

where the rock

walls should be decreased

had

been

greater.

small the thickness of the

steps or scarcements internally,

as in Fig. 14, but never externally. 7.

The

level of the top of the solid part of the tower,

and the thickness of the walls above

it,

shoidd, in different

LIGHTHOUSE COXSTKUCTION.

towers having the same exposure, be determined in each case

by the

level

of the sea. ledge,

But

and configuration of the rock and of the bottom

Unfortunately, in the present state of our know-

no rule can be given

facts, illustrative

tion of the rock itself

for

dealing with such cases.

of the great influence of the configura-

on the action of the waves, are given

in a subsequent section.

Fig. 13.

The best

Fig. 14.

position for the tower

highest part of the rock.

not necessarily the

It should, in each case, be selected

so as to secure the greatest protection in the direction of the

maximum

fetch

and deepest water near the

The tower should not, any chasm which divides the any gully which projects into 9.

if possible,

reef.

be erected across

rock, or in the direction of it,

especially if

be of con-

verging form which would concentrate wave action. 10.

No permanent

fixture of the tower to the rock

GENEKAL PRINCIPLES.

required for increasing the stability of the structure.

foundation-course (unless where a curved profile

adopted)

becomes, indeed, in the end the most stable of

all,

has the greatest weight above

its place.

11. together,

it,

keep

The

because

The stones should, however, be sufficiently connected and fixed to the rock, in order to prevent their

being washed away during the

of the work,

construction

when they have no superincumbent weight

keep them in

their beds.

12.

The tower should

rest

on a truly level base, or on

level steps cut in the rock.

13.

The pressure of

all

the materials within the tower

should act vertically, so as not to produce a resolved force acting laterally as an outward thrust, 14.

with

The tower should be

walls

of such height

and diameter,

of such thickness, as to prevent the

masonry

being disturbed by the impact of the waves. 15.

The entrance door should be placed on that

the tower where the length of fetch

side of

shortest, or where,

from the configuration of the reef and the depth of water, the force of the waves Bell

Eock by the

the lower

parts of the

tion being least

16.

is least.

This was determined at the

distribution of the fucus which

grew on

tower during the winter, the vegeta-

where the waves were

The materials should be

heaviest.

of the highest specific gravity

that can be readily obtained, and, in some special cases, lead, or dovetailed blocks of cast-iron set in cement, might per-

haps be employed.

LIGHTHOUSE CONSTKUCTION.

AlTLICATION OF FOREGOING EULES TO THE CONSTRUCTION OF Towers.

11. There are no better examples of what to avoid and

what

to imitate in lighthouse construction

which were erected on the Eddystone

in the towers

The

rock.

first of these (Winstanley's), both in general design

details,

ing,

than are to be found

and

must be placed in the vetanda of maritime engineer-

while Eudyerd's and Smeaton's contain between them

what seem

be the best models.

Winstanley's Tower. in all

its

parts that

it is

â&#x20AC;&#x201D;

This tower

so consistently

bad

unnecessary to do more than simply

state its defects. 1.

which

failed to take full

the rock, small as

advantage of the available base

it is,

afforded.

This error he no

doubt did his best to rectify during the subsequent years in

which the work was 2.

in progress.

Instead of being circular in the horizontal plane,

was polygonal. 3.

In his blind devotion to ornamentation he violated

the principle of uniformity of external surface. 4.

The only redeeming point

he did not, in the end, throw

in the structure

was that

away the diameter

of the

foundation by adopting a curved vertical outline. Budi/erd's Tower.

â&#x20AC;&#x201D;In whatever

tower, conspicuous for

its

direction

look, this

simplicity of general design

and

careful adaptation of the different parts, will be found to fulfil

the conditions of almost every axiom 1.

was erected on a

level base

we have mentioned. ;

or at least the slant-

ing ledge was cut out in level steps, so far as the hard nature of the stone would allow.

APrLICATIOX OF KULES. It

was

circular in tlie horizontal plane, instead

polygonal, as in Winstanley's tower.

The carpentry was most judiciously constructed

had not

sufficient

weight in

bolted to the rock, and the boltuig

itself, it

was

but,

;

required to be

and

of an mgenious

effective kind.

did not, however, depend wholly upon strength

of fixtures, but

upon

from his placing

weight, as appears

courses of masonry above the lower portion of the timber-

work.

did not needlessly

by the

afforded

throw away

size of the rock,

the

diameter

but adopted the conical

form, sloping from the outside at the bottom to the size

required for the light-room accommodation at the top.

all

preserved a uniform external surface by avoiding

and ornamentation

outside projections

made

shutters were

to close flush

;

even the window

with the outer

face.

There were no arches in any part of the biulding

tending to produce an outward lateral thrust.

The only

defect

some

durability of

catch

used as it

little

in the

want

of weight

and

the materials, and their liability to

may, perhaps, be said that he should have

fire.

was

wood and as much stone as he could, because but it must be recollected that he acted in

was heavier

;

the matter as a contractor, and perhaps thought himself justified in

employing only as much weight as he considered

sufficient

and

;

in this

he judged correctly, for the building

withstood aU the storms of forty-six years, and yielded at last,

not to the action of the waves, but to an accidental

which destroyed the work. in the records of the

There

fire,

probably no instance

profession of such a wholly untaught

engineer being found able

once to

grapple with the

LIGHTHOUSE CONSTRUCTION.

difficulties of so

novel a problem and eventually to produce

a true masterpiece of construction.

further remark-

able that, after having achieved his difficult work, he seems

suddenly to have retired either to his mercer's shop or into private

life,

instead

of,

like his great successor,

So far as

leading engineer of the day.

stone was his

first

and

Smeaton's Tower.

last

â&#x20AC;&#x201D;

becoming the

known, the Eddy-

work.

This tower cannot be said to have

been in any respect more successful as regards resisting the

waves than Eudyerd's, which was the success of Smeaton's Lighthouse tion

but

;

to a very

no doubt

its

best vindica-

should not, as I have elsewhere said, be regarded

model

as a safe

The

earlier of the two.

which are exposed

for imitation at all places

heavy

For rocks

sea.

such small area

would

surely be safer to adopt a conical form like that of Eudyerd's. certain diameter for the foundation

Having once secured a

on the rock, that diameter should

not, in

opinion, have

been suddenly thrown away higher up by adopting a sharplycurved

The

profile.

case

wind has been already

way

an oak tree resisting

shown

satisfactorily

analogous to that of a tower exposed to the surf

greatly as this curve has been admired,

followed in

The advantages

all cases.

the

be in no ;

and

ought not to be

wliich

it is

supposed

to possess seem, in such a situation as the Eddystone, to be

more to be

fanciful

than

compared

real,

for

moment

the loss of diameter resulting from

The arched

adoption.

and certainly not

floors

which the introduction

its

were also a source of weakness,

of the iron

hoop was intended

counteract.

The rock was even

at the

Eddystone, though of great hardness,

in Smeaton's time considerably hollowed out under-

neath by the grinding action of the waves.

In 1818 Mr.

APPLICATION OF RULES. E. Stevenson, on visiting the Eddystone,

made

the following

remarks as to the wasting of the lower part of the rock

—

" I

conclude that,

when

the sea runs high, there

of this house being upset after a lapse of time,

and shingle have wrought away the rock

danger

when

the sea

to a greater extent.

JSTothing preserves this highly important building

but the

hardness of the rock and the dip of the strata, but for long a period this

The and

may remain no one

how

can pretend to say."^

fears thus exjDressed as to the wasting of the rock,

my own

convictions in 1864, and also those of Captain

Fraser^ as to the insufficiency of the tower itself for exposed situations,

said

—

have smce been

verified.

Mr. Douglass^ in 1878

For several years the safety of the Eddystone Light-

house has been a matter of anxiety and watchful care to the Corporation of the Trinity House, owing to the great tremor of the building with each wave-stroke during

heavy storms

from the westward, more especially when from west-south-

The

west.

joints of the

masonry have frequently yielded

the heavy strains imposed on them, and the sea water has

been driven through them to the interior of the building. structure has been strengthened on

The upper part of the two occasions

—

viz.

1839 and

strong internal wrought-iron floor

downwards

last occasion it

ties,

again

1865

—

wdth

extending from the lantern

to the solid portion of the tower.

the

was found that the chief mischief was caused

by the upward stroke

of the

heavy seas acting on the pro-

jecting cornice under the lantern gallery,

portion of the building above this level.

which

lifted

the

After reducing

the projection of the cornice about 5 inches, and well fasten1

By David Stevenson, Edinburgh. 1878. Design and Construction of Harbours. Edinburgh, 1864. ^ Miu. of Pro. of the Inst, of Civil Engineers. Vol. liii.

Life of Kobert Stevenson. ^

P. 47.

LIGHTHOUSE CONSTRUCTION.

ing the stones together with through bolts, no further serious

leakage has occurred at this part."

Mr. Douglass says further as to the action of the waves on the rock

itself:

which the tower

â&#x20AC;&#x201D; "The

portion of the gneiss rock on

founded has been seriously shaken by

the incessant heavy sea-strokes on the tower, and the rock is

considerably undermined at

its

base."

already stated,

the Trinity House have resolved on the removal of Smeaton's

new lighthouse from now being carried out,

tower, and the erection of a of Mr. Douglass,

which

12. Modifying influence of on hreahing waves.

German Ocean upper

the

freestone

of rocks

November 1817, the waves

of the

feet high,

which was the reef.

the configuration

overturned, just after

part

â&#x20AC;&#x201D;In

the design

had been

of the

finished,

Carr Eock Beacon, a column of

and 17

feet diameter

at the base,

largest diameter that could be got on the

curved profile was adopted, but this seems to have

been warranted by the unreliable character of the sandstone

on which

was

The diameter

built.

at the level of high

water was 1 1 feet 6 inches, and at the plane of fracture 1 feet 9 inches.

When

the history of this beacon

contrasted

with the records which have been preserved of the different structures that were erected on the Eddystone

there

by Winstanley,

every reason to suspect that the configuration of that

rock must have the effect of modifying or diverting the force of the waves. to his

original tower

improvements,

may

the additions which Winstanley

were in many respects anything but be questioned whether

accelerated rather than delayed stood,

however, until the

greatest

storm ever

swept away.

From

full

made

fifth

by those winter,

experienced in

its fate

was not

alterations.

when, during the

this coimtry,

was

the records of that tempest I think

INFLUENCE OF SHAPE OF KOCKS.

may

was

fairly

violence

which

questioned whether Winstaiiley's work

knocked down by the sea or overturned by

really

the

its

meter, stood

of the

Eudyerd's tower

wind.^

base was

timber,

only about twenty -two feet dia-

forty-six years

before

was burned.

further remarkable that, while at the building of the Bell

Eock, three stones,

were

lifted after

all

dovetailed, wedged,

they had been permanently

and

trenailed,

and

set,

at the

Carr Eock Beacon twenty-two stones were displaced during construction, there

its

was no instance of any but

loose,

un-

having been moved during the erection of the

set blocks

Eddystone, although the stones were of very similar weight,

and

fixed in

much the same manner. Smeaton mentions that was thus fixed we never in fact had an instance

" after a stone

of its having been stirred It

by any

action of the sea whatever." ^

doubtless, a diflScult question to account for the

is,

Eddystone tower withstanding so many storms, and at

succumbing to their

assaults.

may

last

be that some altera-

tion of the contour of the ledge at the bottom, produced

the erosion of the waves, has changed the direction of their

impact on the upper parts of the tower, so as at the masonry in a

whatever

may

way which

last to

did not do at

shake

first.

But

be the explanation, there can be no doubt of

the fact that the masonry was so shaken at last as to lead to

being strengthened, and ultmiately to the erection of a

its

new

tower.

Additional and very striking corroboration of the influence of the

shape of rocks

on towers has also been afforded

by the experience derived ^

in the construction of the

Historical Narrative of the Great

happened Nov. 26, 1703. Lond, 1769. * Account of the Eddystone, p. 132.

Dhu

and Tremendous Storm which

P. 148.

LIGHTHOUSE CONSTRUCTION.

Heartacli Liglithouse.

During a summer

each of two tons weight, which

g?i\e,

fourteen stones,

had been

fixed in

the

tower by joggles and Portland cement, at the level of 37 feet cibove high water, were torn out and swept off into deep water. It

a remarkable fact {See Plate

I.)

that the level above

the sea at which these 14 blocks were removed by a summer gale, is the

same

Winstanley's

as that of the glass i^ancs in the lantern of

first

Eddystone Lighthouse,

wliich, nevertheless,

And

stood successfully through a whole winter's storms. in

tower,

his

level,

constructed,

as last

there

was

were supported on

pillars,

during four wmters.

and

this

structure stood

fragile

In consequence of the experience of

storms, the solid part of

Dhu

Heartach lighthouse was altered

from the original design, and carried up

the

same

above high loater as the ijrescnt lantern in Smeaton's

These

same

at the

an open gallery, above which the cupola and lantern

level

toiver.

facts surely afford sufficient proof of the great influence

of the form of rocks on the action of the waves.

The following of

Dhu

table from

different lighthouse towers

Name.

Mr. D. A. Stevenson's account

Heartach shows the relative levels of the :

solid at

INFLUENCE OF SHAPE OF HOCKS.

The very remarkable

cases

wave -action exerted

high levels on the rocks of Whalsey, Unst, and Fastnet,

same

are further corroborative of the

The facts

is,

conclusion, then,

that the

level

which

the

phenomena.

class of

deducible from these

is fairly

plane of impact of the ivaves above

high water depends upon the relation subsisting between their

and

height

and

height

the

high

belotv

and

configuration of the rocks above

ivater, as well

as on the configuration of the

bottom of the sea near the lighthouse. at

Dhu

Heartach, from

Thus, while the rock

height above high water, forms a

its

great protection against the smaller class of waves, as

a dangerous conductor

operates

to the largest waves, enabling

much

to exert a powerful horizontal force at a

The

than they would had the rock been lower.

them

higher level facts

which

have been stated do not therefore necessarily prove that the

waves are exceptionally high Tastnet, but

may

these rocks.

Dhu

to the rock,

reach

Dhu

Heartach and at the

be accomited for by the configuration of

no doubt

A. Stevenson,^ that there the sea at

by Mr. D.

true, as discovered

a deep track in the bottom of

Heartach, extending from the ocean nearly

and

that, therefore, a

higher class of waves must

than woidd otherwise have been the case

influence of the configuration of the surface

;

still

sufficiently

obvious both there and at the other places referred is

the

to.

essential that these facts should not be overlooked, for

a rock

may

either shelter a tower

from the waves,

other hand, increase their force against strike higher

up than

it,

or,

on the

and cause them

the rock had been smaller, of a

different shape, or at a less elevation above the sea.

13. the

Centre

building 1

of Gravity of the Tower.

erected

by Winstanley,

Mill, of Pro. of the Inst, of Civil Engineers.

â&#x20AC;&#x201D;

the

except

manner

Vol. xlvi.

LIGHTHOUSE CONSTEUCTION.

destruction of which there

we do not

any information,

possess

no record of a lighthouse of masonry being overturned

en masse

by the

And

force of the waves.

all

towers which are

fully exposed are necessarily of such dimensions as to resist

being overset by the force of the wind.

Hence the

deter-

mination of the position of the centre of gravity need hardly be considered in towers of the ordinary construction.

we have

in the one hypothetical case

But

referred to, of a cylindric

tower being placed on a very small rock and carried high

enough

throw

sufficient

weight upon the lower courses,

that the structure

is possible

may

be overset by the com-

bined impact of the waves below and the

mnd

above.

The

experiments at Dunbar with the marine dynamometer show that on a curved wall the horizontal impact of the waves at

2 3 feet above high water was only -^th of the vertical upward force with

waves of a certain

reduction of force, surface

much

it is

we keep

class.

If,

further very largely reduced, there can hardly be

risk to the finished

masonry at 20 or 30

high- water level with waves of ordinary case

water

are

besides this large

view that on a semicylindric

now

size,

feet

above

when, as in the

considering, the direction of the impinging

Though

not materially altered by the rocks below.

I do not wish to speak decidedly

and where so much yet requires

where

to be

so little

known,

still I

known venture

to express the opinion that, in so far as relates to the resist-

ance to horizontal

force,

no jogglings are needed higher than

about 30 feet above high water in a cylindrical or nearly cylindrical tower on

which the waves

filaments of water are not vertical plane

But

much

strike freely,

and the

altered in direction in the

by sloping ledges outside

of the foundation.

the slope of these ledges directs the mass of water

obliquely, so as to strike far above the

bottom of the tower,

FORCE OF WIND.

Dim

as at

Heartacli or at the Fastnet, additional precau-

tions should certamly be ado^^ted in designing the tower.

14. Force of Wind on Towers.

—Leonor

following formula for the force of the

Where S

Fresnel gave the

wind on towers

"W the weight of a masonry, n the number of cubic yards in

cubic yard of

tower, h the diameter of base of tower in yards, in square yards,

wind per

the stability of the tower,

the height in yards, and

the area

the force of

yard

superficial

Wn4S =

•^3 But

in this, as in other formuloe, the strength of the

assumed as uniform I lately made,

at all heights.

appears to

the land:

whose vertex below the surface of the

a pole 50 feet

—

liigh, is

observations,

and v and

which were

applicable to lighthouses on

the lower point on the tower (not less

than 15 feet above the ground), and point,

be 72 feet for winds of moderate strength.

The formula deduced from these

made on

wind

observations which

appears that the force increases vertically

in a parabolic curve,

ground

From

V the

that at a higher

respective velocities at those points

+ +

In the cases of towers erected in the sea the formula will be probably somewhat

different, for the ratio of friction will

less for a yielding surface

such as water, than for the solid

ground.

15. Relative Forces of Currents of

Equal

Velocity.

—The

relative forces of

Wind and Water

wind and water on

the tower are in the proportion of the squares of the velocities

LIGHTHOUSE CONSTKUCTIOX.

multiplied by the specific gravities velocity of the

;

so that

and v the corresponding velocity of the

air,

water capable of producing the same force as the if

S and

be the

air

be the specific gravity of sea water and of

and

;

air

=Vi And and

the specific gravity of sea water be taken at 1-028,

-001234 and v be taken

air at

as unity, then

V= 29-27

times that of moving water required to produce the same force.

There

are difficulties in dealing with elements so

different physically

as air

and water, but

be regarded as at least approximately

this

result

may

correct.

â&#x20AC;&#x201D;

16. Lighthouse Towers of Cement Buhhle or Coiurcte. The first rock station at which cement was employed, instead of lime-mortar,

was

in Shetland, already referred in

1873

the tower of

1854,

at the tower of

to.

in cement, but

La Corbiere

exposed to the action of the waves.

it is

not

In 1871 I proposed

1871) that monolithic towers

{Nature, September

Unst

Sir John Coode erected

of Port-

land cement rubble might perhaps be employed for towers exposed

the full

force of the

waves.

The advantages

attending such a plan were stated to be the following " 1.

The dispensmg with

squaring or dressing of

all

materials. " 2.

The

suitableness for such

quality, thus rendering

from a distance or " 3.

materials " 4.

No is

work

any stone

hard

unnecessary to bring large materials

open quarries

for ashlar.

powerful machinery for moving or raising heavy required.

savins in the levelling of the rock for a founda-

tion for the tower.

IKON TOWERS. " 5.

The ease

of landing small

fragments of stone on

exposed rocks, as compared with the landing of heavy and finely dressed materials."

How

far this

system

may

be avail-

able in the erection of lighthonses in

the sea

a question which can only

be settled by actual of

this

Messrs.

Stevenson

exposed

situation

Mull, and

trial.

beacon

was erected by

construction

1870,

Island

the

off

has not as yet shown

any symptom of decay. 17. Iron

been

Towers.

constructed

pillars

— Towers

both

and plates in

this

have

cast-iron

country and

in America, examples of which are given in Plates VII. and XXI., and in Fig.

15.

Some

already

of the

been

remarks which have

made

regarding

stone

towers may, imdatis mutandis, be applied to those of iron.

18. Mitchell's Screiv

Pile.

—Where

a light has to be placed on a sandbank, the principle of the screw

pile,

by Mr. Mitchell of Belfast, utility, and many examples cessful application

in this country

may

invented of great

its

suc-

be found both

and in America. Ficr.

19.

Ice.

— The

difficulties

Towers

exposed

American

Floes

Board have

15.

had

contend

with

which are seldom met with in Britam, and never

LIGHTHOUSE CONSTRUCTION,

44 to

the

same

extent.

Floes

ice

present

formidable

difficulties to the erection of towers which are exposed to

such extreme lateral pressure.

show two American

Plates No. VII. and VIII.

towers, the one constructed of iron

the other of stone, both of which have

form of

â&#x20AC;&#x201D;

and

to resist this peculiar

assault.

20. Modes of Uniting the Stones and Courses of Masonry. Plates II. and III. show the mode of combining the stones

during construction at different lighthouses in this country

and in America.

The Dhu Heartach mode

was suggested by Mr. Alan Brebner,

C.E.

of connection

CHAPTER

11.

LIGHTHOUSE ILLUMINATION. 1.

The

best

intricate

and

clearest

method of explaining the somewhat

arrangements of the various forms of lighthouse

apparatus will be to give a continuous historical account, from the earliest period to the present date, of the different optical

instruments wliich have from time to time been invented, so as to

show the remarkably gradual development

ferent systems

many

respects

instructive

of the dif-

These successive, and

illumination.

steps

advancement, extend-

ing over a period of more than a century,

will, so far

consistent with clearness, be described under each separate

head in the order of their publication, or of their employment in lighthouses.

The problem

of lighthouse illummation

threefold,

and

involves to some extent both physical and geometrical optics

but the fundamental principles on which most of the combinations depend, rest really on two or three simple element-

ary laws of catoptrics and dioptrics.

Our

be given,

itself,

1st, to

the source of the light

attention

must

which should

produce a flame of constant intensity, and which should, as

shall afterwards see, be of the smallest possible

Given the source of

light, optical

to collect the greatest possible

the flame, and to direct

them

number

bulk

;

2d,

apparatus must be designed of rays

coming from

certam parts of the horizon

LIGHTHOUSE ILLUMINATION.

and the sea

same

and 3d,

;

When

lights

becomes further necessary

line of coast, it

recognisable from each other. stantly in view

and then waning

a third delivering

its

suddenly eclipsing

;

The progress

fire to

greatest

till

power

the full

till

complete extinction

all at once,

and then

a fourth illuminating only a small sector

and

;

One, for example, being con-

another waxing in strength

;

flash is attained,

from a coal

to introduce

their character, so that they shall be at once

distinctions in

of the horizon

are multiplied on the

so forth.

improvement in the source of illumination

the electric light will be readily conceived

;

but to understand fully the gradual improvement of the apparatus,

explained, and

necessary that some points these will

looked for along different

first

an ever-increasing success in economising the light by

find

the following means cepting more and

â&#x20AC;&#x201D;

1st,

By the

design of apparatus inter-

of the rays,

and directing them with

greater accuracy in the required directions

the

should

show that an advance is to be lines, and that we must expect to

number

;

2d,

reducing

of optical agents which are employed in produc-

ing this result, one

new instrument being made

doing a greater amount of optical work, so as to

capable of

effect singly

what had formerly required two, thus preventing decreasing the amount of absorption, glass

rather than metal, because

causes less wasteful divergence

provements in the application tlie different

;

etc.;

3d,

loss

absorbs fewer rays, and

and 4th,

shall find

im-

of these optical designs to suit

characters of distinguishing lights, as w^ell as

new

certain

of the line of coast, such as

narrow

modifications to meet peculiar wants occasioned

geographical features

By employing

Sounds of varying width, where stronger luminous beams have to

be shown in some directions than in others.

Lighthouse

GENEKAL PKINCIPLES. optics

may

different

with two separate and

said, therefore, to deal

problems

â&#x20AC;&#x201D;

viz. the

equal distribution of light either

constantly or periodically over the whole horizon,

distribution in different azimuths.

and

its

unequal

fully understanding

the different objects which are to be attained, the reader will

the more readily aj^preciate the value of the different steps in the

march

of improvement, from the artless expedient of

or a naked candle, to those optical com-

an open coal

fire,

binations of

maxunum power which

are geometrically

and

minimum number absorptive known material,

physically perfect, because they employ the of agents, consisting of the least

and which condense the whole available sphere of rays diverging from the flame, into one or more beams of parallel rays, or else spread

them uniformly, though with

intensities, over those limited sectors of the sea

which alone require

to be illuminated.

It is

different

and horizon

no more than

a corollary from what has been said, to affirm that every

may

improvement of lighthouse apparatus

simply

he resolved

a method of preventing loss of light ; for, as in mechanics we cannot as follows from the law of the conservation of into

energy

what

â&#x20AC;&#x201D;

bring out more power at one end of a machine than

supplied at the other, nor even so much, for tliere

must always be a surfaces

;

loss

due to the

so in lighthouse apparatus

or combination of appliances bring

friction of the

moving

we cannot by any more

device

light out of the

apparatus than that of the unassisted flame, nor so much,

because there must always be a loss from absorption and other causes.

It therefore follows that the opitimum

that which does the optical

apparatus

by means

of the

minimum

form of

work required

it,

nimiber of glass agents, and pro-

duces that amount of divergence only, whicli

required in

each particular case for the guidance of the mariner.

LIGHTHOUSE ILLUMINATION.

Essential Difference in Poiver of Fixed

2. Lights.

â&#x20AC;&#x201D;

All light

to diverge above the liorizon, so far

below

therefore,

water

must

The kind

sea, or

Every apparatus, on the

it,

can be of no use to navigation.

be used will depend on whether

of apparatus to

be the

everywhere constantly

of the rays

all

first,

then, in order to

the

it is

one which appears

light, or

visible,

make

divergence

natural

round the horizon must not be interfered

but those which would pass to the skies should be bent

down, and those which would

fall

room floor should be bent up.

on the land and the

The

But

the light

light-

fixed light apparatus

ought therefore to parallelise the rays in only.

either allowed

cast its rays as exclusively as possible

only periodically.

;

and Bevolving

because to light up the clouds above the horizon, or

;

wished to show a constant fixed

with

and therefore above the

as to fall short of the sea.

the land and the shore below

which

to the sailor

is lost

be revolving,

the vertical jplane it

has no longer to

illuminate all points of the horizon simultaneously, but only

each point of

at successive intervals of time.

In addition

to the vertical condensation of the fixed light, the periodic flash of a revolving

one should obviously be further strength-

ened by condensing the rays horizontally, and in mediate planes, thus forming of light.

Hence, from

separate solid

beams

all inter-

or columns

more limited requirements, the

its

apparatus for producing the periodic flashes should with the same radiant be enormously more effective than that for

producing a fixed characteristic, because the light which,

when is

fixed, is diluted

by being spread

all

round the

in the revolving collected together into one or

beams, in each of which

all

the

circle,

more dense

rays point in the same

direction, being horizontal or nearly so.

Metallic Reflection.

â&#x20AC;&#x201D;

It is a

well-known fundamental

REFEACTION AND TOTAL EEFLECTION.

law in catoptrics that wlien rays of

light fall

surface they are reflected from

an angle equal

of their incidence

with the angle of incidence, and with the This loss

its polish, is lost

power

reflective

in the act of reflection.

partly due to imperfections in the form or con-

struction of the mirror, reflection,

to that

but a certain amount of light varying

;

of the material and

on a polished

and partly

which produce dispersion by irregular which

to absorption,

arises

From

transformed into heat.

from the

and being there

light entering the substance of the metal,

actual experiments

made on

kind generally used in lighthouse appawas found that at 45° incidence only "556 of the

silver-plate of the ratus, it

was

reflected,^

4. Refraction

and Total

incident light

Reflection hy Glass.

—Wlien

rays

of light passing through air fall obliquely on the surface of

a piece of polished glass, they enter the substance of the glass,

but in doing so suffer refraction, being bent out of

their original direction,

and a certain amount of

which varies with the angle of incidence. refraction refraction,

where fi

is lost,

the angle of incidence, and r that of

the index of refraction of the kind of glass,

and S angle of .

When

light

The deviation by

deviation, is

—

when sm

sin ^

the rays, after being refracted at the

first

surface

of a glass prism, reach the second surface at an angle of

incidence greater than what tion becomes impossible,

called the critical angle, refrac-

and they

suffer "total" or

reflection ; so that, instead of passing out of the air,

they are sent back again into the substance of the

The minimum angle i^

internaP

prism into the

which

Stevenson's Lighthonse Illumination, 2d Edition

" Strictly

speaking, the light reaches the surrounding

glass.

this reflection takes place de:

Edin. 1874, p. 12. but does not enter

air,

it.

LIGHTHOUSE ILLUMINATION.

pends on the index of refraction of the

glass, its sine Toeing

in each case the reciprocal of the index of refraction,

which

for safety should in calculation be taken for the least re-

frangible of the red rays of the spectrum.

When

//,

the

index of refraction of the extreme red rays and r the angle of internal incidence,

i /^

LOSS OF LIGHT BY REFLECTION AND EEFEACTION. apparatus,

appeared that the amount of light transmitted,

after passing

through

From

of the whole.^

follows that

by using

(being 2*6 inches of glass), was "805

these experiments and those of No. 3,

glass instead of metal there is a saving of

about one-fourth {-2 4:9), thus clearly establishing the superiority of glass over metal as a material for lighthouse purposes.

But

in addition to this advantage, curves ground in glass certainly

admit of much greater accuracy of surface-form than those of In the one case

the metallic reflectors used in lighthouses. the result

means

effected

of mierring

a gradual process of grinding by

machinery of

struction; while in the other

it is

rigid

and unalterable con-

attained

a comparatively

rude tentative manual process, and subject, therefore, to

all

the imj)erfections to which such methods of working are

obviously is

The polish

liable.

too,

on which so much depends,

in the glass apparatus given once for all

by the accurately

constructed machinery of the manufacturer metallic polish

atmospheric action, and rec|uires to have

renewed by a succession of

whom

less skilful of

and permanent broken never

whereas the

;

constantly undergoing deterioration from

injuries.

loses its

its

brilliancy daily

different lightkeepers,

may

from the

receive ineradicable scratches

Thus, though the glass while imcorrect form, the metallic polish

may

be deteriorated, and even the curve of the mirror may be altered

by an

shall

accident.

now proceed

to give a historical account of the

application of optics to lighthouse illumination I.

To THE ILLOIINATION OF

E^TIEY AZIMUTH, OE OF CEE-

TAIN AZIMUTHS ONLY, BY LIGHT OF EQUAL POWER,

ACTING EITHEE CONSTANTLY OE PEEIODICALLY ^

LigLtliouse Illumination, 2d Edition, p. 10.

;

aud

See also Chap. VII.

LIGHTHOUSE ILLUMINATION.

II, (in

the next Chapter)

the light to

To the unequal allocation different

azimuths,

either

CONSTANTLY OR PERIODICALLY.

The

means

optical

of producing these different distribu-

tions of the light are the Catoptric System, acting reflection only is

wholly glass

by metallic

the Dioptric System, where the optical agent

;

and the Catadioptric System, which

combination of the two. Before describmg these systems, however, I shall shortly, to

some

of the earlier

modes

of illumination,

refer,

where

no optical arrangements were employed.

The ''^'^M

first

attempt to indicate the position

of the sea coast to the sailor at night

Fig. 16.

Fig. 17.

probably by means of a gTate of burning wood or

on the top of a high tower. of those beacons, wliich

May,

was

Figs. 1 6

and

erected, in

coal, j^laced

represent one

1635, on the

at the entrance of the Firth of Forth.

Yet

it is

Isle of

some-

EARLY MODES OF ILLUMINATION.

possess refers

wliat remarkable that the earliest record

an illuminant in 1595,

to the use of oil as

Voyages,

vol.

448,

ii.

of the Bosphorus there

120

land,

a turret of stone

mouth

upon the main-

diameter and three in height, with a

great copper pan in lights in

In Hakluyt's

stated that "at the

steps high, having a great glass lanthorne in the

yards in

top, four

it,

and

midst to hold

the

oil,

with twenty

serveth to give passage into this Strait in

the night, to such ships as come from

all

parts of those seas

The next record we have

to Constantinople."

of the

Eddystone, where in 1 G 9 6 Winstanley placed tallow candles

on a chandelier, also surrounded and protected from the wind

and rain by a glazed lantern.

may

here be noticed, though

in strictness belonging to the dioptric system, that in

a London optician proposed, as Smeaton glass of the lantern to a radius of

tion

too vague

more than conjecture nature of the

had

admit

tells us, to

feet

;

1759

grind the

but the descrip-

the

apparatus which

The

view.

idea

was, however, an important one,

inasmuch

germ

the

illumination. in

1759

the

contained

dioptric

mode

Fresnel states that

lenses

were

actually

used in some English lighthouses, Fis;.

but were in

all

18.

probability impro-

perly applied, for their use was afterwards abandoned. doubtless one of these which

ing

made

at Portland

is sho-svn

in Fig. 1 8

Hghthouse in 1801.

an assemblage of fixed lenses would be

beams

The

was

from a draweffect of

such

throw out narrow

of light with arcs of darkness between.

It is there-

LIGHTHOUSE ILLUMINATION.

fore not surprising that

a partial

siicli

mode

of illumination

was discontinued. Catopteic System of Illuminating eveey Azimuth with LIGHT OF equal POWEE, EITHEE CONSTANTLY OE PEEIODICALLY. 5.

Parabolic Reflectors.

â&#x20AC;&#x201D;

was apparently not

that optical principles were for the

first

till

1763

time correctly ap-

plied to lighthouses. Mr. Hutchinson, dockmaster at Liverpool, states in his

book on

" Practical

Seamanship)," published in

1777, that lighthouses were erected

at the

Mersey in 1763

;

and, at page 180, that they were fitted with reflectors formed of plates of silvered glass,

and made,

as they can be to the parabolic curve."

Fig. 19.

show these then,

" as nearly

he says, Figs.

19 and 20

Fig. 20.

reflectors as given in

Hutchinson's book.

This,

unquestionably the earliest published notice of the

They

use of parabolic reflectors for lighthouse illumination.

were

first

introduced into Scotland at Kinnaird

by Mr. Thomas Smith, the

first

Head

in 1787,

engineer of the Northern

Lighthouses, and were also formed of small facets of silvered glass set in plaster of Paris.

But

1804 he

substituted

silver-plated reflectors at Inchkeith, in the Firth of Forth.

to this time the wicks

The ingenious Dr.

Ptobert

were

Hook,

all of

so far

the old

flat

in a monograph, called " Lampas," that an oil flame reality a cone of gas, of

kind.

back as 1677, showed

was

which the outside only was on

fire.

CATOPTEIC SYSTEM.

This can be proved by introducing a pipe into the middle of the cone, wlien the gas will escape

what was

and can be burned

as a sepa-

Argand

carried out

so nearly suggested in Hook's paper,

by making

But

rate light.

was not

till

1782

that

the wicks and burners of a hollow cylindric form, so as to

admit a central current of

Eumford ignited

split

air

up the cone

them both on the

through the burner

inside

and the

outside.

Traite d'Eclairage of 1827, states that

parabolic mirror, and

in his excellent

how many

Argand

and

finally

" It is

Memoir

and

Peclet, in his also used a

proposed a revolving light

the chandelier to rotate.

Chance

;

of gas into concentric shells,

by causing

remarkable," says Mr. J. T.

(Min. Inst. Civ. Eng.,vol.xxvi.),

inventors have contributed their respective parts

to the multiple burner

â&#x20AC;&#x201D;Argand,

the double current

;

Lange,

the indispensable contraction of the glass chimney; Carcel, the mechanism for an abundant supply of

Eumford, the multiple burner, an idea made contrivances, and

Fresnel."

finally realised

Teul^re

is also

oil

;

and Count

feasible

by these

by Arago and Augustin

said to have proposed the double

current burner in connection with the parabolic mirror, and this

was applied

about 1786. 6.

Cordouan in France, by Borda, but not

Properties of the Parabolic Bcficctor.

â&#x20AC;&#x201D;

These improve-

ments were undoubtedly of great importance, and must have enormously increased

the power of a light, as will

further appear from the optical properties of the parabola.

As any diameter and

as a

of this conic section

is xDarallel to

the axis,

normal to the curve at any point bisects the angle

between the diameter through that point and the straight line

drawn from

on the curve parallel

to the focus, all rays to the axis will

of light falling

be reflected to the focus;

and, conversely, all rays diverging from the focus will be

LIGHTHOUSE ILLUMINATION.

reflected parallel to the axis, so that a

luminous point placed

in the focus will throw forwards a horizontal

column

of rays, all of which are parallel to the axis.

the radiant used

beam

As, however,

not a mathematical point, but an

oil light

of considerable magnitude, the rays at the outside of the flame

and will

are ex-focal,

divergence

focal distance of the

divergence

emerge as a cone, whose

muTor.

Eor most

reflectors the useful

about 14^Â°, and the power

360 times

after reflection

dependent on the radius of the flame and the

generally taken

that of the unassisted flame.

inclination to the axis of a ray reflected at

be the

any point

the mirror, d the distance between the outside of the flame

and the sin

focus,

A =

â&#x20AC;&#x201D;d

the distance from the point to the focus,

but as the flame

;

circular, the

total diver-

gence

2 A.

Useful Divergence.

â&#x20AC;&#x201D;The

divergence varies therefore,

directly as the diameter of the flame,

distance of the reflector.

and inversely

as the focal

It follows, then, that the smaller

the flame and the larger the apparatus the better, because

Although

the incident rays will be better parallelised. is

no doubt true that a certain amount of divergence

needed

for the sailor, yet the correct j)ractice,

be afterwards explained,

by means

which will

of optical

specially designed for each case, to give the exact

of such divergence, is

required.

But

devices,

amount

and in the direction only in which

we attempt

increase the divergence in altitude,

it.

ixiri jpassio

will

throw more of

we have

said, it is lost,

which

the light above the horizon, where, as

too far to say that for

to increase the divergence in

azimuth by simjjly enlarging the flame, we shall

because the sailor cannot see

It is not therefore going

most apparatus

all

divergence due

CATOrXEIC SYSTEM. to

the

rays of a flame

ex-focal

bulky radiant

simply an

evil.

indeed the sole cause of the difficulty that

has to be encountered in

all

attemj)ts

with the proper distribution of the

deal accurately

to cut

light, or

off

sharply by means of shades or masks in any one direction in azimuth, for guiding

Even with the most

the

sailor

clear of

ments in wliich an ordinary intense part of the emergent

oil

flame

In France,

authorities

question,

and

but the view

may

principal claim

;

now given

lost

be added that on

which

justly

made

still

above the

its

is,

on the held by

optically

most

to the horizon,

especially, a different opinion

subject of divergence has always been, and

some

instru-

used, if the

beam be pointed

very nearly one-half of the whole hght horizon.

hidden rocks.

perfectly constructed dioptric

beyond

truth depends the

for the superior qualities

of the electric light as an illuminant, and also in a certain

degree for the superiority of the dioptric over the catoptric system.

It is right to

add that as every radiant,

however

small,

an object

of sensible magnitude, the

pencil of rays which issues

from every kind of apparatus

and

necessarily divergent,

its

intensity

fore vary

must

inversclij,

there-

as the

square of the distance from the lighthouse. Optical a2J2M7r(tus

can7wt therefore abolish the Fiar.

21.

divergence.

8. Defects of

the Paraboloid.

â&#x20AC;&#x201D;

It will

be noticed, from

LIGHTHOUSE ILLUMINATION.

58 Fig.

21,

that the

parabolic

very imperfect instrument,

mathematical point, only about intercepted

rays

shown

by a mirror

mirror

best

but

for even though the radiant were -^-l-th

of the light

of the usual form,

would be

and the cone of

in Fig. 21, escaping past the lips of the mirror,

would be therefore It is also to

lost.

be noted that photometric observations show

that after reflection the rays are not distributed uniformly

over the emerging cone, the density of which rapidly decreases

towards the edges.

The mode of producing a fixed light on the system was by arranging a number of reflectors o aroimd a stationary frame or chandelier

catoptric (Fig.

22)

As abeady

mentioned, an ordinary paraboloid has about 14-| degrees of divergence, so that

reflectors

continuously (though, as

whole horizon.

If,

were needed to

we have

again, the light

chandelier having 3 or 4

flat

faces

seen,

was

light

not equally) to

up the

be revolving, a

was employed

(j),

Figs.

CATOrTEIC SYSTEM.

23 and 24), on each

of wliich

were fixed a certain number

separate lamps and reflectors,

having their axes parallel to each

When

other.

the

frame

made

to revolve,

sides

was turned towards

sailor,

was

and one of the the

he would, when at some

distance from the

shore, receive

a flash at once from each of the

mirrors which were on that face

;

but when the face was turned Elevation

Fig. 23.

away from him, there would be came roimd. The power of the obviously proportional to the number

a dark period until the next face flashes in this

kind of light

of reflectors on each side of the chandelier. flashing, the apparatus

If the light

Eock,

that

527.)

any angular

It follows

displacement

was

â&#x20AC;&#x201D; (Account

from the laws of of

sides.

Mr. B. Stevenson's focusing arrangement, 1811.

Bell

tion

was made with eight

the

position

reflec-

the

LIGHTHOUSE ILLUMINATION.

flame in relation to the mirror, will produce a corresponding deviation in the emergent rays.

It is important, therefore,

lamp has been removed

that after the

be cleaned or

re-

wicked, means should be provided for replacing the flame exactly in the focus, so as to be independent of the keeper.

Fig. 25.

Fig. 26.

Mr. Stevenson's arrangement Figs. 2 5

burner,

and 2 6, in which

e e

are fixed slides,

reflector is is

shown in elevation

represented as lowered

effected

by the

returned to

c is

and

its

purpose

for this

the fountain for the cl

and

sliding arrangement

oil, h

are guide rods.

in Fig, 25, in

down from

shown

the

The

which the lamp This

the reflector.

which ensures

true position in the reflector.

its

being

Sir G. B. Airy,

the Astronomer Eoyal, in his report to the Eoyal Commission in 1861, says with reference to Girdleness, where this focus-

ing arrangement construction, is

employed, " I remarked that by a simple

which I have not seen elsewhere, great

facility

given for the withdrawal and safe return of the lamps, for

adjusting the lamps, and for cleaning the mirrors."

10.

Bordic7" Marcct's

Fanal a

doiible asjpcd.

â&#x20AC;&#x201D; In

1819

CATOPTEIC SYSTEM. Bordier Marcet, in order to reduce the loss of the instrument shown in Fig.

27.

Two

light,

invented

parabolas,

LIGHTHOUSE ILLUMINATION'.

of roimd a horizontal

axis

vertices of the parabolas

common

reflected

The

reflectors.

off,

permit of

as to

The rays

focus for the flame. this

former

as in

are cut

will therefore be

instrument parallel to the horizontal axis,

but in the vertical plane only, while the natural divergence

of the light in azimuth will not be interfered with. this excellent contrivance the

light was, for the first time,

spread equally round the horizon in one continuous zone.

But a

large portion of the rays in the vertical plane are

allowed to escape past the lips of the takes place

and

reflector,

all

round the

though the radiant

still

this loss

circle,

even

w^ere reduced to

a mathematical point.

12. Mr. 29).

(Fig.

IF. F.

â&#x20AC;&#x201D;In

actions of

London Trans-

1837, Mr. Barlow,

in order to

than by the posed

Barloids Reflector

the

common

to place

instrument

and

F.E.S.,

intercept

light

parabola, ^Drofront

that

opposite

the

flame /, a small spherical mirror

X y of

rays

have escaped

and

send

which

catch

would

without

back

the

cone

otherwise reflection,

through

flame, thence to diverge again fall

The cal

upon the parabola behind defects are,

1st,

The

the

and it.

spheri-

mirror renders almost useless

Fig. 29.

that part of the parabola x x

opposite,

and which might therefore as well be

removed altogether;

but, in addition to this loss, all the

which

it is

DIOPTPJC SYSTEM. rays

by the

reflected

spherical mirror

which

useless part of the parabola are also lost.

fall

2cl,

this

The neces-

extremely short focal distance of the spherical mirror

sarily

occasions wasteful divergence in the horizontal, and vertical,

and every other plane,

Some

3d,

wards by the spherical mirror are

of the rays reflected

lost

down-

on the burner.

falling

DioPTEic System.

In the year 1822, which must ever be regarded memorable

history

the

lighthouse

an entirely new system.

light can be altered

they can

mirror,

back from the

optics,

we come

rays

direction

from a polished metallic

we have

glass.

surface

the

reflection

also,

other directions

seen, be refracted into

reflection

they

are

sent

on which they impinge, and by

made to j)ass through the glass, but in The size of the flame produces divergence and the amount of this divergence for any

refraction they are

altered directions.

with •

refractors,

J-

point or

4-1

tJie

lens

—

Sm •

Radius of flame from centre of

/ (^dlst.

of poiut

flarnej

•

13. Peesxel's Optical Agents. 1.

Annular

Buffon suggested a

In order to save the

— So

far

back as 1748, the celebrated

new form

of lens for turning purposes.

Lens.

loss of heat

absorption,

which must

take place with the sun's rays in passing through the thick of a large lens

glass

whose outer

profile is

continuously

he proposed to grind out of a solid piece of

glass,

a lens in steps or concentric zones, so as to reduce

to a

spherical,

minimum was

Condorcet, the

1773

shown

the thickness, as

carried into execution

Academician, in his Eloge

(Paris edition,

This idea

in Fig. 31.

by the Abb^ Eochon

1804,

1780.

de Buffon, in

35), proposed

the capital

LIGHTHOUSE ILLUMINATION.

improvement of building up Buffon's stepped lens in sejmrate ringSj so as to correct, to a

large

extent, the spherical

aberration,

which

the di-

vergence from the parallel of rays

emitted by a lens

having a spherical surface. Sir

David Brewster

1811

also described in the Edin-

burgh Encyclopsedia "

Section

Fis. 31.

lenses

writers

designed

their

less

for

building

But

these

all

turning purjDoses only,

operating on

to lighthouse illumination.

nel, celebrated alike as a

mode

aberration.

lenses

and not with any view

Burning Instruments " ) the

same

and of correcting the

(article

light,

and

still

In 1822 Augustin Fres-

mathematician and physicist,

a]Dpa-

rently ignorant both of Buffon's and Condorcet's proposals,

un nouveau systeme

described (Memoire

sur

des Phares, 1822),

and afterwards constructed a similar

lens,

d'J^clairage

but for lighthouse purjDOses, in which the centres of

curvature of the different rings receded from the axis of the

instrument according to the distances of those rings from the centre, so as practically to eliminate spherical aberration

the only spherical surface being the central part

{a, Fig.

3 0).

These lenses were used for revolving lights only.

The Cylindric Refractor.

â&#x20AC;&#x201D;

Fresnel further originated

the idea of ]3roducing a fixed light by a refracting agent, that should act in the vertical plane only,

by bending down

the rays that would pass above the horizon, and bending up those that would pass below

it,

so as, witliout interfering

fresnel's dioptric agents.

with the horizontal divergence, to throw constantly a zone of rays of strictly

thus

horizon,

before

uniform power over every part of the dioptrically

effecting

by Bordier Marcet's

fractor, as it is

called, is

hoop of

(Figs.

glass

what had been done This cylindric re-

reflector.

a zone

32 and

33) generated by the revolution

round

vertical

axis

the

Fig. 32.

Fig. 33.

middle section of the annular lens already described, which lens,

on the other hand, being generated by the revolution of

the same lenticular profile round a horizontal axis, parallelises

the rays in every plane.

Prisms.

Totally Reflecting

improvement

admirable

the

â&#x20AC;&#x201D;

internal

The ray F

prisms.

glass

i (Fig.

34),

reflection

falling

on a prismoidal

refracted

on the side critical,

C, at

Fig. 34.

B C

A B

and

E, and falling

of incidence greater

in the direction c,

and emerges horizontally.

refraction to

an angle

totally reflected

J::.

/iv'B"

and bent in the direction

pinging on the side tion,

:;:^:

ring,

Etwation

Section

employing the principle of "total" or

Fresnel next conceived

than the

and im-

undergoes a second refrac-

The highest ray F A after by A C must (in order

reflection

avoid superfluous glass)

pass along

and

after

LIGHTHOUSE ILLUMINATION.

refraction at B emerge horizontally. The lowest F B after refraction by A B must, for like reason, pass along B C, and after reflection by A C and a second refraction by B C also emerge horizontally. Every other ray incident on the prism between A and B is, after one reflec-

second

ray

tion

and two

refractions, emitted horizontally.

The Straight Refracting Prism

which

optical agents,

one plane only.

another of Fresnel's

refracts the rays that fall

it,

but in

no further explanation, as

It requires

it is

simply a straight prism of the same horizontal cross section as one of the prisms of his cylindric refractor.

Gixat

14.

new to

central

1822

the

mode

of getting

will

own

all

now

on to

utilised the four

first

referring

lighthouses

prior

up the required power was

number

sufficient

each of which (unless its

burner system.

by employing a required

â&#x20AC;&#x201D;We

which Fresnel

agents which he originated,

ojotical

to his

Lamp.

Central

manner

describe the

separate reflectors,

except Bordier Marcet's mirror)

But Fresnel placed

separate lamp.

in the

centre of the apparatus a single lamp, which had four concentric

wicks, and was

by clockwork.

with

fed

Surrounding

tliis

oil

by a pump worked

burner was a stationary

cylindric refractor for a fixed light,

and revolving annular

lenses for a revolving light.

The mechanical lamp, jointly

called,

was designed

by Fresnel, Mathieu, and Arago, and

will be after-

wards more

fully described.

FKESNELS DIOPTRIC COMBINATIONS.

Feesnel's Combinations of his Optical Agents. 15. Catadioptric Fixed Light.

â&#x20AC;&#x201D;

This apparatus (Figs. 35,

36) consists of the central lamp, surrounded by a cylindric o

refractor E, while above

low are zones A, similar in

j)rofile

Fis. 35.

the use of the refractor the whole

of the wasteful divergence, which, as

only geometric defect of Marcet's all

to Bordier

Fig. 36.

Marcet's reflector.

vented, and

and be-

of silvered mirror

we have

reflector,

seen,

was the

entirely pre-

the rays are intercepted and spread in a

zone of uniform intensity

all

round the horizon.

have

here, then, for the first time in the history of lighthouse optics,

a combination which, for the purpose required,

geometrically metallic

perfect,

see,

not

physically

so,

This defect, as

shall

Fresnel completely obviated in

his

reflection is

immediately

though

because

employed.

design.

16. FresneVs Dioptric Fixed Light.

First Application

LIGHTHOUSE ILLUMINATION.

of Total Reflection

substituted

totally

liis

Fheed Lights.

â&#x20AC;&#x201D;In

reflecting

Figs.

37 and 38, Fresnel

prisms for Marcet's re-

flectors, so as to spread the light uniformly over the horizon

solely

cation of -

dioptric agents.

This was not only the first appli-

total reflection to lighthouses,

but was the first optical

combination, which, for the purpose required, was both geometrically and physically perfect (excepting of course the inevitable loss due to the divergence of the exfocal rays of

the flame), leaving in fact no room for improvement; and, accordingly, this beautifvil instrument continues

till

now

universal use.

In Figs. 37, 38,

/ the

focus,

and

p p

represent the cylindric refractor,

the totally reflecting prisms placed at

top and bottom.

17. Fresncl's Revolving Light, 1822.

which showFresnel's form of revolving

â&#x20AC;&#x201D;In

Figs.

surrounded by annular lenses L, and

arrangement of inclined trapezoidal lenses mirrors

]\I.

The inclined lenses

fit

39 and 40,

light, the central

burner

compound

and plane silvered

closely to each other, forming

a pjrramidal dome, and the light, intercepted by them,

sent

fkesnel's dioptric combinations.

upwards in beams of a trapezoidal

section, until, falling

B---"

the plane mirrors

M, they

are bent so far do^vnwards as to

emerge with their axes r r parallel to the axes rectangular lenses below. are made, central

If,

by the wheelwork N,

lamp B, the

of the

then, all these optical agents to

move

together round the

sailor will receive the full flash

whenever

the axes of the emerging beams are pointed towards him,

and he will be in darkness when they are turned away from him.

Though

all

the rays are, or as Fresnel pointed out,

might be intercepted and sent in the required directions by placing similar inclined

lenses

and plane mirrors

below

the central lenses in order to intercept the light which

LIGHTHOUSE ILLUMINATION.

escapes striking on the burner of the lamp, yet the design,

unlike that of his fixed light,

very far from being perfect

not only because metallic reflection

two agents are needed

for all

rays, thus causing a large loss, to

fully alive, as appears

1822

(p.

17)

which Fresnel himself was

from this passage in his Memoire of

—"Mais comme on

glaces etamees pour ramener dans les faisceaux

employed, but because

but the central portion of the

est oblig^ d' employer des

une

lumineux qui sortent de

direction horizontale

ces lentilles,

partie de la lumiere incidente est absorbee

malgre leur iuclinaison prononcee, qui

est

UanUes!'

tr avers

The

une grande les miroirs,

de 25°;

que la lumiere incidente doit itre rSduite 2oassage

j)ar

vioitid

des lentilles et sa reflexion sur ces glaces

first

light

this

principle

was that of

Cordouan in 1822.

18. FresneVs Fixed Light, varied ly distinction (Figs. liis

jdstime

^mr son

Flashes.

—

This

41 and 42), Fresnel produced by placing

straight refracting prisms r' (13, 4)

outside

on a revolving frame of

apparatus

his x.

fixed

Now,

light

as that

apparatus paralleKses the rays in

and the straight

the

vertical

plane only.

refractor in the horizontal plane only,

obvious that whenever the revolving straight prisms come in line

with a distant observer, the light must increase

FEESNEL S DIOPTPJC COMBINATIONS.

enormously in volume, giving, instead of a narrow a broad flash like an ordinary lens.

light,

apparatus, although glass only

agents for producing the

The

strip

used, is the use of

two

effect.

19, Sir David Brcluster's Catadioptric Arrangement.

In 1823 (Edinburgh Philosophical Journal) posed

apply

burning

Sir

â&#x20AC;&#x201D;

David pro-

his

arrangement

1811

defect in this

to lighthouse

The only

illumination. difference

between the

design by Fresnel which

was

the

that

first

was made

for

houses, and

light-

pro-

this

posal of Brewster was,

showed

that the latter

arrangement

for

collecting the rays into

a single beam, and em-

ployed a spherical mirror mn,ior

deahngwith

the back rays.^ In Fig.

43, lens

c, h,

;

clined

main

the

the in-

lenses

;

the spherical mirror;

and

%u,

the

silvered mirrors; as

Fig. 43.

plane

and /the flame.

The

defects are the

same

have been already stated in relation to Fresnel's plan.

^ The spherical mirror had previously Thomas Eogers.

(1790) been used for lighthouses by-

LIGHTHOUSE ILLUMINATION. 20. Introduction of Dioptric

after the

Systcr)i.

窶認or

a long period

lamented death of Augustin Fresnel in 1827, his

M. Leonor Fresnel, who succeeded him as Lighthouse Engineer, was largely instrumental in introducing these most brother,

valuable imiDrovements into the lighthouses of the world.

The Dutch are believed having been the

have the honour,

introduce the

first to

new

after France, of

Next in

system.

order come the Scotch Board, who, in 1824, sent Mr. Eobert

Stevenson to Paris, and he recommended

Buchanness Lighthouse in

1 8 2 5.

employment

its

This proposal, however, was

not carried out, as a different character of light was adopted

In 1834 his successor, Mr. Alan Steven-

for this headland.

son, visited Paris, where, through the liberality of L. Fresnel,

he received the

fullest information as to his brother's system,

which he embodied in an elaborate report printed burgh in the same year Inchkeith, and in

made

1836

and in 1 8 3 5 the revolving the fixed light of Isle of

formulae

Fresnel's

dioptric instruments,

for

light of

May, were

Mr. Stevenson also

of catoptric.

dioptric inste^id

published

;

at Edin-

calculating

the different

and he computed and published the

elements of a complete cupola of totally reflecting prisms.

London followed

and employed

The Trinity House

Mr. Stevenson

superintend the construction of a

next,

first-

order revolving light, which was afterwards erected at the Start Point, in Devonshire.

The Americans, considering ment and deep

their

well-known enlighten-

interest in naval matters,

slow in the matter of coast illumination.

been more

up.

seems to have

than twenty years after the invention of the

dioptric system in France that the subject

In 1845

information,

were somewhat

was seriously taken

they applied to M. Leonor Fresnel for full

and in an elaborate report

(December 31st,

INTRODUCTION OF DIOPTEIC SYSTEM.

1845), which he furnished to the American authorities, he conchided:

—

apparatus

present a

effects,

" 1st,

That the

lights

variety in

and may be made

which renders them of an

their

with

dioptric

power and

in their

fitted

produce an intensity of lustre

interest, in

a nautical point of

view, incontestably superior to those fitted with catoptric apparatus.

That,

2d,

cost

of construction

ance,

shall

new system

and

take

the

find, in respect to

The Eeport

the

first

their mainten-

the effect produced, the

from once and a half

is still

tageous as the old."

account

into

expense of

to tioice as

advan-

also contained the results

of photometric observations of the powers of the different

instruments then known, which clearly proved the superidioptric system.

ority of the

was

not, however, until

1852, that a committee of the American Board resolved

— "That cases

the Fresnel or lens system, modified in special

by the Holophotal apparatus

of Mr.

Thomas Steven-

son, be adopted as the illimiinating apparatus of the States, to

and

embrace

all lights

deficient

all

new

lights

now

United

or hereafter authorised,

requiring to be renovated either by reason of

power or defective apparatus."

—

When estab21, Mr. Alan Stevenson's Improvements. lishing the new system in Scotland, Mr. Stevenson made the following improvements 1.

Refractor

of a

tridy

difficulties in construction

Cylindric

Form.

May

ting Messrs.

But

for

the

light, in

1836, Mr. Stevenson succeeded in get-

Cookson

Newcastle to construct a

first-order

refractor of a truly cylindric form. ^

Fresnel had to adopt a polygonal

instead of a cylindric form for his refractor. Isle of

— Owing

Report of the American Lighthouse Board, 1852,

p. 602.

LIGHTHOUSE ILLUMINATION. Helical Glass Joints for Fixed Liyhts (Figs. 44, 45).

Section.

Mr. Stevenson also introduced

tlie

valuable improvement

of constructing the refractor in rhomboidal, instead of rec-

In this way the joints

tangular pieces. of the

stead ;^

glass,

of being

and the

better distributed,

azimuth

(Fig.

were

44), inhelical,

loss of light at the joints

scuration in

amount

upright,

of is

and prevented ob-

any one light

was

The

direction.

intercej^ted

any

inversely j)roportional to the

sine of the angle of inclination of the joint. 3.

The Fig. 45.

were

him

also, in like

Helical

Metallic

Framings.

â&#x20AC;&#x201D;

internal metalKc framings for sup-

porting

the

upper

cupola

prisms

manner, and for the same reason, made by

of a helical form, as seen at the

bottom

of Fig. 45.

MR. ALAN STEVENSON S IMPROVEMENTS. Diago7ial Lantern.

â&#x20AC;&#x201D;The

astragals of the lantern were,

same purpose, made diagonal, and were constructed

for the

of bronze instead of iron, in order to reduce their sectional area

shown

(as

in Plate XI.) ^

small harbour light, with in-

was made by Mr. E. Sang about 1836. 22. Improved Revolving Light of Skcrryvorc. In 1835

clined astragals,

â&#x20AC;&#x201D;

Mr. Stevenson, in Board, proposed to

the Northern Lighthouse

Keport to

add fixed

reflecting

prisms

{p,

Fig.

46) below the lenses of Fresnel's revolving light

;

and he commu-

proposal

nicated

this

Fresnel,

who approved

gestion,

and

of his sug-

assisted in carrying

out the design in

1843.

This

combination added, however, but to

little

the power of the flash,

and produced both a periodically flashing

but

and a constant fixed

light,

must be remembered that

the fixed form was the only kind of reflecting

The

prism then known.

trapezoidal lenses and plane

mirrors were reason,

still

also, for

used.

the same

The prisms

made of the were constructed by M. Soleil

first

that were

Fig. 46.

for

large

Skerryvore were the size

(first

order),

and

at Paris, under the immediate

superintendence of M. L. Fresnel.

Mr. A. Stevenson also extended the helical principle to the astragals of it Avas not

the lantern in a design for Start Point, in Orkney,' in 1846, but adopted.

was the

Mr. Douglass, independently, made a similar proposal in 1864, and Mr. Douglass's publish it and introduce it into lighthouses.

first to

lantern will be described subsequently, and

shown on Plate XII,

LIGHTHOUSE ILLUMINATION.

23. Mr. A. Gordon' sCatadioptric Reflector The next improvement was 1847). for increasing the power of the para-

(Civ,

â&#x20AC;&#x201D;

bolic reflector dioptrically (Figs. 47,

^c."^';;^

Eng. Jour.,

IMPROVEMENTS ON FRESNEL'S REVOLVING LIGHT. Mr. Gordon placed in front of a paraboloid, a

48).

of the outer rings of Fresnel's annular lens,

77 c,

four

the

h, for

purpose of intercepting some of the rays which escape the action of the reflector, wliile the itself

Though

cular rings.

the paraboloid

between the

e g,

lenti-

so far a step in the right direction, the

defects are very obvious, e

beam from

passed through the central void,

—

1st,

The escape

f g through the central void.

2d,

of the cone of rays

The extremely short focal

distance of the reflector, which was found on trial to pro-

duce a cone having a divergence of no

The

3d,

loss

by metallic

24. Lconor

reflection

Fresnel's

less

and

than 60°;

from the paraboloid.

— M.

improved Revolving Light.

Fresnel pulDlished an improvement on his brother's light, in

his " Phares

Fanaux des Cotes de

France,

Paris,

1842," by enlarging and altering the position of the tra-

inclined

pezoidal lenses

and

(Fig.

thus

49),

increas-

ing their focal distance.

was thus

effected,

Fig. 49.

but he

;#-

very considerable improvement still

retained metallic reflection

and double agents for the upper rays. 25. Holoplwtal System

—These improvements come

in order of date, but for the sake of clearness shall first describe the remaining

French improvements on

Fresnel's revolving light.

26. M. Lepaute'sform of Revolving

Light.

— M.

the collaborateur of A. Fresnel, gave a design in

which, in order to avoid

the

and unity we

use

double

Lepaute,

1851, in

agents,

'78

LIGHTHOUSE ILLUMINATION.

increased the height of the lens, and reduced proportionally

the angle subtended by the iixed light prisms above and below.

this

way he extended

light probably farther

the powerful part of the

than was consistent with favourable

angles of incidence of rays falling near the top and bottom of such elongated lenses.

The apparatus could

parallelise the rays in the vertical plane only

and lower prisms.

course, if he

therefore

its

upper

had been acquainted

with the holophotal prisms subsequently to be described, he could have parallelised the light in every plane from top to

In M. Lepaute's

bottom of the apparatus.

letter to the U.S.

Lighthouse Board, of 28th July 1851, he states that his design " received the approbation of the Commissioners of

"The French Adminis-

Lights in France;" and he adds that tration is aljout to order

of the first order

from the undersigned an apparatus

of this

description of flashes for every

minute, to renew the apparatus of the light of Ailly, near Dieppe."

27. Zdourncau's

imjyrovcd Ficml Light, varied hy Flashes.

â&#x20AC;&#x201D;

Fig.

50 shows

in section an-

apparatus,

other purely dioptric

which was exhibited by Letourneau and Wilkins at the great

London

Exhibition

Exhibitor,

drum

(Illustrated

The

1851).

central

of this apparatus consisted

of alternate panels of the cylindric refractor

nular lens I

E, and the an-

which

last

surmounted by straight refractmg prisms

volve together; while above and

all

which agents

below the lenses

and

re-

refractors

IMPEOVEMENTS ON FEESNEL'S EEVOLVING LIGHT. are portions of fixed light prisms

So long as the cylindric refractor

the light appears fixed, but

when

and straight prisms comes round duced singly by the central original arrangement

no doubt very considerable,

light is parallelised

stationary.

opposite to the observer

beam

of light

pro-

and doubly, as in Fresnel's

by the

fixed horizontal prisms

The advantage here gained

and revolving straight prisms. is

which are

the combination of lenses

a solid

lenses,

18),

(ISTo.

for the

whole of the central

single agents, but there

still

the

defect of employing double agents for the upper and lower

which produce the

divisions of the apparatus

28, Improved Fixed structed hy

Tahouret,

â&#x20AC;&#x201D;

flashes.

Light, varied hy Flashes, as con-

This apparatus (Fig. 5 1) was exhi-

Fig. 51.

bited at the

1851 by Messrs. Chance, superintendence of M. Tabouret, who

London Exhibition

and was made under the

LIGHTHOUSE ILLUMINATION.

80 constructed

all

the apparatus for Augustin Fresnel

The

death in 182 7.

was

nearly than was done at Skerryvore

his

till

to equalise

(22), the fixed

and

object of the design

revolving beams, and for this purpose the fixed prisms were

introduced above, as well as below the middle part of the apparatus, as in Lepaute's design, so as to supersede Fresnel's

compound arrangement

of inclined lenses

and mirrors.

Thus,

although double agency was avoided, the power was very largely reduced, for only the central part of the luminous

sphere was parallelised horizontally and vertically.

29. ReynaudJs Improvement, 1851.

which revolving

â&#x20AC;&#x201D;This was a design in

were placed in front of

straight prisms

all

the fixed prisms, for the purpose of increasing the power of

the lenses by suppressing altogether the fixed light which

was employed

at Skerryvore.

a great addition of power, but

The it

effect

would no doubt be

was obtained by means of

double agents.

HoLOPHOTAL System, 1849. 30. The object of the different improvements on Fresnel's revolving light which have been described, was to obtain the

This was perfectly attained

best utilisation of all the rays.

unnecessary

without

It will

explained.

by the system now

agents

be seen from what follows that by

and

be its

rendered optically as per-

further, that,

from the wideness of the

agency the revolving apparatus fect as the fixed,

application of the principle,

its

introduction has led to

many

and various changes in other forms both of catadioptric and dioptric apparatus. 1.

The Catadioptrie Holophote.

ments which Arts, vol.

iv.)

I proposed in

â&#x20AC;&#x201D; The holophotal arrange-

1849-50

show the modes

(Trans. Scot.

Eoyal Soc,

of solving the problem of con-

HOLOPHOTAL SYSTEM. densing the wliole

beam of imrcdlel

rays

of diverging

into

any unnecessary

single

reflections or

Part of the anterior hemisphere of rays (Figs.

refractions.

52, 53)

s])liere

rays, loitliovi

intercepted and at once parallelised by the lens L,

Front Elevation Fiff.

Fis

whose principal focus

{i.e.

53.

for parallel rays) is in the centre

of the flame, while the remainder

made

intercepted and

a, and thus the double agents in and Brewster's designs (17, 19) are dispensed with. The rays of the posterior hemisphere are reflected by the spherical mirror h, back again through the focus, whence

parallel

by the paraboloid

Fresnel's

falls

on the lens and

the paraboloid, so as finally to

emerge in union

passing onwards one portion of them the rest on with,

and

parallel to the

This was the

front rays.

instrument which intercepted and parallelised proceeding from a focal point by the agents. called

It is

employs metallic

But

reflection

it ;

perfect,

reflected

mirror would, as in Barlow's design, fall ^

adopted this name from the Greek words

and hence

not physically

so,

and with an ordinary

and burner many of the rays

first

the rays

minimum number

therefore geometrically

a Holophote}

all

oil

for it

flame

by the spherical upon the burner 6'Xos

and

(pus.

LIGHTHOUSE ILLUMINATION.

and be

lost.

Harhonr which

all

Tliis

instrument was employed at the North

of Peterhead in the

1849, which was

rays lucre geometrically

the first light in

comMned in a

hcam, without unnecessary agents.

Figs.

another arrangement on the same

jDrinciple,

single

54 and 55 show in

which the

HOLOPHOTAL SYSTEM, 2.

Holophotal Catadioptric Apparatus revolving roimd a

Single

Central Burner.

—The

application of the holophotal

principle to catadioptric revolving lights with a central burner is

given in Fig.

Fresnel's

58.

In place of

compound arrangement

trapezoidal lenses and plane mirrors (17), the mirrors

B, instead of

being plane, are generated by a parabolic profile passing round a hori-

zontal axis, and all rays from the L j

focus are thus at once parallelised

The design

a smgle agent.

there-

fore geometrically perfect, like that last described is

still

;

but metallic agency

employed.

was

first

adopted at Little Eoss, one of the Northern Lighthouse stations.

Optical Agents. 3.

Eoy.

Holophotal Brisms. Scot.

Soc.

Arts,

though similar in section

— The

holophotal prisms (Trans.

1850), to those

Section

071'

ai.

4L.

in Fresnel's fixed light, differ in the

mode

of their

generation

and in

their optical effect, for the sections

are

made

revolve

round

Fig. 59.

horizontal instead of a vertical axis, are therefore

made

vertical plane only.

total reflection

and the incident rays

parallel in every i)lane instead of in the

True lenticular action from 45°, which

thus extended

about the limit of

Fresnel's refracting lens, to nearly 130°.^ 1

In 1852

similar prisms

it was for the first time stated that, so far back as 1826, had been made for A. Fresnel by M. Tabouret, and tried, though

LIGHTHOUSE ILLUMIXATIOX, 4.

Double Reflecting Prisms.

â&#x20AC;&#x201D;In 1850

Soc. Arts) I i)roposed prisms for giving

tions instead of one.

The surface

(Trans. Eoy. Scot.

two internal

h c (Fig.

60)

reflec-

concave,

the centre of curva-/

ture

the

Ijeing

centre of the flame /.

The other h of parabolas,

/ will

fall

whose common focus

normal

to the surface I

at its incidence on a

direction r

r' it

where

at rioht

surfaces,

are portions

A ray diverging from

is /.

pass on without any deviation, or loss

and a

and therefore will

superficial reflection

totally reflected in a

angles to the

axis

each zone.

again reflected, and finally emerges in a radial lamp posts for lighting the quays of a canal in Paris, and, by M. Reynaud, they were not designed in such a way as to be

unsuccessfully, ou as is admitted

aj)plicable for lighthouses.*

It is also admitted that

no draiving or

descri2]tion

of them v;as ever inihlislicd, andtlud they loerc never used in any lighthouse.

But

the follo\Ying facts incontestably prove that even their existence was unknown, or entirely forgotten, for quarter of a century after 1826, both by Leonor Fresnel

(Augustin's brother and successor) and by Tabouret,

the canal apparatus dioptric system,

was

â&#x20AC;&#x201D;

1st,

Alan Stevenson

for nearly a

in Tabouret's workshop,

who

said to have

made

in 1834, before reporting on the

month almost

daily in L. Fresnel's office

and

but never heard of them then, nor at any of his

frequent visits to Paris afterwards ;t 2d, In 1842 L. Fresnel published his

improvements on his brother's plan

own

and in 1843 superintended the apparatus of Skerryvore (22) 3d, In 1850 I went to Paris for the express purpose of showing L. Fresnel a model of my dioptric holophotal arrangement, and he did not at either of two interviews question its novelty 4th, All (24),

;

apparatus, even as small as 7 inches radius, where there could be no difficulty of consti-uction, continued to be 5th,

made

at Paris with unnecessary agents

;

In 1851 Tabouret himself constructed and exhibited at the London

own apparatus (28) 6th, In 1851 Lepaute wrote that the French Lighthouse Commissioners approved of and " were about to order " Exposition his

;

* Annales des Ponts et Chaussees, 1855. t Report to Xorthern Lights, Edin. 1834. to Arago by T. Stevenson, Edin. 1855.

Keply to Letter from

Fresnel

HOLOniOTAL SYSTEM. direction at

tlie

point

A ray on

the other side of the axis

will be simultaneously reflected in the opposite direction,

and

also sent

back to the flame.

The mode

of em^Dloying

such prisms will be given in next section.

Optical Combinations of Dioptric Holophotal Agents. 5.

Dioptric Holoplwte with Dio2Jtric Spherical Mirror,

â&#x20AC;&#x201D;

If holophotal rings {p p, Fig. 61), with a central refracting lens, L, together

subtending 180Â°, are

placed before a flame, the front half of the diverging sphere of rays will

be at once condensed by refraction

and

l^arallel rays,

beam

total reflection into

and the back half may

be returned through the flame by a metallic spherical mirror

If,

in-

stead of the metallic spherical mirror, a 62), formed of zones generated

dome

of glass (Fig.

by the revolution

cross section of the double reflecting prisms (Fig. 60)

of the

round

a horizontal axis, be placed behind the flame, then the

back hemisphere of diverging rays falling on

will all be

returned through the flame, so as to diverge along with the front rays, for this

dome

a perfect spherical mirror not only

his form of apparatus for Cape Ailly,

and in tlie same year tliey invited tenders on his plan, which were recalled after

(since published *) for that lighthouse

they had seen the Scotch holophotal apparatus then being made in the workshop of M. Letourneau in Paris ;t 7th, In the same year M. Degrande, one of the Government lighthouse engineers, claimed for himself the invention of the holophotal prisms, not having then, as he afterwards told me, heard

anything either of the canal prisms, or of *

prior publication. %

De I'application du principe de la Eeflexiou

V. Masson.

Ibid.

Totale aux Phares Tournants.

Paris, 1856.

X Reply to Fresnel's Letter to Arago, 1855.

LIGHTHOUSE ILLUMINATION.

86 for

tliat

of the

portion

faiiit

liglit

that

~-~

!</

superficially

reflected

inner

its

surface, but also

much

for the

reflection

greater

portion

enters

the

all

the light

total

which

substance

glass

that

;

reflected

back again through the flame,

the in

and

finally falls

parallelising

agents

the

flame.

front

Thus

the

whole light

entirely

2Jarallclised glass,

metallic

rcfleetion

heing

wholly

dispensed

Fig. 62.

with in every 'part of the apparatus, and refraetlon and total further to be noticed that this

It has

reflection siibstituted.

mirror does not concentrate the heat on the burner, as was

pointed out by Professor

Swan. thermometer placed behind the mirror stood, after 24 hours, 5째 higher than the surrounding

showing that the heat rays are largely

air,

transmitted through the glass, and thus the is

kept comparatively

oil in

This combination should therefore produce

maximum ment still,

and

intensity,

oil

the light

consequently, so far as arrange-

goes, both geometrically

with an

the burner

cool.

and physically perfect

flame of the ordinary

size,

some

;

but

of the

rays reflected from the upper part of the spherical mirror will

not

The

first

structed in

clear

the

double

burner,

reflecting

1850 by Mr.

and will therefore prism was

lost.

successfully

con-

Adie of Edinburgh, and those of

HOLOPIIOTAL SYSTEM.

the improved form proposed by Mr. Chance (which will be noticed at

its

proper date) were made in the most perfect

manner by Messrs. Chance Stevenson,

1861.

Birmingham

a person

who

for

Messrs.

unacquainted with-

optical principles, the action of these double reflecting prisms is

somewhat surprising

;

for,

when they

are placed at the

proper focal distance between the most dazzling flame and the observer, no trace of light can be seen, although the intervening barrier

But

only a screen of transparent glass.

the flame be placed exfocally, or be of too great

magnitude, so as to exceed the limit assigned by the formula, the

rays

exfocal

mirror,

instead of being reflected

will,

transmitted

freely

through

it,

by the

and therefore

lost.

In order at

and I

to ensure that the light shall

(Fig. 63),

be totally reflected

which are the points where the angle

Fis. 63.

of internal incidence

gave

me much

least. Professor

Swan, who kindly

assistance in carrying out this design, has

given the formula 45=

Putting

C F =

rf,

~ :p^ 4

r D =

â&#x20AC;&#x201D;+ /A

sni

^â&#x20AC;&#x201D;\id

= the index

of refraction of

LIGHTHOUSE ILLUMIXATIOX. the glass for the extreme red rays of the spectrum, and

^|r

the

greatest admissible value of the angle subtended at the centre of the flame

by the

face of the zone.

Tor the radius of the circular replaces the parabolic arc

A B

which in practice

arc,

sui

^ —

he obtained the

value 'A

d r

sin

= 2 sin [45°

And

for the co-ordinates of the centre of curvature

~ —

~+f r sin 4^

d cos

45°— —

h- —r 6.

sin

45°

Dioptric Holopliotal Revolving Ligld, 1850.

tion of total Itcficction to Revolving Lights. '

—

Applica-

Sections of the

front half of the dioptric holophote (Fig. 62) are equally

applicable to a revolving light where there

which

by a

is for

light

because

rays are

1850

in Fig. 64, and

intercepts

agents,

single

The 64),

of a rectangular so

-\^

the

all

and these

central

original

tlie

(Fig.

and physically per-

glass.

L L

of those

form of revolving

shown

geometrically fect,

made up

This

a central burner,

that purpose enclosed

glass cage

sections.

lenses,

design

instead of being

form are

circular,

that the reflecting and refract-

ing zones junctions, saved.

are

concentric at

by which some

When

the

light

their Ficr.

64.

light is is

not

required

show

all

HOLOPHOTAL SYSTEM.

round the horizon, a portion of a dioptric spherical mirror

may

be placed behind the flame on the laud

rays

reflected

seaward

used for strengthening

the opposite

Chance, in referring to the relative advantages

J. T.

and of the holophotal, says

of Fresnel's revolving light

of his paper on ojotical apparatus

and the

arc.

Mr.

side,

â&#x20AC;&#x201D; "In

Fresnel's

revolving apparatus, as the focal distance of the accessory lenses

is less

than one half of the shortest focal distance in

the system of reflecting zones, the intensity of the light issuing from the former

would be scarcely

by the

fourth of that transmitted to this cause of inferiority

so that, light

latter

;

more-^ than one-

and in addition

the loss arising at the mirrors

on the whole, the modern plan (holophotal) must give

^ve

more intense than that

or six times

of the former

(Fresnel's) arrangement."

The holophotal revolving It

was

small

introduced in

first

scale, in

now

light is

1850 by

connection with j)arabolic reflectors, at Hors-

burgh Eock, near Singapore, which was ivhich total refiection

on the

was applied

largest scale for

XXIII.)

in imiversal use.

Messrs. Stevenson on the

the first lighthouse in

revolving apparatus

The Horsburgh prisms were completed

by Mr. Adie

;

and

North Eonaldshay, in Orkney (Plate

of Edinburgh,

and those

for

1850

North Eonaldshay

were most successfully and skilfully made in 1851 by M. Letourneau, the well-known manufacturer of Paris. 7.

Holophotal Fixed Light, varied hy Flashes.

â&#x20AC;&#x201D;In

this

arrangement (Eoy. Scot. Soc. Arts, 1850) the double agents used in Fresnel's design (No. 18), and also in Letoiu^neau's 1

"The words

of light caused

scarcely

more are used in order to allow

by the prisms than by the

paths of the rays in glass."

lenses, in

for the greater loss

cousecpence of the longer

LIGHTHOUSE ILLUMINATION.

(No. 27), are altogether disi^ensed with, and the whole effect

produced by the single agency of

al-

ternate panels of fixed light apparatus ])' 'p

with cyliudric refractors L'

and holophotal apparatus

]) 'p,

L',

both re-

volving together round a central lamp, as

shown

in Fig. 6 5. There will in this,

as in Fresnel's form,

be a dark interval

before the full flash comes round.

The reader

have observed

will

that while perfection of design

was

attained in Fresnel's fixed apparatus,

and FiK. 65.

also in the holophotal revolving

described

apparatus

physical defect attaches to every form of

has

The

compress

the

light

into

No.

holophote

single

learn,

that

where

necessary to send the rays back through the flame. defect

one which

not geometric, and therefore

non-existent, where the luminaut

a mathematical point,

and perhaps hardly appreciable where the radiant is so small The existence of this defect is wholly as the electric light. due to the burner, that indispensable adjunct of an oil flame, through which the oil and supply of air are obtained; and the obstruction

is,

of course, limited to those rays only

reflected near the top of the mirror.

which are

This explanation will

prepare the reader for the no doubt unlooked-for and apparently retrograde step of restoring metallic agency in the design next to be described, which possesses, however, the

more than equal advantage of preventing loss of by the obstruction produced by the burner. In 8. Improved Catadioptric Holoplwte, 1864.^ far

â&#x20AC;&#x201D;

light

Figs.

Stevenson's Design and Constniction of Harbours, Lend. 1864, p. 251.

HOLOPHOTAL SYSTEM.

G6 and 67, «

h c

the front half of a holophote, which

Fig. 66.

Fig. 67.

parallehses one half of the light

;

the other half

cepted by a portion of a paraboloid, h dioptric spherical mirror

All

lost.

faultless is

that

none

wanted

whose action

so near the horizontal axis that

are

fall

inter-

and a portion of is

limited to rays

on the burner and

make

instrument

this

to substitute glass for the remaining portion of

metallic reflector

which

used,

and

accom-

this is

plished in the design next to be described.

Back Frisms.

light

by means

glass

angle

limited to about

would be

through

made

maximum 90°,

overpassed,

the prisms,

In 1867 Mr. termed

—The

possible deviation of

crown

of the Fresnel reflecting prisms of

instead

Brebner

beyond which the and the rays

transmitted

of being reflected

by them.

and myself designed what

back prisms," by means deviate

critical

from their

of which, rays

original

direction

may

for

we be

about

130°, so that by their use the lighthouse engineer becomes virtually independent of

the critical angle.

commmii-

cated the description of these prisms to the Eoyal Scottish

LIGHTHOUSE ILLUMINATION.

Society of Arts on 6tli of St.

Andrews

December 1867.

of prism, a description of wliicli he

same

Society on

general formulse for (Fig. c,

commimicated In

its construction.

68) the ray a h

Swan

same form to the

December 1867, accompanied by

9th

and again refracted

horizontal axis.

Professor

also independently proposed the

refracted

this

form of prism

totally reflected at

at d, so as to pass out parallel to the

Prisms of

this

may

kind

be formed by the

revolution of their generating section round either a vertical Ci Tcu Ta rform

Siraiglit foT'Tn.

Elevation

SectioTV

Fis.

or horizontal axis, or

may

be made straight, as shown in elevation and section in Pig.

68.

These prisms were

used

Lochindaal

first

Light-

house in Islay, and were made

by Messrs. Chance in accordance

with Professor

Swan's

formula, which will be found in the Appendix,

10.

Eoloplwte Fig. 69.

Improved (Fig.

combining the

just described, with a semi-holophote a h

Dioptric

69).

â&#x20AC;&#x201D; By

back prisms, c

subtending

MK. CHiVXCE S niPKOVEMENTS. 180°, and a portion of the dioptric spherical mirrov

the whole rays are parallelised, and none of them are lost

on the burner, so that

this

apparatus, being all of glass,

both geometrically and physically perfect.

31. Mr.

J. T. Clianccs

Dioptric Sphcrieal Mirror.

Improvements o/

—Mr. Chance

1 8 6 2 o?i Stevenson's

proposed

(Fig.

70) to

generate the prisms of the spherical mirror round a vertical instead of a horizontal axis, and also to arrange

segments.

says (Min. Inst. Civ. Eng., vol. xxvi.)

them in " The

—

plan of generating the zones round the vertical axis was introduced by the author,

who adopted

plete catadioptric miiTor

which was made, and was shown

in the Exhibition of Secibion^(wU/u>

1862 by

in the

first

com-

the Commissioners of Northern

Fig. 70.

the realising

whom

was constructed, in order to further of what Mr. Thomas Stevenson had ingeniously

Lighthouses, for

suggested about twelve years previously." "During the progTess of this instrument

the

idea

occurred to the author of separating the zones, and also of dividing them into segments, like the ordinary reflecting

LIGHTHOUSE ILLUMINATION.

zones of a dioptric light

;

mecans

this

became

practi-

cable to increase considerably the radius of the mirror,

thereby to render

and

applicable to the largest sea light, with-

out overstepping the limits of the angular breadths of the

and yet without being compelled

zones,

high refractive power."

to resort to glass of

There can be no doubt of the advantage of these improvements, and

without any intention of derogating from

My. Chance's merit in the matter that idea

first

was

also

But the

axis.

it is

added that

generate the prisms round a vertical

flint-glass

which was necessary

for so small

a mirror could not be obtained in large pots, and had to

be taken out in very small quantities on the end of a rod

and pressed down into the mould.

was therefore obliged

to reduce the diameter of the rings as

and

was thought by those

whom

much

as possible

consulted at the time

John Adie, Mr. Alan Stevenson, and Professor Swan),

(Mr.

by adopting the horizontal axis the most important and most useful parts of the instrument near the axis would be more easily executed, inasmuch as those prisms that

were of very much smaller diameter.

Mr. Chance not only

adopted the better form, but added the important improve-

ment

of separating the prisms

ments.

would be

better,

and arranging them in

however,

constructed with a shoulder, as in the original design, I

(Fig.

seg-

the prisms were

63), instead of a sharp edge, as in section, Fig.

70, for the inner concave surface interferes with the passage of the rays reflected

32.

from the parts of the

Professor Swan's

Optical

Mr. D. Heudersou states that M.

however this publication of the improvement.

ment

in 1860,

but,

IMasseliii

may

sides adjacent to

Agents.

bo,

â&#x20AC;&#x201D; In

the

it.

Trans.

and he suggested this arrangeMr. Chance's was the first

PEOFESSOR swan's impeovements.

Swan proposed a new forms of prisms.

Eoy. Scot. Soc. of Arts (1867-8) Professor

number

of ingenious arrangements and

Among these is what he termed the Triesoptric prism, by which the rays wouki undergo two refractions and three reflections. Its execution presents gTeat difficulties in construction,

Swan

Professor

two

pieces,

Among

for separating the front ^nisms,

otker combinations, for wliick the

referred to the paper

by making

suggests might be overcome

which woukl be afterwards cemented togetker by

Canada balsam. reader

which

itself, is

a vakiable suggestion

and sending

light parallelised

prisms placed behind, through the interspaces left between

In Fig. 71, a are the

the front prisms.

are

constructed of

refraction.

front,

and

glass, ha\TJig a higher index

flint

will

the

The two upper and lower prisms,

triesoptric prisms.

ob-

of SD—•

served from the drawing that this ingenious

arrangement

nevertheless open

and of

6 5 ° at the

are lost w^hen an oil

used.

objec-

Professor

Swan

~_>-.^^

back

lamp

;§

sug-

its

^M;vc^^::::::z^:^

j^cptr^-:;^;::::::

>--'

loss

all

'^^

^^Sl

— ^"^

c ;

°'

but in the case of fixed lights

showing

;

'^'"

hori-

zontally instead of vertically

]M^.'.'.'.'.'^'''..-..

y'^jMfSc^:::::^f::^-

electric light,

lamp

SllM^ki^:^^'

this loss might be prevented

and placing

•

^rfi"f^Z'^J.'!!?.'.V.'';!.".'.'.'

""-..

gests that the greater part of

byemploying the

'•.

cones of rays of 30°

tion, for

in front

round, there w^ould

still

be a very considerable

from want of interception, as well as great divergence

from tke skort focal distance of tke central back prisms.

In tke dioptric spkerical

mirror tkis

evil

avoided,

LIGHTHOUSE ILLUMINATIOX.

96 every point in

its

surface

removed

sufficiently far

from

the flame.

have now traced the gradual progress of improve-

ment from the rudest

to the

most perfect forms of apparatus with light of equal power, both

for illuminating the horizon

constantly and periodically.

FIXED LIGHT

Tlie

ivcis

showii (16) to have, heen 'perfected

hy Fresnel hy 7neans of his two new optical agents, including the first introduction of the ^rinci])le of total reflection to this

hind of

light.

Tlic Icen,

REVOLVING LIGHT

iccis

cdso shoivn

hy single agency, perfected

hy the

(Fig.

holophotcd principle, hy which total reflection to

into

of light diverging

single

of agents.

from a flame may

heam of parallel

rays, ivith the

have

of the

was first applied

moving apparatus, and hy which, as shown in

the rays

64)

cqjplication

Fig. 69, cdl he condensed

minimum numher

CHAPTEE

III.

AZIMUTHAL CONDENSING SYSTEM FOR DISTRIBUTING THE LIGHT UNEQUALLY IN DIFFERENT DIRECTIONS EITHER CONSTANTLY OR PERIODICALLY. 1,

have now to consider certain requirements of

ultimate destination

for the

the rays, of a different and less

simple nature than those which were dealt with in the

former Chapter.

having

the

1855, lighthouse apparatus,

Previous to

same illuminating power

was used not only

at places

every azimuth,

where the distances from which

the light could be seen were everywhere equal, and where the employment of such apparatus was therefore quite legitimate, but also for places having a sea range in

some

directions

than

This

others.

much

greater

indiscriminate

application of apparatus of equal power to the illumination of our coasts necessarily involved a violation of economic principle, for the light

was

either too

weak

or else unnecessarily strong in another. there were several lamps, each with its evil

one direction

At stations where own reflector, the

could no doubt be obviated to a certain extent by

employing a greater number of such instnmients to show in the direction of the longer, than of the shorter range.

But

from what has been already explained in the previous Chapter, the reader will see that by this method the light could not

be distributed uniformly over any given

sector.

In other

LIGHTHOUSE ILLUMINATION.

where perhaps only half of the horizon had

cases,

to be

a single flame in the focns of a fixed apparatus

lighted,

could also be strengthened by a hemispheric reflector placed

on the side next the land, so as

send back the rays, and

thus increase the power in the seaward arc, but no attempt

was ever made the

to allocate this

auxiliary light in proportion

varying lengths of the different ranges, and the

of the arcs all

and

the horizon,

ivith the

which,

power.

illuminated

to he

round

nor, where

;

wxaken

rays so abstracted,

its

light

intensity

amiMudcs

had

show

in one arc,

strengthen some other arc,

from its range being longer, reqidred to be of greater As none of the agents or combinations which we have were

as yet described

sufficient for dealing

of lighthouse optics, I found

with this branch

necessary to devise eight

new

agents, possessing special optical properties, for distributing

These new agents, and

the rays not equally but equitcd)ly. their combinations with

some

which

of those fornierly in use,

are required for dealing with special cases, will be described.

The most important

of the problems

which have

solved are the following, and, from the principles involved in them, the

modes

of dealing with different modifications of

such requirements can easily be deduced, and examples will be given in which some of these modifications have been carried out.

Eequikements for Condensing Fixed Lights.

2. 1st,

AEG of the

Where a

light

the horizon, the

rays

amplitude

from

may

the be,

has

flame

and

to be seen constantly over

apparatus mttst xoitliin

to sp)read

2d, IVIiere a light has to

made

only one

compress all

that one arc, whatever its

them uniformly over

illuminate constantly the

it.

WHOLE

horizon, but has TO BE SEEN AT GREATER DISTANCES OVER

AZIMUTHAL CONDENSING SYSTEM.

SOME PAETS OF THE SEA THAN OVER OTHERS, must

made

to abstract

the shorter ranges,

and

so as to allocate

longer,

much

divert

the light has to

strengthen

one that has

and

squares

the

he seen

lie

spared

from

compound

ratio

of the distances

And

in each arc.

the

has thus heen diverted from any arc in order

light, lohich to

can

apparatus

the

the illumination of the

the rays in the

all

of the number of degrees, and

from which

light as to

another, must to he

spread

strengthened.

uniformly

this

mode of

over

the

ahstraction

addition, there is therefore produced a constant equitable

distribution over the lohole horizon of all the rays

from any

single flame.

Eequirements for Condensing Eevolving Lights.

Is^,

Where a

light

has

to give its flashes periodically over

only one arc of the horizon, the apparatus must the rays

power

and send them

collect

all

out periodically in solid beams of equal

over that one arc only.

2d,

WHOLE

JVJiere

light

has

WHERE

illuminate pcriodiccdly THE

FLASHES HAVE TO BE SEEN AT GREATER DISTANCES OVER SOME PARTS OF THE SEA THAN HORIZON, BUT

OVER OTHERS,

the

ITS

apparatus must

the condensing fixed light) to

nuide (as in

JSfo.

vary proportionally the

power of

the flashes whenever they pass over those parts of the sea vjhere the ranges are of different lengths, so as to produce

equitable

periodic distribution of all the rays over the whole horizon.

4. According to the hypothesis which has been generally

advanced as probable, the through the however,

loss

air increases in a

one

by absorption

geometric

which very

little

ratio. is

" that

passing subject,

known

mentally, and Sir John Herschell remarks that

on the supposition

The

experi-

proceeds

the rays in the act of traversing

LIGHTHOUSE ILLUMINATION.

100

one stratum of a medium acquire no additional

facility to

If the absorption were neglected,

penetrate the remainder."

when more than one azimuthal angle we have said, be so calculated as to distribute the rays in the compound ratio of the squares of the distances and the number of degrees in Thus, if 7i be the number of degrees in an arc to each arc. condensing aj^paratus,

has to be strengthened, should, as

be illuminated, and d the distance in miles to be traversed

by the

then,

light,

atmospheric absorption, the

neglecting

quantity of light to be apportioned to that arc will be pro-

portional to

d'^.

But,

take account of atmospheric

absorption, supposing q to be the quantity out of a unit of

transmitted

light

through a mile of

which escapes absorption air,

arc to be illuminated will be proportional to

Supposing now that

passing

after

then the whole light needed by the

L is

emitted by the lighthouse

d'^ q^.

the whole 360째 of available light

quantity to be

apparatus, the

apportioned to the given arc will be

mL where puted 5.

denotes the

sum

of the several

numbers

for the respective arcs of the horizon to

What

follows will

show the modes of

com-

fulfilling

various conditions which have been enunciated.

entering upon this description,

be illuminated. the

But before

well to notice, in case

any misconception, that the form assumed by the emer-

gent

rays

of a fixed light

neither a solid

beam

on the condensing system,

of parallel

rays

that from

annular lens, nor yet a zone of rays diverging naturally in

azimuth ratus,

all

but

round, like that from an ordinary fixed appa-

intermediate between these, being a solid angle

AZIMUTHAL CONDENSING SYSTEM. or

vMge

of light, strengthened

101

by those rays which would

naturally diverge in other directions, but which are diverted

and spread over the given

This

sector.

position of the lighthouse,

radius of the

circle

the

sea

from which the light

range

be seen, and

distance

i.'

horizontal angle

the solid

that

illuminated, and into which all

the light

D',

/ //-''

can

which would

the

pressed,

must

cu'cle,

^^/

X^^""^"

â&#x20AC;&#x201D;-"^^

and over which

must be uniformly

The condensing system different heads

^^^" '^"

com-

dji

naturally diverge over the rest of

the

"\P^ ^^ "^^"'

/'''

per-

represents

^,_-â&#x20AC;&#x201D;...^^^ ^\, ^

the

D, ox

shown

spectively in plan in Fig. 72, in which

spread.

divides itself into the following

Fixed Lights. 1st,

Wliich compress

the rays from the flame into a

all

single sector in azimuth.

than one

2d, Wliich compress all the rays into more, sector of

unequal range.

3d, Wliich illuminate constantly the

which vary the

strength

of the light

liorizon,

ivliolc

the

sectors

but of

different range.

Eevolving Lights. 1st,

Which

compress

periodically over a single 2d,

Which compress

all

the rays into flashes passing

sector.

all

the rays into flashes of unequal

power, which illuminate periodically more

than one

sector

of unequal range. 3d,

Which

periodically iUimiinate

the whole

horizon,

102

LIGHTHOUSE ILLUMINATION.

but which vary the strength of the flashes whenever they pass over sectors of unequal range.

Optical Condensing Agents.

Condensing Straight Prisms^ which, either by reflection

or refraction or both, cause a ray

in any

r (Fig. 73), proceeding

compass

bearing

main apparatus

the

emerge parallel tion

from

the direc-

(in

g) to the corresponding ray

which proceeds in the same compass bearing from another

/ 1,

f^-

part of the apparatus of â&#x20AC;˘A

any other ray

Prisms (introduced at the Tay leading

\\\

^ Section

lAvii^n

"nab.

i i

:|-l

;

and so is

/ a.

Bight-angled Expanding

l^yji'v'-.

;

which

bent parallel to the ray

Fig. 73.

i^.yi;i;:::;:;

lights in 1866).

These prisms, shown in Fig. 74, are rightangled vertically

them IS

while in Tplano two of

;

0, C,^ are semi-rings, ^

a semi-cone.

vertically

and the third C,^

Parallel rays in passing

upwards in a semi-cylindric

beam, and falling normally on the bases

a of the prisms, enter the flected

by the

sides

glass, are

re-

and pass out hori-

zontally and normally at the other sides

Plan

but

the prisms

circular rays, ^

segment

when they emerge,

a simple

method

in piano, the

will

be spread

of distributing naturally-

diverging rays of light over any azimuthal angle. T. Stevenson (Edin.

New

;

bent through a

are

Phil. Journal.

April 1855).

103

CONDENSING AGENTS. over the same angle for

d' h

d in azimuth by

reflection only,

both the immerging and emerging rays enter and leave

the prisms without being refracted. horizontal

Thus, whatever be the

by the prisms, the incident

subtended

angle

parallel rays will be spread over the

same

angle.

the

prisms shown in the figure subtend 180°, the light will in this case be spread over half the horizon, d' h d. 3.

Tivin Prisms}

—

This kind of prism was designed for

carrying out within the smallest possible space the ingenious projDOsal of Professor

Swan

(described

in Chapter

II.),

placing prisms behind others, so as to cause light coming

from those behind,

to pass

between those in

through spaces

left for

The twin prisms

front.

the purpose

(Fig.

75) are

formed by cutting out the apex (shown black in diagram) of a straight prism so as to reduce size

and provide more space between

its

and the

next prism for the passage of rays coming from another prism behind. effects a

This arrangement not only

^'"'*'''"'

saving of light by shortening the length

of glass traversed

by the

^°"

rays, but it also admits of a very

great diminution of the size of the apparatus and lantern. 4.

Differential Lens.

—The

differential lens

which

I pro-

posed in 1855 should have no divergence in the vertical plane (excepting that due to the size of the flame), while the horizontal

divergence

may

be adjusted

any required

amount, by simply varying the radius of curvature. face of this lens

(Figs. 76, 77,

One

and 78) remains the same

as in the ordinary annular lens, while the other face, though

straight in the vertical plane,

ground

to different curves

in the horizontal, so as to produce the required divergence. ^

On an improved optical arrangement for azimuthal condensing apparatus By Thomas Stevenson (Nature, 26th August 1875).

for Lighthouses.

104 Fig.

LIGHTHOUSE ILLUMINATION.

76 shows in front elevation the

inner face

Fig. 76.

and

outer,

Fig. 77.

which

Fig.

77 the

while Fig. 78 gives the middle horizontal section

;

after passing

rays

are

Fig. 78.

coming from the flame, which,

through the lens

focal plane /',

converge to the vertical

and afterwards diverge through the smaller

x which the Kght has horizontal angle

x'.

When

to be seen

is,

the angle

over

as in this case, smaller

than the angle x f x which the lens subtends from the flame, the radius of convexity of the inner face must be greater

than that of the outer 5.

and

Differential Befractor.

refractor

above

face,

of the

vice versa.

â&#x20AC;&#x201D;

the application to the

same principle which has been described

for the lens.

Differential Reflector of single agency.

â&#x20AC;&#x201D;

It is possible

to condense the rays over a horizontal sector

agent. bolic,

For

tliis

must be of such hyperbolic,

other curve, as will most advantageously give, in

each case, the required horizontal divergence. in

single

purpose the vertical section must be para-

while in the horizontal

elliptic, or

by a

The

construction might be most easily overcome

the old form of mirror in which reflection

difficulty

by using

produced by

small facets of silvered glass, bent or ground vertically and horizontally to the parabolic

and

elliptic curves.

If the

edges of the facets were cemented together with Canada

CONDENSING AGENTS.

105

balsam, the large loss of light which takes place at the edges of each facet in the old reflectors will in great measure be

There will not, as formerly, be any refraction of the rays in passing through the edges of the facets, and thus saved.

the whole mirror will become practically monodioptric, other words, optically nearly the same as

of one whole sheet of glass.

ment

or,

had been made

if it

would be a further improve-

to select for the foci of the facets different points in

the flame, so as to secure the accurate destmation of the

When

brightest sections of the flame.

wanted the

would

facets

required colour, so as to render unnecessary, thus

enough

coloured lights are

consist of glass tinted with the

stained

saving light.

glass

chimneys

Professor Tait

was kind

mathematical conditions of the

investigate the

and in the Proceedings of the Eoyal Society of Edinburgh of 1871 he gives, by a quaternion differential

mirror,

integration, the formulae for its construction.

by the ordinary methods Appendix in more detail. gation

The

difficulty of construction

employment perfectly

the

formed

formulae;

and the

found

investi-

the

has hitherto prevented the

One mirror was made

of this instrument.

accordance with

will

but the surface was im-

Light

was not very accurately

directed. 7.

Condensing Catoptric Spherical Mirror.

represents different

modes

â&#x20AC;&#x201D;

of condensing light

Plate

XVI.

by means of

a spherical mirror which has a sector cut out opposite to the arc that

may

weakened.

The rays which thus

escape are received (Fig. 1) on an ellipsoidal mirror e

which converges them

to its other conjugate focus P, wliile

an interposed plane mirror

deflects

them through a

smaller aperture in the spherical mirror, opposite to the arc

LIGHTHOUSE ILLUMINATION.

106

This aperture

that requires to be strengthened.

reduced to is,

still

smaller dimensions by a lens

of course, desirable to diminish as

size of this opening,

much

may be It

(Fig. 2).

as possible the

because the condensation of the rays

depends on the relative

sizes of the

two apertures.

If they

were equal, as many rays would pass in the one direction as in the other,

same

result

and no condensation would be

may

different focal lengths,

and Plate XXXII.,

which an

similar design, in

C D,

ellipsoid

Spherical Mirror of unequal area.

mode of allocating the light

Fig. 2,

shows a

B, and hyperboloid

are used.

The

effected.

be obtained by means of paraboloids, having

â&#x20AC;&#x201D;A

very simple

to the different sectors

would be

place between the lamp and the apparatus a spherical mirror

varying in height inversely with the lengths of range in the different azimuths, as

shown

in Plate

XV.

That portion of

the light which can be spared in one direction to assist in another

where the range

mainder of the rays passing above the mirror

where

made

at the places

has been lowered, are allowed to continue in their

original directions.

down

thus

greater, while the re-

that

The mirror should

its different

therefore be so cut

heights will represent inversely the

varying distances of the neighbouring land from the lighthouse.

7. 1.

shown

Fixed Condensing Lights for a single Sector.

The

first

in section

design^ which I

and elevation

parallelised into a single

beam by

made

for this

purpose

(Figs. 79, 80),

where

a holophote a &

a, is

light,

made

converge in azimuth to a vertical focal line/' by the straight ^

Edin.

New

PhU. Journal, 1855.

107

FIXED COXDEXSING LIGHTS. refractor c

G/'

so long as

which

after

This design it is

over the sector

diverges over any horizontal sector

the conditions of the problem

satisfies

not essential to spread the light uniformly

/' H.

G-

But in almost every case uniformity

Fig. 80.

Fig. 79.

of distribution

essential.

To provide

for this, a

num-

Cb Middle Horizontal

Fig. 81.

ber of independent refractors,

c c (Fig.

81), are substituted

LIGHTHOUSE ILLUMINATION.

108

for the single refractor.

the rays incident upon is

Each it

of these refractors spreads all

over the required sector, wliich

These were the

thus illuminated equally throughout.

and are indeed the fundamental designs spreading

may

all

the light over

any

for

But the same

sector.

first,

condensing and result

be effected in other ways, examples of wliich will next

be described. Condensing Qimdrant}

—

This combination (Fig. 82)

consists of a central or

main

apparatus of the ordinary kind, h

prisms

with auxiliary straight

p and ;p'

(6, 1) so as to

intercept on each side 4 5 ° of the light

from the main apparatus,

and cause the to

emerge parallel to

rays

different

other

coming from the unob-

Fig. 82.

structed

the main apparatus.

central

quadrant of

Those prisms marked p' send out rays

in the same compass bearings as those from the arc a a' ; while those marked p send their rays parallel to those from

the arc

The

a' a".

straight prisms are in this case

cave on the reflecting side, sides.

The whole

light will therefore be

condensed equally

over 90°. 3.

con-

and straight on the refracting

Condensing Octant (Tay Leading Lights, 1866).

apparatus was remarkable, at the time

—

This

was made, from its known, viz. the

containing every hind of dioptric agent then lens, reflecting prisms,

IL);

and cylindric refractor

of Fresnel (Chap.

and the holophotal, the condensing, the rightangled expand-

ing, and the double reflecting 1

prisms (Chap. II.)

Holophotal System, London, 1859.

The apparatus

FIXED CONDENSING LIGHTS. is

109

sliown in vertical and horizontal sections (Figs. 83, 84), and

in front elevation (Plate XX.)

The main apparatus

circle of a fixed light instrument,

ale

Vertical Section,

45째

a halfh h h

Horizontal Section.

Fig. 83.

(Fig. 84),

and

(Fig. 83),

Fig. 84.

which spread the

agency over the required

arc,

light directly

^ gp

(Fig. 84).

single

either

side of this middle arc are placed in front of the

paratus,

straight

spread over 45째

condensing prisms, the light which

all

ing the main apparatus.

hemisphere of rays

this

way

is parallelised

falls

p p

on them

main apwhich also after leav-

the whole of the front

in the vertical plane, and

spread equally over 45째 in azimuth.

Let us next consider

how

also to

the hemisphere of back rays

into the same seaward

part

by the

Fig. 83,

arc.

be condensed

These rays are received in

dioptric spherical mirror

Fig. 84,

and d d

and are returned through the flame, where, mingling

with the front rays, they are finally distributed over the 45째 which are to be lighted. The light which passes above the spherical mirror (Fig.

83) (which

parallelised

by a

half-holophote, i

fixed above the flame),

and bent by

h it

110

LIGHTHOUSE ILLUMIXATION. upwards, when, falling

vertically

prisms (6,

Thus with

2), it is finally

the

whole

on g

light

expanding

the

arc.

condensed and distributed

equality over the 45째

strict

spread over the required

by means

of six optical

agents, involving in no

case

four

For the upper double agents, the

total

back (Chap.

reflections.

prisms II.)

afterwards

more than four

invented

might be substituted.

refractions

and

and already described

This apparatus, w^hich was

manufactured by Messrs. Chance in the most perfect manner,

was erected exhibited

at the

mouth

of the Tay,

by the Commissionei-s

Paris Exhibition of 1867, and

burgh Industrial Museum.

and a duplicate was

of Northern

now

Lights at the

placed in the Edin-

The only other

case

the expanding prisms have as yet been employed,

which

at Souter

FIXED CONDENSING LIGHTS. Point electric

Mr.

J. T.

Plate

light,

the apparatus of which was designed by-

Chance, for the Trinity House, in

XXXI.)

Ill

this design

1870

(Fig. 4,

Mr. Chance employed these

prisms for carrying out a suggestion of Mr. Douglass, to send a portion of the light from the principal apparatus, down-

wards from the lantern

the bottom of the tower, where

the expanding prisms distribute the rays over a danger lying at

some distance from the 4.

shore.

Condensing apparatus for Y2^/t of the

85 and 86 represent the condensing

Van Diemen, New

light of

Zealand, in which

circle.

â&#x20AC;&#x201D;

FifTS,

Cape Maria

the whole light

condensed into a sector of only 30Â°, being

the greatest

compression hitherto effected by the condensing system.

112

LIGHTHOUSE ILLUMINATION. 5,

Twin

Pris7ii Condenser.

—

Fig.

87 shows the apparatus

Fig. 87.

for Lamlash, near the island of Arran, in the Firth of Clyde,

constructed in 1876, in which the twin prisms (6, 3) were first

Their action will be easily understood without

applied.

further description,

by the numbers shown on the diagram.

Fixed Condensing Lights for more thaj^ One Sector OF 1.

Isle

Oronsay

Oronsay is

Unequal Eange.

Apparahis.

situate

(Lighted

in the narrow

1857.)

—

Sound of Skye, and

throughout nearly the whole of the illuminated arc of 167°, it does not requu-e to be seen at a greater distance than three or four miles, while in one direction

wards

and B,

Fig. 88), it

(down the Sound

to-

can be seen for about fifteen

FIXED CONDENSING LIGHTS. miles,

113

and in another up the Sound (towards C and D),

can be seen for about seven miles.

Fig. 88.

The apparatus shown

in horizontal section (Fig. 89)

was

designed in order to reinforce the ordinary fixed light apparatus over the two sectors of greater range.

This

effected

dis-

tributing in those directions the spare light of 193째, which

would otherwise have been

lost

on the landward

side, or, if

returned through the focus in the ordinary manner by means of a spherical mirror,

would have equally strengthened that

portion of the light which

The formula arcs

for

already sufficiently powerful.

condensing the light unequally for different

and distances has already been given

magnitude

the

apparatus on which

But the

(4).

the

visual

angle

depends, forms an element of some importance, especially in narrow seas, such as the

Sound

of Sleat.

It is obvious,

however, that the influence of this element must be circum-

The Oronsay

scribed within certain limits. fore, after

light

due consideration, allocated nearly

metical ratios

of the distances,

appears warranted from nautical

with the

locality.

struction,

and

the arith-

and some such

allocation

considerations

connected

Moreover, there are

also in connection I

was there-

difficulties

in con-

with the amount of available

LIGHTHOUSE ILLUMINATIOX.

114

space in the lantern which had to be taken into account.

The

arc

down

the Sound could not have been

enough without of the lens,

and

through

cutting this

the

central

made small bull's

eye

would not have been advisable.

Fig. 89.

Fig. 8 9,

A is a sector of

1 6 7° of a fixed light apparatus.

This sector subtends the entire arc

C, in Fig. 88,

which

is all that has to be illuminated, so that the rest of the

light light,

on the landward of

side,

which 129° are

amounting

parallelised

193°,

spare

by the portion

of the

holophotal apparatus B, and after falling on a series of twelve

equal and similar straight prisms the horizontal plane only

a focal

;

and

«, are

again refracted, but in

lastly, after passing

through

plane (independent for each prism), they emerge

in a series of

12 equal wedges, each having a horizontal

115

FIXED CONDENSING LIGHTS. divergence of about 10째, and in

of wliicli the rays

all

are

respectively parallel to those of the directly diverging sector

from the main apparatus, which

Each

ened.

therefore strength-

of these supplemental prisms spreads

over the whole arc

A B

(Fig.

its

light

and the portion a of

88),

the main apparatus does the same directly by a single agent.

the light

139째

manner condensed

this

an arc of about 10째, the power, disregarding the transmission through

amply

by the other

lens C,

so as to strengthen the arc

which has a range of 7

The

number

greater

/3,

of rays

refracted

by the prisms

from the main apparatus,

between C and

miles, viz.

times

In hke manner, the

sufficient for a range of 1 5 miles.

light parallelised

loss in

about fourteen

and should doubtless be

a'pparatus,

of the unassisted

that

the prisms,

into

(Fig. 88).

which are represented in the

chart (Fig. 88) as passing through the arcs

and C D,

intended to indicate the additional density due to the action of the auxiliary prisms.

The condensed

light at Isle

Oronsay

appears to be equal to a first-order apparatus, so that, with a

lamp consuming only 170 is

gallons of oil per

annum, a

required), equal in effect to that of a first-order light

ing about It

first

800

may

made on

consum-

annum.

the trial of Isle Oronsay light, which

by the

of its kind,

but to

whom

calculations.

gallons per

be satisfactory to add the observations which were

not only took

Light

obtained (in the only direction in which great power

charge is

late

was the

Mr. James M. Balfour, who

of the

erection

of the

apparatus,

due the execution of the drawings and

In a

letter,

I7th October 1857, he

says,

Friday I got the apparatus permanently fixed at Isle

Oronsay, and at night

we steamed

More I cannot say than that

was

out to try the Hght. satisfactory

beyond our

LIGHTHOUSE ILLUMINATION.

116

The prisms throw a

utmost expectations.

Sound superior

any

iirst-class

light

down

light

Sleat

the ISTorthern

Bay from

Lights Service, and the Kght up to Glenelg

the

at all short of the

power

of a first-class light, although a large portion of the

beam

smaller set of prisms

by a

intercepted

little

^''

^'^

crossing of astragals

have

not the slightest hesitation in asserting that the Oronsay light is the best in the Service, both as respects actual

where power

Lights, on the

maintenance."

same time erected

same

principle,

Kyle Akin and Pama

power

economy of

wanted, and also as regards

were at the

Gall,

and shortly

Macarthur's Head in

Argyllshire, and at several

other parts of the coast of Scotland.

This kind of condensing

afterwards at

light,

which

also

been

in general use where oil

now

employed

successfully

for

electric

light

Dr. Hopkinson, 2.

The

J. T.

Chance.

the Lizards, which was designed

also

on the same principle.^

Zochindaal Co7ide7ising Apparatus. (Lighted 1860.)

single acting prisms for operating

68, 69) were

first

employed

on the back

light (Figs,

at Lochindaal, in the island of A

Fig. 91.

Fig. 90.

Islay.

used, has

the electric light of

South Foreland, which was designed by Mr.

The

The apparatus shown *

in elevation

Min. Civ. Eng.,

vol. Ivii.

and

vertical section

Il7

FIXED CONDENSING LIGHTS. (Figs.

90, 91) consists of the

surmounted by a panel of

mented by by

at once,

the "

main

fixed light apparatus,

back prisms " A, and supple-

straight prisms P.

strengthened by

The

arc of greatest range

back prisms," which throw the light

single agency, over tlie arc.

These prisms were

next used at Stornoway Apparent Light, where they were of a straight form (Chap. V.

159).

They have

also lately

been employed by Mr. Chance and Dr. Hopkinson

for the

electric light.

Fixed Condensing Lights of Unequal Eange, which constantly illuminate the whole horizon.

The mode

of illuminating the

whole horizon by means

of a fixed apparatus, which will allocate the rays unequally

among the

different ranges, will easily be understood

from

the descriptions of the condensing spherical mirror (6, 7),

and the spherical mirror in Plate

of unequal area (6, 8), or as

shown

XXXVII.

10. Condensing Appaeatus for Steamers' Side Lights.

The condensing principle is equally available for steamers' side lights, for which purpose the apparatus is placed in a small tower in front of the paddle boxes, so as to distribute all

the rays with strict equality over 112째 30', which

the arc prescribed by the Board of Trade regulations.

order to give a steady light, the apparatus and burner are

This application of the condensing hung on gymbals. principle was first introduced into the " Pharos," Northern Two of Lights Tender, in 1866 (Plates XVII. and XVIII.) the " Anchor " Line of Transatlantic steamers were also, in

118

LIGHTHOUSE ILLUMINATION.

1873, furnished with similar condensing apparatus placed in iron towers, large enough to admit a sailor to trim the lamp,

and having means of entrance during bad weather, provided below the deck.

11. Eevolving or Inteemittent Lights, which condense

the light into one sectoe. In light

Fig.

92 a

dioptric holophote throws its parallelised

on curved condensing prisms

P, which are so con-

Fig. 92.

structed as to give the required

amount

masks M, turning horizontally on either slowly or suddenly, as

may

of divergence, while

hinges, cut off the light

be wanted

for

producing

a revolving or intermittent light, both of which will thus

be made to condense the whole of the rays uniformly over this

one

sector.

CONDENSING EEVOLVING LIGHTS.

119

Unequal Eange, which do not illuminate the whole hoeizon.

12. Eevolving Condensing Lights of

Repeating Light.

designed

â&#x20AC;&#x201D;

Fig.

1860,

which plane mirrors

revolve

endless

chain passing over

roll-

ers placed outside of the

main apparatus,

for alter-

ing the direction of the flashes

after

they

pass

into the dark arc on the

landward

L L

side, so

cause the lenses

repeat their flashes

over

the seaward arc which requires strengthening.

The

only objection which was

93 shows an apparatus which I

120

13,

lighthouse illumination.

Condensing Eevolving Lights which periodically illuminate the whole horizon, but which vary the

strength of the flashes in passing over certain SECTORS.

The

spherical mirror of unequal areas (6, 8)

obviously

By means of may be satisfac-

equally well suited for those which revolve. it

or the condensing mirror (6, 7),

any case

torily dealt with.

14.

Condensing Intermittent Lights.

Although the distinctions about strictly fulfil the

definition

to be described

do not

which we have given of the

condensing principle, inasmuch as they do not show light of unequal intensities in different azimuths, they nevertheless require condensing agents, and will

therefore

most appro-

priately be described here.

94 and 95 show

1. Figs.

straight refracting or reflecting

prisms, which revolve and intercept certain of the rays from

central

fixed light apparatus, so as to

produce perfect

darkness over the sectors they subtend, while they spread the rays which they intercept, uniformly over, and thus strengthen

the intermediate sectors, which are illuminated directly the central apparatus.

ment

that the

duration

the

The peculiar property

power

of this arrange-

increased in proportion to the

the intervening periods

neglecting the loss

when when

absorption,

etc.,

Thus,

darkness.

the power

doubled

periods of light and darkness are equal, trebled

the dark periods are hvice as long as the light, and

on in

proportion,

while

in every

spread uniformly over each illuminated

case

the rays

sector.

are

The con-

CONDENSING INTERMITTENT LIGHTS. densing intermittent apparatus which was

first

121 proposed in

f4 \\

122

LIGHTHOUSE ILLUMIXATIOX.

of darkness alternately

;

and Plate XIV. shows an example

of a similar light, where the continuity

broken up by one

long and two short periods of darkness alternately. light periods in each case are of precisely the

and of equal and

definite duration,

same

and are in the

The

intensity

case

first

two and a third and in the latter three times the strength of the fixed light.

In these forms of apparatus the vertical

prisms only are made to revolve, and by their adoption two

new

characteristics are

added to the lighthouse system inde-

pendently of the changes which might be produced by varying

Any

the speed of revolution of the prisms. light can easily therefore

power by the simple addition

win be

existing fixed

be altered in character and increased of the condensing prisms.

readily seen that these lights are different in character

from the gi-oup flashing light of Dr. Hopkinson (Chapter in which

the

flashes

solely

IV.),

the duration of is

dependent

on the natural diver-

gence due to the size of the

The drawings

Jlame.

will

be readily understood without further explanation. 3.

tent

Condensing Intermit-

Light with Differential

Bcfr actors

A still

(Figs. 96, 97).^

more perfect form of

the condensing inteiTaittent light

can be produced by

availing ourselves of the pro-

perty of the differential lens (6, 4), which, in this case, takes

the form of the differential refractor (6, ^

Min.

5).

Ins. of Civ. Eng., vol. Iviii 187S-9.

This refractor

CONDENSING INTERMITTENT LIGHTS. having

its

centre of inner curvature at 0, Fig. 9 7,

123 is

substi-

Fig. 97.

tuted for the ordinary refractor of Fresnel, and the central

drum, instead

ABA haK

of being a continuous hoop,

(Figs. 96, 97).

becomes polygonal,

The design shown

in section through the focus. Fig. 9 7)

(half in plan

and

that of the

new

apparatus at the Mull of Galloway, Wigtonshire, where the parts revolve together.

the differential refractor

refractor, while the inner is

The outside the same

as that of Fresnel's

curved horizontally to the radius

A, and the upper and lower

intercepting light,

all

vertical profile of

joints,

order to avoid

are concave conoidal and convex conoidal

124

LIGHTHOUSE ILLUMINATION.

respectively.

compound

the

horizontal

and

vertical

action of this single agent, the whole intermittent effect

produced, so that double

agents

are

no longer required

The

excepting for the upper and lower reflecting prisms. joints are also

made

after refraction,

an improvement which was previously sug-

coincident with the paths of the rays

prisms

may

by M.

The

straight

be also adjusted so as to counteract the

effect of

gested for ordinary apparatus

Allard.

unequal incidence of the rays on each side of the centre of the refractor. of

The lightkeepers

Man, twenty -one miles

light of

Mull

at Point of Ayre, Isle

report

distant, "

Galloway

new

that the

appears to us to be nearly

double the strength in intensity of what the old reflector Plate

light formerly was."

XXXIV.

shows another design,

in which dioptric spherical mirrors are used. 4.

two

Fixed

White Light changing

sectors of a fixed apparatus (Plate

with coloured

glass,

and

placed in front of so

when

Fixed Coloured.

XXXVI.)

much

of the intermediate sector as,

in intensity to the white light

uncovered part of the coloured glass

coloured condensing prisms be

spread over the adjoining coloured sectors, will

them equal

â&#x20AC;&#x201D;

be covered

intermediate

and prisms be made

make

coming from the

sector,

then,

the

to revolve, the effect will be

a fixed light of equal power constantly in view, and changing its

character from white

red alternately, without any

intervening dark period, or any waxing or waning of the light,

and capable of illuminating simultaneously the whole

horizon. 5.

(See page 132.)

Condensing Intermittent Light, shoioing different

with dark

intervals.

â&#x20AC;&#x201D;

Colotirs,

Equality of power can be produced by

continuous straight prisms condensing the red arcs in the required ratio.

CHAPTER

IV.

DISTINCTIVE CHARACTERISTICS OF LIGHTS.

increase in the

necessarily to

number

of lighthouses

an increase in the number

on any

coast, leads

of distinctions.

following are the principal characteristics

now

The

in use, or

which have been proposed, but there are some modifications and combinations of these, to which we shall also have occasion to refer. I.

The Fixed

Light,

which remains constantly in view,

and invariably presents the same appearance, and is always of the same power in the same direction, whether it be condensed or naturally diverging in azimuth. II.

The Double

Light, fixed or revolving, either

from separate towers or from one tower III.

The Fixed

steady fixed

light,

Light, varied hy Flashes,

and

which shows a

at certain recurring periods

gradually for a few seconds of

much

shown

at different heights.

greater

becomes

volume and

in-

tensity.

IV. The Revolving Light, which at equal and comparatively long periods (such as once a minute), comes slowly and gradually into full power, and then as gradually disappears.

V. The Flashing Light, which at short periods (such as a

few seconds) comes very quickly, though gradually, into view,

and

as quickly

VI.

and gradually disappears.

The Coloured

Light,

which

obtained by using

LIGHTHOUSE ILLUMINATION,

126

coloured media in connection with any of the varieties of characteristics described.

VII. The Light of Changing Colour.

—

See pp. 124, 131.

VIII. The Intermittent Light, which bursts instantaneously into full power, and after remaining as a fixed light of the same power for a certain length of time, such as one

minute,

and succeeded by a dark

as suddenly eclipsed

period, such as a

of a minute

and these phases, which

;

occur at regular periods, are therefore unaccompanied by any

waxing or waning,

as in the revolving

and flashing

lights.

IX. The Intermittent Light, of Unequal Periods, which differs

from the ordinary intermittent light just described, in

showing from the same apparatus

different durations of the

light periods and different durations of darkness.

example,

may show

a fixed

seconds, then be eclipsed for

light

two seconds

Thus, for

constantly for five ;

revealed again for

two seconds, eclipsed again for two; and then continue steadily in sight for five seconds as at

first.

X. The Group Revolving and Flashing Light, which

from the common revolving and flashing

lights

differs

by show-

ing a group of waxing and waning flashes at unequal periods.

XI.

Separating Lights, in which lights

separate and then gradually to coalesce either or vertical

are

made

by horizontal

movement.

Fixed Lights.

—Nothing more need

be said regarding

the fixed light than has been already explained in previous Chapters, except to point out that, from

regards power,

its

inferiority as

should not be adopted where revolving or

condensing lights of greater power can be made available. II.

lights

Double Lights.

may

—The

risk of confusion

be prevented by showing two

among

lights,

fixed

separated

127

DISTINCTIONS OF LIGHTS. vertically from each other in the

1810

May

for the Isle of

(Fig.

same tower,

as proposed in

98) by Mr. E. Stevenson,

who

afterwards introduced this distinction at Girdleness, Aber-

deenshire, in 1 8 3 3.

of irradiation

The

effects

tend to blend

together the visual images of

such

lights,

long before their

distance apart has

small a fraction server's

tower as

distance

become of

from

longer

the

ob-

the sub-

tend the angle of the mini-

micm

visible.

The distance

which the two separate

lights

appear

can be found from

the following

table

Fi?. 98.

by Mr.

Alan Stevenson,^ founded on the

fact that

two

lights

6 feet

apart are seen just separate at a distance of a nautic mile.

Distance of the

Observer in Nautic Miles.

128

LIGHTHOUSE ILLUMINATION.

III.

Fixed Light, varied ly Flashes.

â&#x20AC;&#x201D;The mode

produc-

ing this distinction has already been described (Chap.

30,

7).

is still

largely used

II. 18, on the coasts of France, but

The

has seldom been employed in this country. is

a serious one,

fixed light

seen for a

and the revolving

much

which

defect,

the great inequality of power between the flash, for as

the former can be

shorter distance, the distinction

must cease

at that distance, and the light then becomes simply a revolv-

ing one to an observer

who

farther

off.

IV, and V. Revolving and Flashing Lights.

â&#x20AC;&#x201D;The

time

during which an observer will continue to see a revolving or flashing light at each of its luminous periods will

on the distance and the condition

some variation

in the duration of the light-period

able in all lights

depend

Hence

of the atmosphere.

unavoid-

which wax and wane, and which are therefore

But

in each

maximum power

of the

of unequal strength throughout their flashes.

case the times of appearance of the

flashes are nevertheless of exactly equal recurrence,

whatever

be the state of the atmosphere or distance of the observer.

The varying duration

of the flashes possesses, moreover, one

advantage, for it gives the sailor a

which

to the lighthouse,

rough idea of his proximity

always a matter of importance.

of producing these distinctions has been explained

The mode

in Chapter II.

The revolving

light

was used

at Marstrand,

1783, but it is not known by whom. was proposed by Mr. E. Stevenson, who

in Sweden, prior to

The

flashing light

introduced

at the Ehins of Islay, in Scotland, in

It gives

a flash every 5

seconds, and

striking

and

At Ardrossan

effective.

its

1825.

phases are very

Harbour Messrs.

Stevenson introduced, in 1870, a flashing light of only one second period, but

was found

be unsatisfactory, and was

altered to its present period of one flash in every

two seconds.

DISTINCTIONS OF LIGHTS.

—

Other Modes of 'producing Revolving

Besides the

II.,

common arrangement

129

and Flashing

the following methods have been proposed 1. Reci'proeating

Apparatus.

—

Lights.

explained in Chapter :

Captain Smith proposed,

with a view to economy of construction and maintenance, that

when only half the circle is to be illuminated should move forwards and backwards over objection

the apparatus

180°.

The

the varying inequality of the periods in the

different azimuths. 2.

Mr.

J. T.

Thomson's Reeiprocating Apparatus.

possesses all the advantages, without tages, of Captain Smith's plan.

any

by means

of rods

This

Mr. Thomson describes his

contrivance in the Trans. Eoy. Scot. Soc. Arts, 1856, in which,

—

of the disadvan-

and a pin at

(Figs,

308,

99 and

Fig. 100.

Fig. 99.

100), the reflectors are reversed in position while traversing the dark arc. 3.

Reversing Apparatics.

—

This plan, which I proposed in

1851, produces, in a different way, the same results as that In Fig. 1, Plate XXIV., c are the revolvThomson.

of Mr.

ing holophotes, which have a vertical spindle passing through their centre of gravity, with a small pinion at their lower

LIGHTHOUSE ILLUMINATION.

130

E F

end.

are concave toothed segments, wliicli turn eacli

reflector over

180Â°

comes round, so

point seawards again, and in this

way

as to

half the

holophotes which would otherwise be required 4.

make them number of

is sufficient.

Fixed Lights illuminating the whole horizon, hut shoiv-

ing Bcvolving or Intermittent Lights over small

arcs.

â&#x20AC;&#x201D;

found that where only a small arc of a fixed light

made

have

revolving or flashing, in order to cover an outlying

danger, the effect can be satisfactorily produced

a straight upright

mask

to revolve horizontally

by causing

on a vertical In

axis in front of an ordinary dioptric fixed apparatus.

this case the mask must subtend at the flame the same horizontal angle as that which is to have a characteristic differ-

ent from that of the main apparatus. will be

The dark periods

extremely short at the limits of the danger -arc,

and will gradually lengthen danger, which

as the vessel nears the line of

in the centre of that arc.

Fig.

101

and d the

fixed

Elevation

the

mask

apparatus.

thus

made

in elevation,

and h in

Flashes of equal power to

plan,

may

be produced, and

show over the whole of the small

using opaque leaves,

a, Fig.

arc,

102, arranged either vertically

or horizontally, like the Venetian blinds for house windows.

DISTINCTIONS OF LIGHTS.

131

proposed to alter a sector of 79° at Dhii Heartach on

It is

this principle, in order to

and dangerous reef the south

mode

sector, is

mark by red

called the Torrin

of the Eoss of Mull.

colour the extensive

Eocks, which

The

lie

superiority of this

marking dangers, as compared with a fixed red about 4 to 1, for the full power of the light is

preserved, wliile a red shade causes a loss of about threefourths,

VI. and VII.

The French

Alternating Red and

White

Lights.

—

alternating red and white light has an upper

cupola of holophotal prisms, each of which parallelises 45° of light.

The

refractor

and lower prisms give a

Each

the

about

5°,

and has a red shade placed in

revolve, in front

of the

refractors

shades, subtending an angle of

5°,

There also

front.

and lower prisms, red being the same as the

angle of divergence of the upper panels 5° of red

fixed light.

upper panel of prisms has a divergence of

and thus arcs of

;

and 40° of white are simultaneously illuminated,

and shown alternately when the apparatus ing an alternation of red and white

revolves, produc-

light

—

say 40

seconds white and 5 seconds red, or any other periods in these proportions.

and

But these proportions cannot be

as the light in the axis of the red arc

altered

stronger at the

132

LIGHTHOUSE ILLUMINATION. time during which the red will be visible must

sides, tlic

vary with the distance and state of the atmosphere. Condensing Intermittent Light of changing apparatus described at from, and far are, besides,

124

colour.

—The

are obviously very different

more powerful than those

and

described,

last

independent of the divergence of the flame.

VIII. Tlie Intermittent Light.

—The

Catoptric intermit-

tent light

was introduced by Mr. E. Stevenson in

the Mull

Galloway.

The

183

occultations are effected

the sudden closing and opening of two intercepting opaque

drums, Tigs. 103

and 104, which inclose the apparatus.

v y V

Fig. 103.

Fig. 104.

and are moved vertically in opposite directions by means of macliinery.

IX. Intermittent Light of Unequal Periods. 1.

Professor Bcdtbage's Mode,

1851.

— The method

pro-

posed by Babbage for producing occultations was by successive descents

and ascents of an oj^aque shade over the

flame, instead of

Mr. Stevenson's system of drums moving

outside of the apparatus.

The

inequalities

the periods

133

DISTINCTIONS OF LIGHTS.

were not introduced

for

producing optical distinctions, but

in order to sliow the differences of numbers,

by a

to indicate

example,

larger interval that three succeeding occult-

meant 300, and not 30

ations

as, for

tion to Babbage's proposal

or 3.

A great practical

the obstruction which

to the access of the keeper to the burners for

objec-

opposes

trimming the

flames.

Mr. B. L. Stevenson's Strengthened Intermittent Light

Yis. 105.

1871.

Unequcd Periods,

This

was

the

first

j)roducing an op-

proposal for tical

only

not

distinction

unequal

periods, but also the first

pro-

posal to save light in the dark

and

sectors it

strengthen with

the light sectors, so that an

existing

only

fixed

have

changed, but

light

its its

could

not

characteristic

power

tain extent increased.

Fig 107.

to a cer-

This plan

produced by interposing

134

LIGHTHOUSE ILLUMINATION.

between a fixed dioptric apparatus and

its

flame a spherical

mirror, cut vertically or obliquely into separate sectors of

may

such equal or unequal breadths as coloured lights lights

(Fig.

be wanted, and thus

105), and intermittent or revolving

(Figs.

equal periods (Fig. 106), and of unequal periods

may

107),

be aU produced from the central flame,

while advantage was for the

time taken of the spare

first

which spreads uselessly over a dark sector by strength-

light

ening a light sector in another direction, so far as the natural

In this way a fixed light

divergence of the flame admits.

—

may have its power doubled miri'or.

mirror,

In Fig. 105,

less the loss

due to the spherical

are the portions

the strengthened red arcs, and arcs.

are the mirrors.

In Fig. 107

W W the B

In Fig. 106 C C,

strengthened white

of spherical

B',

C C, and

B',

and

D, are

I)'

the mirrors which subtend the larger and smaller arcs. 3.

un-

Dioptric Condensing Intermittent Lights, 1872.

—In

the holophotal condensing intermittent apparatus (Chap. III.

14) the power, as has already been explained,

by simply varying the duration dark, so that the

power

may be

exalted

of the light period to the

in each case inversely ^woportional

to the duration of the dark period. 4.

Sir

Henry

occultations

are

Felly's

Form

produced

of Intermittent Light.

this

—The

apparatus by causing

opaque shades to circulate outside of a stationary fixed dioptric apparatus.

Sir

their inner surfaces light falling

William Thomson

might be

on them, by

also proposed

that

silvered, in order to save the

reflecting it

back again through

the fixed apparatus to the flame, so as finally to pass through

and be

parallelised

plan

obviously not so good as the previous designs of Mr.

E. L. Stevenson, in

by the other

side of the apparatus.

which the rays have

This

pass through

135

DISTINCTIONS OF LIGHTS.

only one side of

tlie

fixed

apparatus instead of through

two, or as the condensing apparatus (Chap. III. 14, 2) in

regard to waste of light, while the power cannot,

be increased in any required

ratio,

but

on the natural divergence of the flame.

lilve

it,

dependent solely

There

also the

waste of light due to the spherical mirror in Mr. E. L. Stevenson's,

and

to the cylindrical

one in Sir

Thomson's,

arrangement.

X. Mr. Wigham's Group Flashing Gas Light.

in 1878.

The

effect is

This use-

in Ireland,

obtained not by optical combination,

but by operating mechanically on the source of is

â&#x20AC;&#x201D;

was originated by Mr. Wigham,

ful distinction

light,

which

a gas flame placed in the centre of 8 revolving lenses.

This mode of producing occultations by gas was

duced in 1827 by the

late

Head

the gas

work while the

intro-

Mr. Wilson at Troon harbour,

but not in connection with revolving apparatus. continually turned

first

At Galley

up and down by

clock-

lenses revolve, so that the ordinary flash

broken up into a

series of shorter flashes.

The automatic

gas meter (Chapter V.), in which the flow of the gas itseK regulates

its

supply, so as to produce an intermittent or

revolving Hght, the alternations certainly

however a simpler method of giving

than any kind of clockwork, and could

advantageously

The

Mr. Wigham's plan. varying the flashes

applied objection

in to

connection with the

mode

at GaUey Head, which does not however

appear to have been found of practical importance by the sailor, is,

and

that the

effect

varies

the periods

raising

lowering the flame, depending on the position of the

revolving lenses

the

time.

Messrs. Stevenson's report of 9th

by Dr. Hopkinson, who remarks,

This was

pointed out in

November 1870, and "

also

In the case of an ob-

136

LIGHTHOUSE ILLUMINATION.

server at a

when

little

distance

—

flash

tlie

might reach him

had just been turned down, and then he

the gas

would receive only 6 of a

off,

flashes,"

instead of 8 in the period

whole revolution of the flame. Dr. IIo2)1dnson's Group Revolving and Flasliing

Lights.

This distinction also has been found to be well marked and

effective.

The apparatus

shown on Plate XXII.,

Fig. 3.

Instead of the ordinary long flash produced by a large lens

when

revolving, the

portions of lenses, as

three flashes

beam

may

rapid

after

which there

The means of producing

longer period of darkness.

and

An objection

of the flashes,

this

also not very expensive,

and has already been introduced successfully places.

or three

be desired, so as to give two or

succession,

efiect is optically simple,

up by two

split

at five different

to this plan is the inequality of

when more than two

power

are adopted, arising

from the variation in the angles of incidence on the difierent lenses

;

but

not

does

aj)pear

have been found a

practical eviL

Mr. Brehicr's form of Group Flashing A2')2')a7'atus (Plate XXXV.) In this better form of apparatus the lenses are

—

cut horizontally in sucli proportions

upper prisms

for the

as,

with the help of the

one half and the under prisms

other half, to produce equal flashes.

If then these

for the

two por-

tions be separated horizontally, a small arc of darkness will

intervene between the two flashes

;

or,

diagram, the upper and lower prisms

may

shown on the

be made to act

together to produce one flash, and the lens, reduced in size so that its light shall be equal to the

combined

light of the

two

sets of prisms, to

two

different sections should be inclined to each other at

angle of 10° to 12°.

produce another.

The axes

of the

137

DISTINCTIONS OF LIGHTS.

Separating Lights

(Fig.

108).

gested the employment of

â&#x20AC;&#x201D; In

1873

sug-

.-y^jj^,

a wholly different element, viz. the

two

relative

tained

motion of

which

lights,

ob-

two

causing

fixed light ajDparatus, each

illuminating horizon, to

the

whole

revolve hori-

common At distances suffi-

zontally round a centre.

ciently near, the

ance

appearFig. 108.

that of one light

separating gradually into two, wliich two again approach each other

they coalesce into one as before, a characteristic

till

which

preserved in every azimuth.

Distinctions by Coloue.

XII. 1.

The

first

proposal to adopt coloured media for produc-

ing distinctions in lighthouses seems to be due to Benjamin Mills of Bridlington,

who

introduced red shades at Flam-

borough Head, some time prior to the year 1809. optical action of coloured shades is

extent the transmission of

excepting the one which of

stained glass

which

the colours of the spectrum,

transmitted.

in every kind

of the one colour

This

proved by

white light be passed through

panes, the emergent rays will be of

hue than

But

wanted.

more rays than those

required are

the fact that

all is

after passing

The

to j)revent to a great

two red

deeper and purer

through a single pane.

pane had transmitted none but purely red

If the first

rays, the inter-

138

LIGHTHOUSE ILLUMINATION.

position of a second red pane, though

would have lessened

the intensity, could have had no effect in making the emer-

gent light redder than before.

more is

From

obtained.

red

But by adding a second pane,

of the other rays are screened out,

and thus a purer red

the greater loss of light by green and blue,

the only colour employed, unless in certain cases where

the range

employed resorted

very short, or where condensing apparatus

for increasing the

power,

when green

Wherever white and coloured

to.

sometimes

flashes are to

shown

alternately to great distances, they cannot be seen

equally

far,

unless they be derived from sources of illumination

The

of different intensities.

first

red and white lioht in which

the transmitted beams were approximately equalised was the Bell

Eock

1842.

But with such an apparatus, where the

produced by several independent

flash is

possible,

reflectors, it is

im-

already explained, from the optical nature of

these instruments, exactly to equahse the two beams, and

the usual size of the lightroom also precludes the employ-

ment

of the additional

reflectors

number

of independent lamps

which would be necessary in order

fect equalisation

;

whereas,

when

there

and

to insure per-

only one central

burner this difficulty disappears, for the apj)aratus can be divided as the

red

easily into

panels

unequal as equal

sections,

can be made to subtend

zontal sector of the Kght than the white.

quite

possible

case that

certam

so that

larger

It is

hori-

however

conditions

of the

atmosphere the equalisation of the red and white beams

may

be destroyed, and that one or other colour 2.

M. EeynaucVs Experiments on Coloured

most exhaustive investigation of this subject naud.

The more important

may

prevail.

due

â&#x20AC;&#x201D;

The M. Eey-

Lights.

of his results, obtained with lights

of equal initial power, are given in the following table

DISTINCTIONS OF LIGHTS,

COLOURED LIGHTS OF EQUAL INITIAL INTENSITY.

139

140

LIGHTHOUSE ILLUMINATION.

In other words,

metres.

equal power

The reverse

greater distance in a fog.

and

blue,

red and wliite lights appear of

at short distances, the red will be seen

which diminish in intensity

far

the white as the distances are increased, and

still

glass tinged with copper, of such a shade as

the

light

more than from ^

duce equality

But the

so in

not to reduce

The amount

-|ds.

in practice allocated to the red to pro-

from two to three times that of the white.

would seem

results of the experiments

smaller disparity than

â&#x20AC;&#x201D;The

to justify a

this.

3. Messrs. Stevensoti's Exfperime7it

White Lights.

than

raj)idly

In the French lights they use red

a thick atmosphere.

white light which

true of the green

on the Foiver of Bed and

apparatus used in these experiments for

comparing the power of the

Alan Brebner, and consisted

lights

two

was suggested by Mr. first-order lenses,

four- wick colza burner in the focus of each.

with a

The corners of

the lenses were covered up, so as to leave circular discs of light,

whose diameters were equal Over the

lenses.

placed a fan,

thin sheet iron.

the breadth of the

(shown in Plate XXII., Fig.

The centre

the centre of the lens, and

of the fan

each cut

off

and strong It

must

in the

light in the

was capable of being

were sectors of the

s^^read

light. circle, so

that

same proportion

as the

whole

disc.

be understood that the burner did not come of

any part of the

illuminated equally over

The

made

from the whole, a portion which contained weak

also

way

of the fan

1),

was coincident with

out so as to cover up the whole disc of white

The arms

was

disc of the white light lens, there

light

its

whole

of one of the

XXII., Fig. 2) by one

lens,

which was therefore

surface.

lamps was made red (Plate

of the ordinary red glass chimneys.

DISTINCTIONS OF LIGHTS.

The

lights

were exhibited from lanterns at Granton,

near Edinburgh, the distance betwixt

100

141

them being about

and the observations were made from the Calton

feet,

Hill, 2-1 miles distant, at a point to

which the two lenses

were directed.

When

the

two

lights

were

exliibited

first

their

normal condition, that from the white lens appeared greatly larger and more powerful. The fan was then sjDread out over the white lens and the effect observed, the result being that the white maintained

showing 4 to

equal, thus 4.

Bed

its

superiority until l^ths of the

was covered up, when the two

circle

Experiments on

Light.

â&#x20AC;&#x201D;A

light

lights

appeared about

1 in favour of the white.

tlie

Penetrative Foiver of a IVJiite

end of the scale of a bunsen photometer

at each

these lights

and

from a gas Argand burner was placed

was made red by means

one of

;

of a red glass chimney,

and the other had the usual white

glass chimney.

this state the two lights were equalised, so that the disc

of the

photometer stood exactly midway between.

The

small

distance trough,

betwixt

made

the

flames

of plate glass,

was

inches.

was next

filled

with a mix-

ture of milk and water, so as to endeavour to resemble

a fog, and

when

this

was placed next the red

light the

white light was 2'015 times stronger, and when placed next the white light the red appeared 1*96 stronger.

With another mixture

was

times

containing a larger pro-

portion of milk placed next the red, the white was 5*22 times stronger,

times

and when placed next the white the red was 5*006

stronger.

After

this the

red glass was taken

and a white one substituted, when light

now

off,

was found that the

from which the red chimney had been removed was 4' 9

times stronger than the other.

This last result

142

LIGHTHOUSE ILLUMINATION.

by the experiment

agrees pretty well ^vith that obtained

with the It

glass

which was

fan,

thus

which

of the rays,

4"0.

appears from these experiments that the red

used in the Scotch service absorbs about |ths

or,

must have an

in other words, the red

initial

intensity /owr times greater than the white in order to pro-

duce equality.

were warrantable to regard the mixture

of milk and water as possessing the as fog,

same absorptive power

would further follow from the experiments that the

penetrative

Eeynaud's experiments give a different be accepted as more it

follows that the

different

But

powers of red and white were equal.

correct,

milk-and-water mixture

from fog in

his fog,

essentially

selective absorption,

its

and

result,

because made in actual

and the

last

experiments cannot therefore be founded on.

In order to equalise red and white to think that the relative

amoimts of

lights I

inclined

light required for that

purpose should either be spread equally over equal areas, or else over areas of corresponding intensities, for

it is

quite

possible that the ratio of loss in passing through the atmosj)here

for

may

vary with the intensity of the

an intense than

The

results

for a

more diluted beam

which the of red to is

of rays.

Bell

of a

that, as seen

1876

from

January

1096 observations, there was only one when observations were made, on The ratio white was seen when the red was not. 1-7 The Wolf to 1. white at the BeU Eock is

all

the times

in the ratio of 2 '2 3 3 to

to 1,

the

miles distant, from June

1880, being in occasion,

less

Eock the red and

have been so nearly equalised

Arbroath, 11

and be

which have been given are certainly

very conflicting nature. wliite lights

light,

The French in the

ratio of 3

while the experiments with the fan gave the ratio of

DISTIXCTIOXS OF LIGHTS. to

Making every allowance

exist in the tints of the

there

tried,

143

for the differences that may-

shades that were

different glass

seems to be great need

a further set of

for

experiments on this important subject.

Mode

XIII.

of Producing Coloueed Lights.

Where the back mirror and made again

rays are not acted on

by a spherical

to enter the flame, but are passed

at once to the apparatus,

method of colouring the

and thence

the

sea,

the best

light is to tinge the glass

chimney

with the required colour.

But many coloured

some

returned by a spherical mirror

the rays to be

lights require

through the flame before they pass out seawards.

In such

a case the emergent light wiU not be aU of one shade, for the portion that passed

through the coloured chimney

tvjice

will have attained a deeper shade than that

once through one of

its

So that

sides.

which passed only

those rays which

pass once through the chimney be sufficiently distinctive,

which they ought

to be, then

follows that there

unnecessary loss of light occasioned by the transmission of

any of the rays a second time

thi-ough the coloured chimney.

In order

avoid this loss

better to use a colourless

chimney, and to place plain coloured shades either immediately in front of the apparatus itself or else

panes of the lantern of the lightroom.

owing there

In the

close to the latter case,

to the close proximity of the shades to the lantern, is

often imperfect ventilation, and in certain

of the weather atmospheric

volving a very great loss of If,

as has

states

condensation takes place, in-

light.

long been practised in the preparation of

certain optical instruments,

two pieces of

glass

be united

144

LIGHTHOUSE ILLrMlXATIO^\

together

by means

of a transparent cement wliich has nearly

the same index of refraction as glass, a ray will pass through

such a composite substance without suffering any notable refraction at the

inner

surfaces

;

and on looking through

such a combination of materials the two inner surfaces of the plates of glass will be invisible, simply because the light passes from the

first

piece of glass into the cement, and from

the cement into the second piece of glass, without suffering

any appreciable change in

would had

glass of the

cement

is,

direction,

its

any more than

passed through a sohd piece of homogeneous

same thickness

as the

as already stated, the

compound mass.

Such a

well-known Canada balsam,

which has very nearly the same index of

refraction as

crown

glass.

If plates of coloured glass

faces of lighthouse apparatus

from the glass

sam

of the

be cemented to the outer

the rays will therefore pass

apparatus through the Canada bal-

the coloured glass, practically speaking, without

into

refraction, and, therefore, without appreciable loss ficial reflection,

and in

Taking the

be saved.

this

way

loss for a polished surface at -g^^th of

the whole, probably -g^ths or

But

be saved. glass

mode

which of

this

by super-

the loss at two surfaces will

employed

-j^sth of

the incident light

would

not the whole gain, for the kind of for colouring cannot

by the present

manufacture receive a poKshed surface on both

its

sides, so that the real loss due to irregular reflection and

scattering

must be considerably more than j^th.

Eumford

found that the

thin pane

loss in passing

of clear white or colourless

through

window

Count "

a very-

glass,

not

ground," varied from â&#x20AC;˘1213 to "1324; so that, allowing for

the loss by absorption in passing through this " very thin ^

Philosopliical Papers

by Count Eumford.

London, 1802.

Yol.

p. 299.

DISTINCTIONS OF LIGHTS.

pane,

mode

of this

more than y^^th

shall certainly save

incident light.

of the

order, however, to take proper

of combination,

side of the glass

145

necessary that

it is

whole

advantage

tlie

coloured

should be placed next to the outer face of

the apparatus, so as to admit of the outer or uncoloured surface of the shade being polished easily be accomplished after the

prism by the Canada balsam.

—

a process

which can

plate has been fixed to the

may

perhaps be supposed

amount

that there will be an increase in the

of polishing,

because the outer surface of the coloured glass will require to

be polished as well as the surfaces of the prisms themThis, however,

selves.

not necessary, because, although

the outer face of the prism be left unpolished, the Canada

balsam which cements the pane of thin coloured glass

its

practical abolition of the

larities of

will,

surfaces, render the irregu-

real consequence.

mode

It has been found that this light

is efficient

large

pane was

for surfaces

tried, separation

producing coloured

moderate

was found

size,

but when a

to take place after

Messrs. Barbier and Fenestre of

a lapse of a year or two.

Paris have constructed, apparently with considerable success, pieces of white

and red glass united together by

Coloured Silver -plated Reflector.

producing coloured light

by means

of small facets of mirror glass. facets

may

— Another

mode

chimney

XIV.

As already

noticed, these

—CoLOUK-

are individuals

glass,

required.

Blindness.

objection to all coloured lights

fact that there

of reflectors constructed

be easily made of tinted instead of white

so that no coloured

fusion.

who

founded on the

are unable to detect

LIGHTHOUSE ILLUMIXATIOX.

146

any difference between some one colour and another. George Wilson, of Edinburgh, was one

late

Dr.

first

writers

who went

fully

into

statistics

Holmgren

Professor

physical imperfection.

the

Tlie

the

this

of Upsala has

recently subjected from 60,000 to 70,000 persons in Europe

and America

them

to the

match

inquiries,

of colom-perception,

test

differently tinted wools.

which confirm that

The

requii-ing

results of these

of the earlier writers,

is,

that

on an average 4'2 per cent (ranging from 2 or 3 to 5 per cent of males)

mal

were congenitally colour-blind It unfortunately

extent.

vision (or the

all colom^s) is

to a greater or less

happens that the defects from nor-

power of discerning the difference between

generally an inability to distinguish between

red and green, which, as

we have

only tints that

said, are the

There can be no

are employed for lighthouse distinctions.

question that this jjhysical defect

an objection to employ-

ing colour distinctions, yet most sailors are not so disqualified,

and to them

will generally be intrusted the important

duty of looking out vessel

XV.

for the

seeking the coast.

different sea

marks when the

Professor Bahbage's numerical system of distinguish-

ing Lights, as modifieclhy Sir William Thomson 1873.

â&#x20AC;&#x201D;The

essential principle of the simple characteristics at present in

use

that of optical distinction, and of strongly marked,

and

therefore very obvious divisions of time, in the exhibition of

the different phases of light and darkness cated ^

Sir

The Board

W. Thomson

;

while that advo-

the alteration of

all

fixed lights

of Trade, in 1877, introduced the " colours test" in the exa-

minations for masters' and mates'

certificates,

and in two

j^ears it

has been

found that about half per cent (â&#x20AC;˘43) of those examined -ft'ere decidedly colourThe Board of Trade has very judiciously come to the important blind. decision that all masters and mates must now pass the "colours test" as a preliminarj' part of their examinations for certificates in

na-\-igation.

DISTINCTIONS OF LIGHTS. to rapid eclipses, which,

147

understood, are intended to

represent letters of the alphabet, as in the Morse system of

signalling.

proposal:

â&#x20AC;&#x201D;"To

William Thomson thus

Sir

describes

distinguish every fixed light

group of two or three dot-dash

eclipses, the

his

by a rapid

shorter or dot,

of about half a second duration, and the dash, three times as long as the dot, with intervals

of light of about half a

second between the eclipses of the group, and of

five or six

seconds between the groups, so that in no case should the period be more than ten or twelve seconds."

present the

once recognises

white

sailor,

when

falling in

with a

light, at

as fixed or revolving, or revolving red

and

in periods, such as once a minute, or once in half a

;

minute

;

or flashing, in such short periods

twelve seconds

;

or as intermittent,

length of time a steady fixed light, which

ceeded by a dark

jDeriod,

showing is

from

five to

for a

certain

instantly suc-

and therefore readily distinguishable

from the revohdng, by the much longer duration of

by the suddenness

of the occultations.

termittent condensing light can be

made

light,

and

Or, again, the in-

(Chap. III. 14, 2)

to exhibit certain intervals of fixed light of unequal duration

and

lastly, the

flashes,

;

group flashing revolving lights exhibit two or

appearing at short intervals from each other,

which are succeeded by longer intervals of darkness.

In order watch purely

to recognise lights

such as

these,

any appeal

hardly required, because the characteristics are cither

optical, or

the periods are so widely different as to he

at once apiparcnt.

The upon an

origin of proposals to alter the present system rests

entirely erroneous conception regarding facts

are well established.

which

The discussions on the subject

are

leading to a widely diffused notion that the great cause of

148

LIGHTHOUSE ILLUMINATIOX.

shipwrecks

the mistaking one light for another

by the

Mr. Alan Stevenson, in an exhaustive report in

mariner,

1851 on Babbage's

proposal,

showed by actual

statistics

that for the four years immediately preceding that report

the

cause

true

Scotland,

of shipwrecks at night, on

was the

coasts

non-visibility of the lights, and not the

confounding of one with another.

133

the

occurred by night; and in

alleged that the lights

when

Stevenson showed that the

been grossly ignorant.

Out

203 shipwrecks,

two of these was

07ily

even

seen were not recognised.

sailors

must

Mr.

in both cases have

In only one of these two cases were

the lights specified that were said to have been confounded, viz.

the revolving light of Inchkeith with the fixed light of

Isle of

May.

If allegations of mistaking such radically dif-

any value, what,

ferent characteristics are to be counted of it

may

be asked, would become of

system, with

its intricate

seems

discussed in

Morse dot and dash

tlie

and minute distinctions

unnecessary to

over

periods of the recurring appearances. corollary, the fatal

intermittent their

2).

mistake of altering

character,

all

proposed

also shows, as

]\Ir.

I entirely agree

with Sir

Babbage, by II.

dis-

improvement or

W. Thomson

desirable to compress the whole

the

power would be prodigiously reduced (Chapter

must not however be supposed that the present

extension. it

of different

the lights to

tinctions are to be regarded as incapable of

ing

questions

all sea lights is jpewe-

power, and not an increased number

trative

the

Mr. Stevenson's report, in which he clearly

pointed out that the grand requisite of

which

all

in think-

features of distinction

within the smallest space of time that

reaUy consistent

with clearness of recognition and simplicity of nomenclature. This had indeed to a large extent been carried out by the

149

DISTINCTIONS OF LIGHTS.

Scotch Board after 1870, on engineers,

â&#x20AC;&#x201D;

recommendation of their

tlie

that the slowly revolving and intermittent lights

which were erected early in consequence of the

in the century should be accelerated,

much

higher speed which has of late

years been attained by ships, and especially by

The Trinity House

London have

also, it

steamers.

understood,

adopted the same improvements in the speed of revolution of I further concur, so far, in opinion

the apparatus.

William Thomson,

altermg the present fixed dioptric

might in some

as to think that fixed lights

cases be advantageously dispensed with.

apparatus,

The

without

lights,

with Sir

best

mode

the

cancelling

make them condensing

inter-

mittent by means of revolving straight condensing prisms

placed outside, wdiich will not only alter the characteristic,

but

also,

power

has been shown, increase very greatly the

of the

was proposed 1872,

Sir

Though Mr. E.

light.

L. Stevenson's apparatus

1871, and the condensing principle in

W. Thomson

did not adopt either of these plans,

The

but used Sir H. Felly's plan of revolving opaque masks. rays intercepted by such masks are therefore

must remain

light

fixed one,

of the

same power

which has been shown

Even though

lost,

as that of

and the

an ordinary

be the weakest of

all.

the masks were made, as he once proposed,

to reflect the rays back through the flame, the loss would still

be great.

been optically

had

His Belfast apparatus would certainly have far superior,

and therefore

far

periods that

may

more powerful,

the condensing principle been adopted.

be employed for intermittent

regards the

lights, it

may

be mentioned that, in the case of Ardrossan, already referred to in this Chapter, where a flash once in every second was tried,

and the physiological

cause of imperfection

effect

found unsatisfactory, the

sufficiently manifest

;

for it is obvious

150

LIGHTHOUSE ILLUMINATION.

that the waxing and

waning

and lowering of the gas

flashes

remamed once a

would continue

for only a fraction of a second,

would vary with the

state

But

apply to lights in which there period

light, I

objection does not

this

a one-second inter-

must be

From

so uninterrupt-

actual trial with a

disposed to expect that

even of only a single second of duration,

a light -period,

be found practically available. It

very

difficult

of distinction.

The preference

we have wreck of the

advocated as superior to of such a

grounded on the

seen, be

Eeference has been

statistics.

system to the peculiar

London, which, owing pipe,

what principle the

discover on

Morse alphabet

made by

effect

has well observed,

effect,

steady.

lamp in supply

that arrested

the monotonous

But, as Mr. Morris

the adjoining lamps had twinkled, in

slightly differing times, the attempt to distinguish one

another would surely have been very perplexing, lutely bewildering. objection to the licrc

and

on the

coast, as at Belfast

from

not abso-

This case illustrates very clearly

Morse alphabet system, which

there

shix?-

the advocate

of a street

much more than

which were

if all

modes

facts supplied

produced a twinkling, intermittent

effect of the others

other

all

system cannot, as

to the presence of w^ater in its

the attention of a passer-by

tion

and

of the atmosphere

for, if it

all,

edly during the whole second.

condensing intermittent

and that

no diminution of the light

by waxing and waning,

mittent light be visible at

may

of recurrence of the

second, the full i^owcr of the light

the distance of the observer.

fraction

to the raising

would reduce the duration of

Although the time

the light period.

due

of tlie flash,

flame,

tlie

real

not the exhibi-

and the Clyde, of a

rapidly intermitting apparatus, whose phases are either equal or unequal,

and which phases or the order of

then' succession

151

DISTIXCTIOXS OF LIGHTS.

need not be particularly noted by the adoption of such

lights,

sailor,

which would require a

but the general

strictly accurate

discrimination of all the phases exhibited at each station on the

The

coast.

sailor, in

coming over

where he should

least,

at once coast,

be,

remembers what

but

sea,

knows, approximately at

and when he

sees a flashing light,

light is flashing

on that part of the

the Morse system were in general use, he would,

without a book of reference and careful watching of the number

and order of recurrence

of the long dashes

and short

saw

entirely misled, so as to mistake the light he

immediately adjoining, or

The

else for

conclusion, therefore,

what has been adduced as at present, either

is,

dots,

for the

some other not very

one

far

off.

which seems warrantable from

that lights should be distinguished,

by purely

optical characteristics,

i.e.

appearances which are at once appreciable by the eye, or else

by widely

different periods of time,

phenomena exhibited

and not by minute

in rapid succession,

optical

which require

be counted, or at least to be very carefully noted, so that

meaning may be understood.

their

lights,

such as suggested by Mr. E.

way

A certain

intermixture of

showing minute rapid alternations of irregular periods, L, Stevenson, is

however in no

objectionable, but, on the contrary, advantageous, so long

as they are liglits

each other by several

separated from

other

of the ordinary character, so that a careful study of

their phases is not required.

closely adjoming

But rapidly intermitting

lights

each other, whether in narrow seaways or

on the ocean seaboard, could not but occasion great anxiety,

and probably lead

serious

mistakes.

therefore

decidedly of opinion that our coasts should continue to be

marked

in the

manner which

out the world.

now

in universal use through-

Similar views have been very ably advo-

cated both by the Trinity

House and the

Irish Commissioners.

CHAPTEE

BEACONS AND BUOYS. Our

marked

shores are

ways, one of which

for navigation in

— the lighthouse — — the beacon or

Wlierever a rock or projecting spit

of use

night; and the other

buoy

two distinct

by day and

— by day

only.

too small to allow, or

too unimportant to warrant the erection of a lighthouse with

tower, apparatus, and lodging for the keepers, the engineer

can only substitute a beacon or perch, consisting of a bare

framework or skeleton of iron, which

by a cage

for

the

use of

usually surmounted

shipwrecked

they be fortunate enough ever to reach is

seamen,

it.

shovild

This, however,

a very indifferent expedient, and does not satisfactorily

indicate a danger except in daylight. lies

But when the rock

constantly under water, the means that can be adopted

are yet

more unsatisfactory

;

for,

unless recourse be had to

a floating lightship, equipped and maintained at great expense, a floating buoy, which

than a beacon, was

till

generally

lately the

still

less

obvious

only alternative.

The

importance of exalting this subordinate class of indications to the all

rank of illuminated night marks, must be apparent

who

are acquainted

with coast navigation.

The great

expense of erecting lighthouses like the Eddystone and Bell Rock, accounts for there being so few of such structures in existence.

Hence many dangerous rocks

are

still

unlighted.

DIPPING LIGHT.

153

and there are many more that are without even beacons or perches.

seems only natural that some share of the

attention which has been so long and so successfully devoted to lighthouse optics should be

those dangers, and

which have

will

bestowed on the marking of

now

explain the different methods

as yet been either

employed or suggested

for

increasing their efl&ciency.

Dipping Light. 2.

all

object, as has

rays,

and

the apparatus employed for lighthouses, the

been seen,

to direct

to gather together the diverging

them towards the

axis of the apparatus

and lamp

angles to the plane of the horizon.

the contrary, should have

its

sea, so that

the vertical

placed nearly at right

The dipping

axis inclined

light,,

downwards

at a

certain angle, so that the rays, instead of being projected

nearly horizontally, will be thrown downwards upon the sea as represented pictorially in Fig.

109, and in Figs.

110

Fig. 109.

and 111, which show a dipping

light in plan

and

represents a lighthouse placed upon a headland

;

section.

a sunken

154

LIGHTnOUSE ILLCMINATIOX.

rock

;

C, the outline of the nearest safe offing, within

which no vessel should come during the

If the

night.

lamp

Fi-. 110.

Fig. 111.

and

reflector are inclined at

shall

such an angle to the horizon as

throw dow^n the rays on the space C C, C C, then this light it is time to put alout,

whenever a vessel makes as the

margin of safety has been

apparatus

crossed.

When

dioptric

employed, each part must therefore be made to

give the rays the required dip in the vertical plane.

The dipping

light is

on the same principle as a night-

which was proposed by Mr. Alan Stevenson in 1842 use in railways/ by which the engineman was warned

signal for

of his

dipping

approach to the station whenever he crossed the

beam

principle w-as not difficulty

of rays.

said that a light

was many years ago used

found to answer the purpose.

of trouble in all apparatus. its

axial

If,

this it

Very probably the

arose from too great divergence, that

holophote has

upon

Beachy Head, but

fertile

source

for example, a small-sized

beam pointed somewhat seawards

the reef, so as to throw the strongest light in that direction,

the base of the diverging cone will then cover ^

Trans. Eoy. Scot. Soc. of Arts, vol.

ii.

p. 47.

many

miles

DirriNG LIGHT.

beyond

of sea

tlie

great dip, which

safe offing, unless the apparatus has

only possible

high, or where the danger

lessen the evil; but there

such a mode of illumination

The following modes

may

danger rig.

when the tower

electric

are not is

light

many

angle.

amount can

also

XXXII. and a

prisms be placed above the ground on

down on The

may

many

the sea by the reflectors at the

exactly

amount

increasing or dimi-

nishing

because

have

the

distance,

rays which

all

greater

gence than will

low and the

divergence

adjusted

If a holophote (Plate

a stage, the parallel rays from the holophote situations be sent

where

likely soon to be provided.

3) be inclined to the horizon at a certain angle,

required

doubt

of reducing the divergence of the

the shore.

series of reflectors or

very

would greatly

situations

be found useful where the land

lies far off

close to the shore.

the small divergence of the

light

151

diver-

escape

wanted

upwards,

while those only which

have the proper amount will

be thrown down-

wards on

sult

the

distant

The same

danger.

may

also

re-

be pro-

duced by pointing the

';->'--'-'--''"'

'''

axis of

the holophote

directly to the place of danger, in

which case the amount

LIGHTHOUSE ILLUMINATION.

15G

can be obtained by interposing between the

of divergence

mask

apparatus and the sea at the proper distance a screen, having an aperture

made

through which those

rays will pass which have the required divergence, and

all

the others which have more divergence will be stopped by

Another device

the screen. wdiich

H, S

represented in Fig. 112, in

a lens with its axis pointed to the danger,

a holophote pointing upwards, and

reflector

throwing

wards on the

]\I

a plain

only the slightly divergent rays down-

In order to prevent confusion in the

sea.

diagram, those rays only which diverge from the edge of the

holophote are shown.

Apparent Light. There are

many

a short distance. rocks

the

fairway

mouth

is little

sea-room,

must be passed by

vessels at

are

included

places where there

and the danger, whatever

it be,

this

class of

localities

bays and roadsteads where

not broad, narrow

Sounds,

between the piers of harbours.

the

and the entrances

In cases such as the

last

named, sometimes one pier-head must be hugged, sometimes and so the other, according to the direction of the wind ;

critical in

stormy weather

even a single yard of distance only

who know

felt in

the

may

taJci7iff

of a harbour that

be of consequence.

Those

from actual experience the anxiety which

entering a narrow-mouthed harbour under night, with

a heavy sea running, can appreciate the vital importance of

knowing

at as great a distance as possible the exact position

of the weather pier-head.

The dazzling produced

l^y

reflections of the sun's rays,

which

are often

fragments of glass or glazed earthenware

when

157

APPARENT LIGHT. turned up by

mode

tlie

plough, are accidental examples of the

of illuminating beacons placed on rocks at sea,

1850;^

I proposed in

which

the apparent light consists

for

certain forms of apparatus for reflecting

and redispersing

of at

the beacon, parallel rays which proceed from a lamp placed

on the distant

shore.

thus redispersing the rays an

optical deception is produced, leading the sailor to suppose

that a lamp of this

burning on the beacon

kind,

shown

The

itself.

pictorially in Fig. 113,

first light

was established

Fis. 113.

in 1851, in Stornoway Bay, a well-known anchorage in the

Island

Lewis, possessing shelter which has

highly prized by the sliipping frequenting those

measures about a mile in length, and half a mile in breadth

entering

114), lying entrance.

;

For

seas.

entrance

about

but available sea-room for a vessel

materially reduced off

its

long been

by a submerged

reef,

(Fif.

Arnish Point, on the south side of the the purpose of rendering tliis sheltered

anchorage safely available alike by night as by day, a ^

Trans. Roy. Scot. Soc. Arts, vol.

iv.

re-

158

LIGHTHOUSE ILLUMINATION.

volving light was "beacon

and apparent-light

sunk reef which

The

on the Arnish Point, A, and a

erected

lies off

apparatus were placed on the

the Point.

erection of a lighthouse for opening

harbour of refuge was long talked of

;

this valuable

and the danger

of the

Fig. 114.

Arnish Eock was so much

felt

that

was proposed that the

tower, instead of being erected on the mainland, should be built on the rock

itself,

which would

of course

attended with very great expense, for the reef a heavy sea, and

low water

quite inaccessible, unless

spring tides.

The diverging

have been exposed to

by a boat

or at

lines represent

the directions of the rays after being reflected at the beacon.

APPAEENT LIGHT. â&#x20AC;˘while the

thick line

the

159

beam

of parallel rays pro-

ceeding from the lighthouse on the shore.

apparatus for is

The revolving

general use of seamen navigating the coast

tlie

placed in the light-room at the top of the tower on the

shore,

and the

beacon at sea beacon

apparatus for throwing the light on the is

530

window

placed in a

at the bottom.

distant from the lighthouse,

feet

truncated cone of cast iron, 25 feet high. the beacon a small lantern

and

The is

the top of

fixed, containing the optical

part of the apparent light, the centre of which

exactly on

the level of the axis of the apparatus on the shore. to " Tlie

In the Notes which were appended

System of Illumination," of Arts for 1850, lights,

and

in the Trans.

the possibility of employing

different

apparent

methods of constructing them, were

It will, however, be

proposed.

Holophotal

Eoyal Scottish Society

enough

to describe the

mode

which was ultimately found best at Stornoway, where, for nearly 30 years, the apparent light faction to seamen.

The

lias

given perfect satis-

optical arrangement

on the beacon,

Fig. 115.

shown

in Figs.

prisms

115 and 116,

(Chap. II. 30,

9),

consists

and

of straight

"back"

straight prisms of the ordi-

LIGHTHOUSE ILLUMIXATIOX.

IGO nary form 2^ V

(Figs.

115, 116), both of which cause the beam of 62째.

single

agency over the seaward arc

Apparent

lights

have since been established at Grange-

to diverge

month 535

feet

and

also

light

;

ment

from the

light,

and

Ayr 245

feet

from the

by Messrs. Chance, for the Eussian GovernBlack Sea, which is 300 feet from

at Odessa, in the

the light, and at

Gatcombe Head, Queensland, where the

intervening distance

also

300

feet.

The peculiar advantage which the apparent light over both the electric light by wires and the gas

possesses light

submarine pipes, both of which will be afterwards described, is its

absolute freedom from risk of extinction

of illumination being on

Unless the beacon

the shore are

itself is

;

for the

means

always accessible.

knocked down by the waves, no

storm or accident can interrupt the exhibition

of the light.

Designs foe increasing the Density of the Beam of Light sent to the Beacon.

The great

and

difficulty to

to a less extent in all other lights, is the diffusion of the

rays by divergence, for there of the light that really falls

purpose, but the cost of

only a very small proportion

on the beacon.

others, the best

clearly, therefore, of all

its

be surmounted in the apparent,

The

electric is

luminary

for the

maintenance has hitherto prevented

adoption for apparent lights.

One mode shore

causing

of increasing the intensity of the light

to increase the density of the different

flames

for this purpose,

luminous beams, by

converge to the same

Brewster (Edin. Trans., vol. xxiv.,

focus.

1866) gave two designs

though not with reference

tion to apparent lights.

on the

to their applica-

In his catoptric design a flame

APPARENT LIGHT.

was placed which the

one of the

foci

was conveyed

light

161

of an ellipsoidal mirror,

so as

strengthen another

flame in the conjugate focus, which was also in the focus of

another apparatus in front of it, but past the lips of the reflector was design fall

upon the

spherical

while in his dioptric

shown

lenses, as

in Fig. 117, in wliich

C, etc., are

show the balance

of the light

emerges in a beam

which

Such arrangements

parallel

were therefore very

as these

David

being holophotal, and Sir

indeed

states

rays.

from

far

that " the

holoplwte principle is inapplicable " to such a purpose. is

mirror,

the lost rays, and

the light which escaped

lost,

the light was lost except the small cones which

all

quite true that, besides the waste of light from employing

agents for

that

are

parallelising

But

purpose.

not needed, the

explained (Chap.

could

rays

the

agents abolished, the problem

least,

number

be conveyed

efiect

the

already

30) be employed, and the unnecessary

II.

the light from any

not possibly

correct holophotal principle,

the

mode which he adopted

the primary

enough solved, and

easily

of flames can, geometrically at

In the para-

flame.

boloidal holophote, A, Fig. 118, all the rays proceeding from

the primary flame are directly parallelised, and sent forward

upon the focus

distant beacon {Ist)

in the flame

;

{2d)

by the lens

whose principal

by the paraboloidal

strips

P and ;

{?>d)hj the spherical mirrors, with the exception of the small

cone P' escape

P'',

which

not intercepted, but

allowed to

backwards through a hole in the mirror, and

finally sent

back again

ellipsoidal holophote B,

to F, after being acted

which F'

upon by the

an auxiliary flame,

162

LIGHTHOUSE ILLUMINATION.

L' a leus having

its

conjugate foci in F' and F, ellipsoidal

a::

Fig. 118.

strips

P' having their

mirror

S'.

All

tlie

foci in

F' and F, and lastly a spherical

rays proceeding from the auxiliary flame

F' are therefore conveyed in a dense cone to F, which once the centre of the muTor

lens L', the principal focus of the lens

common

the conjugate focus of the L, one of the

foci

This apparatus,

common focus of which may be constructed

entirely of glass, can be applied to

any number of auxiliary

to the ellipsoids P',

the paraboloids P.

and

lastly the

flames placed in line behind each other, all the rays

which

will be ultimately parallelised in union

from the original flame

from

with those

was found, in constructing a

catadioptric apparatus of the kind (Fig. 118), that, from the

imperfections in the form of the reflectors (which were of a

somewhat temporary copper),

character,

with the diameter of the rays

and made of very

combined with the small oil

With more

compared

flame which was employed, the

were not so accurately converged

adoption.

light plated

focal distance

justify its

carefully constructed optical agents,

or with the electric light as the illumiuant, the result

would

unc|uestionably be very different.

Another plan

consists

a single

ellipsoidal

mirror

truncated at both ends, with a holophote and a spherical

ELECTEIC ILLUMINATION OF BEACONS. mirror attached, and a small lens having

its

163

conjugate foci

coincident with the foci of the ellipse, as shown in Fig. 119.

Fig. 119.

The

action

of this

instrument

will, after

what has been

described, be ob\dous on inspection.

Illumination by Electricity conveyed through

SUBMAEINE WlKES. 5.

Before adopting the principle of the apparent light

for Stornoway, it occurred to

(Trans, of the

Eoyal Scottish

Fig. 120.

Society of Arts, vol.

iv.,

1854) that

might be possible

illuminate beacons and buoys by electricity, conveyed through

164

LIGHTHOUSE ILLUMINATION.

submarine wires, as shown pictorially in Fig. 120, but I dismissed the plan as inapplicable in the then existing state of electrical science.

In 1865

when

attention

was again directed

to the subject

considering Professor Holmes's magneto-electric light,

and reference

made

plan in Messrs. Stevenson's

this

For such a purpose neither Holmes's

report of that date.^

nor Wilde's light could be employed, as they are produced

by the rapid consumption of carbons, and require the employment of delicate lamp-machinery, which, though to a large extent automatic, involves the constant presence of a light-

keeper in the lantern

so instead of these the electric spark

without carbons was tried in the focus of optical apparatus.

In order

to increase the brightness of the flash, Pihumkorff's

induction-coil

was combined with a Leyden

suggestion of Professor Swan, of the coil on the beacon,

who

jar,

on the

also advised the placing

and the interrupter on the

shore.

submarine cable was next procured by the Commissioners

of Northern Lights.

and Dr.

Mr. Hart designed an improved break,

Wright proposed the employment

Strethill

small coils instead of one large

volume

coil,

two

in order to increase the

of the flash, but the current failed to pass through

the water as required.

In these circumstances, in

Professor

Swan's absence,

Messrs. Stevenson applied to Dr, Siemens of London, the

eminent electrician from

whom the

submarine cable was

got,

and he suggested another and very ingenious, though some-

what complicated, arrangement he thus describes consists of a ^

Report ou

Illumination.

â&#x20AC;&#x201D;

for

producing the

heavy electro-magnet, the tlie

light,

The apparatus upon the beacon coils of

which

buoy

which are per-

Application of the Magneto-Electric Light to Lighthouse

D. and T. Stevenson.

Edinb.,

November

1865.

ELECTEIC ILLUMINATION OF BEACONS.

165

manently connected with the conductmg wire of the cable on the one hand, and with a contact-lever on the other hand,

which contact -lever circuit

manner

of the

The

hammer.

of a Neft's

with the battery (consisting of from ten to twenty

Bunsen's elements) on land

When

by the armature

actuated

electro -magnet in the

completed through the

sea.

the current has had time to excite the electro-magnet

sufficiently for it to attract its

of the latter breaks the

heavy armature, the motion

circuit,

which breakage

accom-

panied with a spark proportionate to the accumulated magnetism, and in some measure also to the capacity of the cable, which, in this apparatus, does assists

the

The luminous

effect.

not destroy but rather

effect

increased

combustion of mercury, which latter

slight

by a

continually

renewed by a circulating pump worked by the armature, by

which arrangement a good and permanent contact

insured,"

This method was tried at Granton Harbour, and the current

was passed

by the result,

easily through the wires,

deflagration of the

and the

light

mercury was very

produced

The

vivid.

however, was otherwise unsatisfactory, owing to the

deposition

of the

products of combustion on the

optical

apparatus, and the existence of some mechanical difficulties

which interfered with

its

This instru-

continuous working.

ment, therefore, which as respected the flash was very

was

still

found inadequate

efficient,

for practical purposes.

back on the

Being thus again thrown

first

plan

employing the induction -spark, the earth terminals were, on the advice of Professor

G-.

Tait, increased in area,

when

the current was at once passed through the cable, and thus Professor Swan's original suggestion out.

the

was successfully

carried

this occasion the battery and break were placed on

eastern

pier of

Granton

;

and the submarine

cable.

166

LIGHTHOUSE ILLUMINATION.

which was upwards Chain Pier coils

of half a mile long,

at Trinity,

extended to the

on which were placed two induction-

with condensers, and a holophotalised reflector to

ceive the spark.

The battery

consisted of sixteen

re-

Bunsen

elements, and two additional cells w^ere used for working the

The

break.

coils

contained each about four miles of wire.

The spark produced was about quarter

an inch in length,

bluish-white in colour, and very striking and characteristic in effect.

the British Association meeting at

Dundee

in 1867,

I stated that " the effect of the light might be also increased

without using additional

the same current could be

cells, if

again utilised so as to generate a second spark in the focus."

This proposal first,

may be

carried out in one of

generating additional sparks in the

using additional coils

;

sparks in the foci of

or, second,

two ways

same focus by

by producing the separate

separate reflectors contained in the

lantern on the beacon, just as in some lighthouses several separate lamps are employed for increasing the

may

effect.

be useful to state that wires of different metals were

tried for electrodes

was found Trials

;

and

that, of all those tested,

bismuth

to give the brightest spark.

were also made with the simple spark afforded by

Wilde's electro-magnetic machine without carbons, and the result

was highly

greater

satisfactory.

The spark possessed much

volume and power than with the Bunsen

cells.

illumination of beacons by gas.

167

Illumination of Beacons and Buoys by Gas.

As mentioned

in the description of the

Stornoway

apparatus in the Transactions of the Eoyal Scottish Society of Arts,

might be

laid

occurred to .

" that

some

cases gas-pipes

so as to illuminate a lantern placed

At Stornoway,

beacon or buoy."

already

on a

stated,

all

these plans were rejected in favour of the apparent light,

from their as

liability to frequent

an insurmountable objection,

of our

extinction,

which I regarded

" at least in the present state

knowledge "

The

Admiral Sheringham,

late

1852, and therefore

not very long after the erection of the Stornoway Beacon light,

not only thought of applying gas to buoys, but

experiment of illummating one near Portsmouth, in

the

description of his interesting experiment "

Admiral Shermgham kindly sent

1853.

process

works on shore

was simply

as follows

the following

窶認rom some convenient

From

say to the low-water line. ;

by means

thence I connected,

laid over the

bank

to the edge of the channel,

the buoy, to about 3 or 4 feet above

provided with the necessary burners, etc. " Copper wires were passed through the tube,

ends connected, at a

;

this tubing

where the buoy was

The tube was

placed for marking the limits of the fairway.

carried over the gas-burner

of a

the one I used I think was about 2 inches

in diameter, and perhaps about 50 or 60 fathoms long

up through

gas-

I laid a series of ordinary iron pipes as far as possible,

union, a gutta percha tube

was

made

it,

i.e.

carried

to the lanthorn

inside of

it,

and

here the wire was broken, and the two

suflBcient distance apart,

by means

of platiimm.

The shore ends of the wire were in communication with a small galvanic battery, which, when in action, heated the platinum to a red heat. The

gas was turned on

instantly produced.

working "

;

by a tap on

shore,

and a most

was quite marvellous

brilliant light

see the effect of its

in one instant the light was put out and relighted. experiments were made, I think, for three or four successive

168

LIGHTHOUSE ILLUMINATION.

nights, at the entrance of Portsmouth Harbour, the gas being supplied

from Mr. Hollingsworth's Assembly Rooms on the beach. The experiments were witnessed by many, and certainly were eminently successful,

and were continued until a drifted foid of

it,

and tore

to pieces."

certainly to be regretted that these interesting ex-

periments, in which

also used electricity,

though not

light

parted from her anchors,

rascally barge

it all

but

heat,

informed

purpose of producing a fact which he

;

not due to any doubt in the success of the

actual

first

Admiral Sheringham

for the

were not prosecuted further

me was

system but owing to

The

will be noticed

difficulties of a legal nature.

employment

of gas for beacons

1861, on the Clyde, near Port-Glasgow, and the

was in

light has ever

about 300 feet

since been regularly maintained.

It is situated

from the shore, but instead

method adopted by Admiral

Sheringham

of the

for insuring the regular exhibition of the light,

the supply of gas during the day

by shutting a valve, and the Wlien the pressure of gas

extent, night.

Glasgow

reduced, to a certain

full

supply

in the

is let

town

reduced in the morning, the sinking of a

on at Port-

float

the beacon shuts off the supply from the principal burner

while a small centre jet

supply derived cubic

feet,

from

which

lighted all day,

night,

burner main.

kept lighted by means of a

From

this source

the float which regulates rises,

of the capacity of ten is

also con-

the small burner

and when the pressure

increased at

the supply to the large

and thus opens the communication with the

Owing

to the fact of the river

quay on the Port-Glasgow is

gasholder

placed on the beacon, and

nected with the main. is

side,

the

fall

running close to the

on the main (which

of copper, and two inches in diameter)

landward from

the beacon, so as to admit of a well for collecting the con-

ILLUMINATION OF BEACONS BY GAS.

169 The

densed water being placed at the shore end.

goes far to prove that, in

been over-estimated

The

for

method

by the waves, and

are used

the extinction of the light

in all cases the accumulation of water in

would however be easy

were noticed from the

to re-light the gas

by means

Admiral Sheringham's plan of heating a platinum

many

There are

had

localities.

If the extinction of the flame

the main. shore

some

1851, the prac-

of illumination

that are likely to attend the illumination

difficvilties

by gas where buoys

my first thoughts in

objections attending this

tical

fact of

having been now for nineteen years in existence

this light

places, however,

wire.

where these objections

when Port-Glasgow, and when

to the use of gas could be easily overcome, especially

the station it is

near the shore, as at

not exposed to a heavy

sea.

Pintsch's Gas-Illuminated Buoy. This system of illuminating buoys promises to form a

most important means of marking dangers by

night,

have hitherto been altogether unprovided

The

minated buoy

inch.

gas and

water

lbs.

fixed light apparatus.

may

In the lantern, which

diameter, the gas-burner

Plate XXIV.), which

above the water.

spring,

plates,

tight,

and

on the square

pipe leads up from the top of the buoy to the

lantern (Fig. feet

of standing a pressure of

which

gas-illu-

an ordinary one, of wrought -iron

about 60 feet capacity. capable

for.

be about

about 8 inches

placed, surrounded

by a small

Pintsch regulator, worked by a

placed on the supply, so as to keep a constant

pressure on the jet of -^-inch of water.

holds four months' supply,

The buoy, which

charged by a boat coming along-

side with a reservoir of the compressed oil -gas,

which

170

LIGHTHOUSE ILLUMINATION.

made from

The

sliale.

cost of the light

stated to be from

3d. to 6d. per twenty-four hours.

Automatic Meter for producing Intcrinittent Lights hy of the Gas.

flow

â&#x20AC;&#x201D; As

the

regards the difficult problem of distin-

guishing one buoy from another,

it is

be feared that

it is

not very practicable, and certainly far from desirable, to apply

clockwork for producing rotation of the apparatus occurred to

intermittent action of

make

the

and

;

flow of the gas produce automatic

by means

Such a form

of a dry meter.

meter must always pass a

sufficient

secure the constant burning of a small

jet,

quantity of gas to

situate either

imme-

diately above or in the socket of a larger burner provided

with a separate tube, for giving at regular intervals an increased supply which goes to the ignited

by the small

jet.

The

main burner, and

full

there

flame continues to burn

until the action of the meter cuts off the larger supply

the small jet of course go is

again left burning alone.

and

This process will

on continuously so long as the gas in the buoy

not exhausted. I applied to Messrs. Milne, the well-known gas-engineers

of Edinburgh, for their assistance in the matter

have,

answer perfectly

all

the requirements.

ducing an intermittent

and they

was tested continuously

effect

twenty-eight hours, which to the

;

made it to This mode of pro-

altering the valves in a dry meter,

for

sufficiently proves its applicability

production of intermittent light at ordinary light-

houses placed on the land, or at beacons, where a gasholder

can be used.

whether

Actual experience

which are so much If colour be

may

desirable in order to

show

be equally applicable to floating lighted buoys, affected

by the motion

of the waves.

adopted as an element of distinction, there

be obtained, by placing a lantern with coloured panes

ILLUMINATION OF BUOYS BY GAS.

above another which teristics

l7l

uncoloured, the following charac-

Fixed Eed and Green

If,

„

Eed and Wliite;

„

Green and White.

again, only one lantern be used, the following dis-

tinctions

may

be produced by the automatic meter

Eevolving or Intermittent White Intermittent

Eed

„

„ Intermittent Green.

;

Crymhals for preserving the Verticality of

Apparatus.

—Another

difficulty

Buoy Light

which has been found

interfere with the proper exhibition of the light

buoys

on floating

the inclination of the buoy itself to the horizon,

when

acted on by strong winds and currents; and Mr. Cunningham, C.E., of

Dundee, has designed a form

pected to preserve

and

sea.

is to

much

its verticality

simpler

mode

buoy which

in all states of the

ex-

wind

of obviating the objection

adopt gymbals, as employed in the Azimuthal Condens-

ing Apparatus used in steamers' lights already described (Chap. III. 10).

The gas may be very simply conveyed

the burner by making the journals on which the lamp and

apparatus swing hollow, so as to afford a passage for the gas.

Unifoem System of Beacons and Buoys. The

earliest proposal to

adopt a local buoyage system

appears to have been that by Mr.

Stevenson in his

Eeport on the Improvement of the Forth Navigation in

1828, where black and red buoys and beacons were recom-

mended is

for the different sides of the channel.

thus described in the report

of river pilots, buoys

—

This system

For the use and guidance

and perches or beacons are likewise

172

LIGHTHOUSE ILLUMINATION.

intended to be placed in the positions shown on the plan, those coloured red are to be taken on the starboard, and

those coloured black on the larboard side in going

up the

This system has since been adopted by the General

river."

Lighthouse Boards in the United Kingdom.

In 1857 Admiral E. system to be extended to this

was introduced

into

Bedford suggested a uniform the coasts of the country, and

all

Scotland by Mr. A. Cuningham,

Secretary to the Lighthouse Board, and Trinity for

House

the English

mode than any buoy

in use.

In 1875

coasts.

The

suggested a simpler

of these, but afterwards found

previously proposed a

is still

adopted a uniform but different system

also

(in

shall indicate

1859) by Mr.

its

J. F.

had been

Campbell, "that

shape and colour the compass

direction as well as the existence of a danger."

which I suggested

for carrying out

The mode

Mr. Campbell's system,

which has since been adopted by the Norwegian Government, was the following

Black Can Buoy will indicate dangers lying to the '

westward of the buoy.

Red Nun Buoy

will indicate dangers to the eastivard

of the buoy.

A Horizontally Striped Black and on

its top,

A on

Horizontally Strijjed

the top,

From

Bed and White Buoy, with a

cross

will indicate dangers to the northward of the buoy.

these

simple sailing

characteristics

directions

sail to the westiuard

red

White Buoy, with a globe

will indicate dangers to the southioard of the buoy.

nun buoy ;

are

the

following exceedingly

obviously derived

of a black can buoy ;

to the

not

eastward of a

southivard of a horizontally striped black

and white buoy surmounted by a ball ; or to the northward of a and white buoy surmounted by a cross.

horizontally striped red

173

courtenay's signal buoy.

Courtenay's Automatic Signal Buoy.

XXIX. shows

Plate

low cylinder

that can occur in the locality where the buoy

thus the surface level of the water within

buoy

ary, while the

passing waves.

with

;

be moored

A remains

attached rises and

rising

and

a hol-

any wave

have therefore the conditions

immovable column encompassed by a velope

the Courtenay Buoy.

of a length greater than the height of

;

stationfalls

of a fixed

falling en-

we have a moving cylinder and a by which we can compress air by wave power.

in other words,

fixed piston,

The tube

powerful whistle

extends to the top of the buoy, where a

N is

placed.

a diaphragm in A, be-

tween which and the plate on the top of the buoy are

two tubes G, open above, and having ties ball valves F, as

shown.

from the diaphragm to the whistle. to be carried

above the

at their lower extremi-

central open tube

of water in A, to the

level, that is the level of the

summit

leads

Suppose the apparatus

from a position where the diaphragm

mean water

H E

just

column

of a wave, then the space be-

and the top of the enclosed water column will have been greatly enlarged, and air must have been drawn in through the tubes G to fill it now, as the instrument descends

tween

;

to the trough of the wave, the

diaphragm must descend on the

top of the water-piston, and the air compressed, being prevented by the ball valves F from escaping through the tubes G, is driven out through the central tube H, and so sounds the whistle N. It is obvious that

water must produce

must do

any disturbance of the surface this

as well as short

effect.

of the

Long low ground-swells

chopping waves

;

but of course the

higher the wave the longer the duration of the sound.

The

174

LIGHTHOUSE ILLUMINATION.

buoy may be usefully employed wherever an undulation of 12 inches

exists.

The buoy

by a

fastened

suitable anchor

attached to the mooring-shackle B, and

whirling motion by the rudder C.

and chain

kept from any

To guard against leakage,

the tube I provides a passage from the space beneath the

diaphragm pressed air

E is

main

that enclosed in the

This tube

float,

maximum

the pressure in the latter equals the

below the diaphragm.

Com-

shell.

forced through this tube into the

until

pressure

provided with a check-

valve.

Courtenay buoy, and also a common bell buoy (beU

weighing 6

cwt.),

water, about

were moored, in 1879, in

1^ mile

the action of both buoys,

when

after

is "

much

quicker than the bell buoy."

buoy

of 5 miles.

General Eemarks.

The

3 7 days.

always the loudest

and most continuous, attracting the attention of the that the Courtenay

may

also be

listener

remarked

sometimes heard at a distance

â&#x20AC;&#x201D;The

experiments that have already

been made on the application of electricity, and the

facts

which

have been adduced regarding the present emplo}Tnent of

seem

four

was heard on 108

buoy was only heard on

keeper adds that the Courtenay buoy

observations on

was found,

that the Courtenay buoy

days, while the bell

The keeper

make

Inchkeith Lighthouse was directed to

trial,

fathoms of

off Inchkeith, in the Firth of Forth, in

order that they might be tested as signals.

months'

V-g-

gas,

to show, so far as they go, the feasibility of the suhmarine

conduction of other light-producing agency, and afford good

hopes that by such means, and that of the Apparent Light, and Pintsch's gas buoy, an important system of coast-illumination

may ultimately be

introduced.

The time

perhaps not far dis-

DIFFERENT PLANS FOR BUOYS.

l75

tant wlien the beacons and buoys in such a navigation as the

entrance into Liverpool

may be lit up by submarine

from a central station on either shore,

wliile the

conduction

whole manage-

ment may be trusted to the charge of one or two light-keepers. The fact of a beacon having been illimiinated for twentynine years on the Apparent Light principle at Stornoway, and

another for nineteen years by gas conveyed under water at the Clyde, and the success which has attended the trials of Pintsch's illuminated buoys,

seem

ther and

practice.

The encouragements

them more

to bring

sufficient to justify a far-

general application of these principles

distinctly

to proceed are

many

;

and,

under view, I shall not only

briefly recapitulate the various methods already described,

and which

afford

very considerable scope

for

choice

applying them to the varying peculiarities of different ties,

locali-

but will also adduce methods for the employment of

sound, and other plans which have been elsewhere proposed.

Different Sources of Illumincdions for Beacons 1st,

The adoption

2d,

The use

and Buoys.

of apparent lights.

of dipping lights for indicating the position

of shoals, so as to cover with the light the ground that

dangerous. ?)d.

The conduction

electricity, or that

either of voltaic, magnetic, or frictional

produced by the efflux of steam, through

wires, either submarine, or air,

where practicable suspended in the

so as to produce a spark either with or without vacuimi

tubes, or

by means

of an electro-magnet

and the deflagration

of mercury. 4:th,

pipes.

The conduction

of gas

from the shore in submarine

LIGHTHOUSE ILLUMINATION.

176

bth, Self-acting electrical apparatus, produced by the action

of sea-water or otherwise at the beacon itself, so as to require

no connection with the shore.

This method, which I sug-

gested for beacons in the Society of Arts' Transactions

1866, derives additional importance from the Mr. A. Bain, the eminent

late

1848 he produced a good

electrician,

light

informed

by using

for

fact that the

that

sea- water at

Brighton. Qth,

The illumination

of buoys

vented and introduced by Mr. II.

by compressed

gas, in-

Pintscli.

Different Applications of

Sound for Warnings

during Fogs. 1st,

The propagation of sound during

communicating with the the beacon or or

buoy

fogs through pipes

shore,^ or the origination of

itself,

sound at

by compressing a column

air,

by acting on a column of water contained in the pipes. Mr. Wilde of Manches2 c?, Bells rung by electricity. to

ter,

whom

kindly informs

applied for his advice on

diameter, placed on

10 miles from the minute by means

this

proposal,

that, in his opinion, bells 1 2 or 1 8 inches

different

beacons, and as

shore, could be tolled a

of a

3-|-

far

off

hundred times a

4 inch electro-magnetic machine,

worked by an engine of about two horse-power. Mr. Mackintosh

of Liverpool

suggested, in 1860, that

fog warnings might be made by a tide-mill, or a stream of

land-water, for sending a current of air to

engine so as to ring a

may

work a pnermaatic

Bells or whistles at a beacon

be also sounded, either by those means or by a hydrau-

ram connected with a

lic

bell.

pipe, or

In one of the Paris water-pipes, 3120

by the

feet long,

rise

and

fall of

M. Biot was able

a conversation, in a very low tone, with a person at the other end.

the keep

DIFFERENT PLANS FOR BUOYS. tide lifting a weight

on the

machinery on the beacon. also

shore, connected

177 with a train of

The same moving power might

work an electro-magnetic,

machine on

or other electric

the shore. Sd,

The

tolling of bells

the tide, as proposed in

1810 by Mr.

E. Stevenson for the

Professor Fleeming Jenkiu has lately suggested

Carr Rock. to

by the hydrostatic pressure

the use of

this

method

for

working the contact-

breaker for an electric current at the beacon, instead of on the shore, as in the experiments which have hitherto been tried.

Which

of these

many ways may

be judged the best will

depend on experience and the nature of each

locality.

science of beacon illumination and identification

we cannot limit its application. In Plate XXVIIL, Figs. 1, 2, 3, 4, and

The

yet in

its

infancy, and

buoys

in general use

on the coasts of Scotland

spar buoy used in the United States,

Fig. 7

show the

5, ;

Fig. 6, a

shows a pro-

posal for reducing the force of the waves upon the buoy in

very exposed situations, where one of the ordinary construction

would not be

ride

likely to

XXX. and XXXI. show

during storms.

Plates

malleable -iron and cast-iron

beacon, originally used on the coast of Scotland.

CHAPTEE

VI.

ELECTRIC LIGHT. The

history of the invention of the Electric Light and the

modes

producing

are very fully given in Mr. Douglass's

valuable paper in the Minutes of Proceedings of the Institution of Civil Engineers, vol.

Ivii. p.

77, from wliich are taken

the following facts relating to the introduction of this most

important luminary

The arily

electric light, as

employed in lighthouses,

due to Faraday, who discovered that

prim-

a coil of wire

were brought near to or drawn away from either of the poles of a permanent magnet, a current of electricity

The

first

was induced.

magneto-electric machine capable of generating a

sufficient current for lighthouse

purposes was made by Pro-

tried by the Trinity House at 1858 they first showed the elecIn 1847 Staite devised a lamp in tric light to the sailor. which the upper carbon was made to approach the lower carbon by a current of electricity acting on an electro-mag-

fessor

Holmes

London

in 1853,

1857; and

and

net combined with clockwork

produced the

first

;

and Serrin appears

have

complete automatic lamp for alternating

The Trinity House subsequently introduced the currents. electric light at Dungeness in 1862, and at Souter in 1871. The

optical apparatus for these lights

Mr. James

was designed by

Chance, and constructed by Messrs. Chance

l79

ELECTEIC LIGHT. of Birmingham.

1867

(but

it is

Siemens and Wheatstone discovered in stated that Mr. Varley

had made the same

discovery in 1866), that powerful electric currents could be

generated without a permanent magnet structed, in

1869,

machine on

this

and Holmes con-

Trinity House, a dynamo-electric

for the

new

;

principle, which, according to photo-

metric measurements by Dr. Tyndall and Mr, Douglass, pro-

duced a light of about 2800 candles.

The power South Foreland

of the apparatus at the high tower at the is

estimated at 152,000 candles, or twenty

times that of the old dioptric

by Dr. Tyndall

dynamo-electric machines following Table

oil light.

Trials

were made

at the Lizard of various magneto-electric

;

and the

and

results are given in the

180

LIGHTHOUSE ILLUMINATION.

181

ELECTRIC LIGHT. It is a fact of great significance that,

House, the cost per unit of light produced has

of the Trinity

been reduced to 1-2 0th of what

The French introduced the

Heve

owing to the progres-

have taken place under the auspices

sive imj)rovements that

at Dungeness.

was

light

electric

in 1863, and at Cape Grisnez in 1869.

apparatus for

these

Sautter, Lemonnier, results

with the

lights

Cape La

The

optical

was manufactured by Messrs.

and Company

Gramme

of Paris.

Sautter's

dynamo-electric machine, as reduced

to the usual standard of 9-5 candle

by Mr. Douglass

for a Carcel burner,

show the

per horse-power for a

2275

intensity to be

power

candles

horse-power machine, and 3221

2-|-

candles per horse-power for a 1 3 horse-power machine. effectiveness of this

than that of the

kind of apparatus

Gramme and

The

therefore greater

Siemens's machines at the

South Foreland. In 1866, Mr. Henry Wilde furnished

for

the

Commis-

sioners of ISTorthern Lightliouses one of his electro-magnetic

machines

for

producing the electric

light,

of experiments, about to be referred

Wilde kindly furnished me of this invention.

discovery that,

with which a series

to,

was made.

Mr.

with a statement as to the origin

says

1864

that "in

made the

electro-magnet was excited by the

when an

current from a magneto-electric machine, the magnetism of

the electro-magnet was series of steel

also

much

greater than that of the entire

magnets employed

to generate the current.

found that when an armature was

poles of the electro-magnet a current of

was produced from magneto-electric

this

machme.

rotated before the

much

greater energy

armature than from that of the

Expressed generally, this

covery consists in developing from magnets and

dis-

currents

indefinitely weak, magnets and currents indefinitely strong.

182

LIGHTHOUSE ILLUMINATION. "In the course of some experiments with a pair of

magnetic machines for producing electric discovery that

when two or more machines

are

about the same nominal speed, the combined

of belts at

them

currents so control the armatures as to cause as synchronously as

electro-

made the driven by means

light, I

to rotate

they were mechanically geared together.

perfect synchronism of the rotations of the armatures

when

absolutely essential

machines current

required, this electro-mechanical property of the

an important one, as

the combined current of several

allows of the machines

being arranged without reference to their respective positions, or to their distances from each other."

The Merits of

the Electric Light.

â&#x20AC;&#x201D; From

the frequent

occurrence of fogs on the coasts of this country, and the

suddenness with which they come on,

importance that

power

all lights

But there

as possible.

others

marked by

obviously of

are certain headlands which,

from their commanding geographical than

should be of as great penetrative

position, require

lights

higher power.

Although such sea-stations could, without doubt, be lighted

up with

oil so as to

radiant than an all

oil

be made of great efficiency, yet flame can be got

from

its

a better

ought to be applied at

those great salient points of the coast.

electric light,

Now

the magneto-

remarkable brilliance and splendour,

But the mere

presents the strongest claims.

fact that a

Kght

has been discovered of far greater intensity than others, though

important and encouraging, inasmuch as hopeful anticipations for the future, to

warrant

essential

its

that

it justifies

not in

immediate general adoption. its

application

should be fully tested. guiding the mariner,

the most

itself sufficient

It is therefore

lighthouse

illumination

There are certain requirements all of

which must be

electric as in every other light.

fulfilled

for

in the

ELECTRIC LIGHT.

That

1st,

shall

183

be constantly in sight during those

periods of time at which

advertised to the mariner as

it is

being visible. 2d, That

shall be seen in a thick

and hazy atmosphere

at the greatest possible distance.

Sd,

That

shall

shall never be 4:th,

mamtain the

constantly

character of the station where

mistaken for any other

That when the light

remain long enough in view

light.

revolving

distinctive

employed, so that

its flashes

shall

to enable the sailor to take the

compass bearing of the lighthouse from the ship. In considering how far these conditions are

fulfilled

the electric light our attention must be directed to what at first

sight appears to be its peculiar advantage, viz. its great

power.

all

near observers

are most striking and is still

its

But

and intensity has been and

asserted, that at great distances the oil light maintains

power more

certainly,

fully than the electric.

seems to be very improbable

present state of our knowledge tion,

brilliance

its

undeniable.

because

from the

may

not one for dogmatic asser-

be the case that the rays proceeding

light

electric

;

Such a phenomenon, yet the subject in the

suffer so

much

greater loss from

absorption in passing through an obstructive those

from an

initial

power,

oil

may

flame, that

after all

at great distances.

the

medium than

Kghts, if of equal

oil

be the more powerful of the two

If this were really so,

would foUow

that the application of electricity to lighthouse illumination

was based upon a

fallacy.

The mere

glare

or splendour

of effect to a near observer, so far from being an advantage to the mariner, is a positive evil,

mitted to only because

paniment

it is

and an

evil

which

sub-

believed to be a necessary accom-

of the property of visibility at great distances.

184

LIGHTHOUSE ILLUMINATION. All that the mariner can in any case require

Anything short

visibility.

than this

of this is useless

really mischievous, because

to destroy his

;

its

distinct

anything more lustre

tends

powers of perception of dangers in the water

that are nearer his view, and which, therefore, from their

proximity, threaten more immediate peril to the safety of

The really useful poioer

his vessel. tion through

measure of tlu usefulness of any it

remains distinctly

then, is that

medium, and,

obstructing

visible,

of 'penetratrue

therefore, the

light is the distance at ivhich

and

preserves its characteristic

appearance.

The few

made

obser\^ations

Edinburgh seem rather

to lead to the belief that the electric light

amount light

but, as the original

;

suffers a

gi-eater

of loss in penetrating the atmosphere than the oil

nevertheless, be visible at

power

much

vastly greater,

it will,

greater distances than

oil light.

The other

qualities

which have been

specified are the

uninterrupted exhibition of the light and the preservation of its

There can be no doubt that the

distinctive character.

both at Dungeness and Havre, especially

electric lights,

\dewed at a distance, are

liable to

when

very great fluctuations in

volume and

intensity.

likely, if of

no greater duration than what was noticed, to

lead

any doubt

Cape

Heve

Although such fluctuations are not

the

lighthouses, yet

identity of it

Dungeness

would undoubtedly be of

importance that the light should be rendered more certain in its is

exhibition,

and

less liable to variations in its

known

that

all lights are liable to

well

states of the atmosphere, but the cause

which produces the

flickering

and momentary extinction of the

though in

many

instances arising from

power.

twinkling in certain

electric

light,

imperfections in the

ELECTRIC LIGHT. quality of the carbons and from action in the

mechanism

to a large extent

want

185 of proper

of the lamp,

due to the variation

compensative

also,

undoubtedly,

in the positions of the

carbon points, and the consequent vertical variation of the plane of maximima intensity of the flame.

This exfocal

must necessarily remove the beam of rays from the horizon and throw it either too near

position of the flame parallel tlie

lighthouse or upwards to the sky.

While electric

it is

lamp

not to be disputed that the perfection of the

so as to ensure that the flame shall

remain in

a given constant position

be solved, and

is the great problem which has to which Foucault, Duboscq, Holmes, and

have devoted much labour and ingenuity, there

Serrin,

nevertheless, other

working the lamp

means by which any

may

to a great extent

irregularities

be obviated.

are,

important recommendation of the electric

light,

according to

Professor Holmes, was that the apparatus

may

be held in a

man's

hat.

But the

electric light loses

one of

its

greatest

advantages by the employment of too small a size of optical apparatus, which, though less costly,

otherwise unsuitable,

because every unit of surface in an apparatus of small size operates on a pencil of light of a greater angle of divergence

than

would in larger apparatus.

Hence, in the small

apparatus, the grinding of the surfaces and fitting of the glass

work become

of the greatest importance,

executed in the most careful

manner.

and must be

Moreover, slight

deviations from exact focusing will, with the small apparatus,

produce greater deflections of the beam of parallel rays, and therefore those vertical displacements of the electric light

which are constantly occurring, owing

to the imperfection of

the regulating machinery, will be especially injurious, because the angular displacement of the

beam

of emergent rays from

186

LIGHTHOUSE ILLUMINATION.

the proper direction, for a given linear displacement of the

carbon points, will vary inversely in the simple ratio of the

In addition to

linear dimensions. as

disadvantage

further

apparatus, that for

this, it

attending

amount

a sufficient

the proper illumination

be remarked

use

small

of divergence be obtained

the

may

the

ocean, this

condition

upwards upon

necessarily involves a corresponding divergence

From

the sky, so that a loss of light inevitably occurs.

what has been

stated

will be seen that the emioloyment of

small apparatus, such as was first adopted in England and France, for the purpose of producing vertical divergence upon the sea, is

a retrograde movement.

In order

to take

f idl

of the electric spark,

used of such a

size as

practically sj^eaking

apparatus of the

advantage of the valuable properties necessary that apparatus should be

it is

would,

made

no divergence

For

all.

order should, as

thii'd

by Messrs. Stevenson

of the ordinary form, give at

in their Eeport of

this

purpose

recommended

first

27 th November

1 8 6 5, to the ISTorthern Lighthouses, be adopted in preference to

the smaller orders.

Mr.

Chance

that

states

also

suggested the same -vdew to the Trinity House in 1862.

For fixed

lights the fourth order should for similar reasons

be substituted for the sixth order.

made

But the

light proceed-

ing from such apparatus,

of course be useless to

the mariner, inasmuch

of the usual form, as,

would in the

case of a fixed light, no rays would be \dsible, excepting at or near the horizon

apparatus,

the

beam

;

so in the case

which there would

the

of a revolving

further evil of

of light sweeping past the eye of

so quickly as to prevent to the lighthouse.

optical

and

the mariner

him from taking compass-bearings

What

required then

to give

the

apparatus such a form as to produce the amount

ELECTRIC LIGHT. of horizontal divergence wliicli bearings, and also such an

187

needed

amount and

for taking

compass-

direction of vertical

divergence as will illuminate the sea from the horizon to the shore,

and not waste any of the rays by illuminating the

above

arc

the

horizon.

an arrangement alone

such

will the mariner be enabled to reap the full benefit of the

new radiant. In the recently erected lights of the Trinity House this has been most skilfully provided for by Mr, Chance (Plate XXXII.

peculiar and valuable properties of

Kg,

The only cause

4),

tliis

he should have

for regret is that

adopted double agents from their being more easily constructed, but there is

no one more able

overcome the

difficulties of construction of single-acting differential

appa-

ratus than himself.

Experiments with the Electric Light.

—

Three

different

kinds of annular lens were tried at Edinburgh with Wilde's

magneto -electric

light

1866.

These instruments had

each a focal distance of 500 millimetres, and were constructed of such forms as to produce different amounts of divergence.

The experiments were thus made

not only the

trial of different

determination of

to include

forms of agent but also the

the amounts

and

of horizontal

vertical

divergence that are best suited for this radiant.

These instruments were,

1st,

plano-convex lens of the

usual construction for revolvinsj ^ certain modification lights; 2d, of the differential scription ^

lens,

which

the

de-

was given "

rephes to the scientific

queries issued

Differential Lens. ,.

•-;^3^;^»^^^

^^^^^/v Hqri-zontql

':::.'-'.''-'--'-H

by the Eoyal Como-

mission on Lighthouses in 1860.

The

-r,.„„

principle of this instrument

shown

in Fig.

•

12 1,

in hori-

188

LIGHTHOUSE ILLUMINATION.

zoiital

and

vertical section.

profile as Fresnel's

outer face has the same

Its

annular lens, but

so as to give different horizontal

inner face

its

and

ground

vertical divergence.

the lens which was used in these experiments, however,

judged better, in order to vary the

results,

was

to confine the

curvature to the horizontal plane, so as to give a divergence in

azimuth of

XIX.)

(Plate

2-^°

Zd,

;

suggested by Mr. Brebner, C.E., which

The form

of lens

was divided

halves (Plate XIX.), and the rings of which

into

their con-

struction and exfocal position, produced a divergence of 6°

in azimuth, and

3° in altitude

;

or,

in other words, it

calculated to give the same horizontal

with the

common

as the

electric light

and

was

divergence

vei^tical

lens gives effectively

with

oil light.

Experiments were successively made with these three different instruments, for results

—Wlien

with the following somewhat unlooked-

the frame revolved so as to give a flash

every minute, the ordinary plano-convex lens was found, as

had been expected, in

to be

divergence

horizontal

wholly unsuitable. rendered

The

deficiency

impossible

take

a bearing to the light at the distance of 2^- miles, where the

were placed

observers

;

while the

defect in vertical

divergence was so great that the light was visible only for a very small

distance

above and below the focal plane.

The horizontal divergence that had been given differential lens

being somewhat

first-order revolving oil light,

The the

vertical divergence

common

lens.

was

divergence as a first-order

regards

was found scarcely

power,

lens

the

than half of that of a

of course

The double

to give in both planes about

less

sufficient.

insufficient as

was found,

with

as designed,

the same amount of effective

oil light.

the

common plano-convex

lens

ELECTRIC LIGHT.

proved of course

somewhat very

less

much

the most powerful, the differential was

hut the douhle lens was not only

effective,

inferior in power,

liarity of the electric flash it

189

hut the distinguishing pecu-

was

so far lost as to

in a great degree to an oil light.

of the instruments, as

shown hy the

photometer, were as under

assimilate

The penetrative powers liquid ink-and-water

.......

Plano-convex rays

lens, constructed to

give parallel

Differential lens, constructed to give

=1-00

diver-

2-J-°

gence in azimuth, and parallel rays in the vertical plane

Douhle

lens, constructed

in the horizontal,

The flash

results

the

apparatus, as to the

seem

electric

not so

-90

vertical plane =:

"75

6° divergence

to give

and 3° in the

show that the highly

light,

much due

when

amount

more complete parallelism of the rays

the smallness of the radiant.

acted

to a greater

distinctive optical

of Hght

arising

from

Apparatus of a small

size,

which necessarily produces a wasteful

vertical divergence,

should therefore be altogether discarded.

The general

result of these investigations

is.

First,

that

the apparatus for revolving lights should be not less than

the third order of 500"™- focal distance; and, Second, that

the annular lens should be either differential or of the double form.

The

differential

may be

either curved in both planes,

or if only in the horizontal, then

the carbons

should be

placed exfocally, as in the double lens.

There has lately been of

the

much

discussion at the meetings

Institution of Civil Engineers as

penetrating powers of the electric and different views

oil

have been expressed on the

to the lights,

relative

and very

subject.

But

LIGHTHOUSE ILLUxMIXATIOX.

190 appears to

fruitless.

The only mode

these discussions have been practically

tliat

of settling the question is to

make

observations at different distances on electric and other

lis^hts

The lights being equalised by observa-

of equal initial power.

tions near at hand, observations at greater distances

show whether experiment

disappeared

separately

distance,

they

of penetrability

at different

would

tried, it

together

same

the

Were

distances.

at once settle the

would this

vexed question

but unless such experiments should prove

;

that the electric light

very defective in penetrating power,

which, from the practical success which has abeady attended its

introduction seems very improbable,

immeasurably better

fitted

than any other radiant, from apparatus.

Petit,

The cliief

actual

oil lights

reaches

results

its

Dunkerque, and

Dr. Tyndall, are, that " the first reaches

75 times out

range

only

electric light,

100, while the

times out of superior

100."

power of the

do not of course prove any

superiority in specific penetrative power, as the light emitted

Antwerp,

compared with the

Calais,

But. these facts, while proving the

South Foreland

illumination

ascertained

service

electric light, as

range

geographical

second

must be regarded

small divergence in optical

North Foreland,

of the

Ostend, as stated by its

its

practical

of the Hydrographic

on the South Foreland

for lighthouse

amounts of

from the different stations are not

initially

equal.

The following ness," has

clearly

table

what he

calls "focal

compact-

been computed by Mr. Douglass, and shows very

how much

the electric surpasses

in the ratio of intensity to area of flame

all :

other illuminants

ELECTRIC LIGHT.

compaeative focal compactness of lighthouse Luminaries.

191

CHAPTEE

VII.

STATISTICS OF LIGHTHOUSE APPARATUS. (Extracted from M. AUard's M^moire sur I'Intensite

Phares

The

following

Cliapter

la Portee des

Paris, 1876.)

contains

the

principal

results

the very valuable researches of M. E. Allard, by whose kind

permission

has been compiled.

Dimensions of Wichs and Burners,

Order of Apparatus.

etc.

LIGHTHOUSE STATISTICS.

Volume of Flame in

193

Consumption of Oil for the

relation to

different Burners.

u denote the volume

per gi-amme of

oil

of flame in cubic centimetres

consumed

an hour, d the diameter of

the burner in centimetres, then

^^= 0-2917^ Â°'^^-

Imminous

carcel burner

Intensities.

consuming 40 grammes of colza

per hour being taken as unity,

mineral

oil

I denote the intensity for

in a burner of diameter d, then I

= 0-22cZ2-^-

The Transparency of Flames. Considering a horizontal cylinder of flame of length

the flame were perfectly transparent the intensity of light

would be value

directly proportional to

a, of

but in reality for any

the coefficient of transparency there

intensity

which may be called the intensity

Now

â&#x20AC;˘^ a

for

less

= '8

= '7

the value

than the limiting intensity

the corresponding value of

appears that an intensity differing

a limiting

for

:= co

give an intensity only is

2 '5 8 inches;

is 4*

little

and

for

13 inches: thus

from the limiting

intensity can be obtained from a flame of no great thickness.

Coefficient

The

fraction of the

through a sheet inch)

thick =

-|.

of Transparency.

whole incident light which passes

of flame

one centimetre (about

-|-

LIGHTHOUSE ILLUMINATION.

194

Consumptio7i of Oil in relation

Number burner

.... wicks

the

Intensity of Flame.

LIGHTHOUSE STATISTICS. Total loss due to these three causes for fixed lights

1st

Order

(

Upper rings

Drum

(

Lower rings

195 :

196

LIGHTHOUSE ILLUMINATION.

are in the ratios 2, 7,

and

where 10

that of the whole

apparatus. Coefficients

of the Different Fixed Light A'p'paratns.

These are the is

ratios in

which the intensity of the lamp

increased by the apparatus.

cient,

/ the

focal distance,

of flame, they can be calculated

Where

from the formula

-,(f\ (^J^orÂť = 212(/f

Order of apparatus

the coeffi-

d the diameter, and h the height

LIGHTHOUSE STATISTICS.

197

Tlie accuracy of the observations in the horizontal focal

plane

may

be tested by adding the intensities obtained from snm by 43Â° 'S the angle

degree to degree and dividing their of the lens,

when

the quotient ought to be the same as the

axial intensity found for the

drum

The sum

order fixed light.

of the corresponding 1st

32910, and

this divided

4 "3 gives 760 (see observed axial intensities, Hence a relation can be established between the

195).

intensity

of a fixed light and that of the corresponding flashing light.

For

a straight line through the focus be taken as axis of

abscissoe,

and from

drawn representing the

ordinates be

intensities in the focal plane, the curve obtained

joining

the extremities of these ordinates approximates to a parabola.

Hence

denote the axial intensity of the

distance from focus to apex, y

flash, or

the

the intensity at another point

in the focal plane situated at x degrees from the axis, a the

we have

horizontal semi-divergence,

The sum

of the intensities or the quantity of light corre-

sponding will be represented by

which = ^

the relation

Aoc.

the fixed light, and

tlie

the other hand, </>

surface of this parabola, if

a be the intensity of

the angle of the lens, the quantity of

Light in this angle will be

(f)a,

and

must equal that which

the lens concentrates into the angle of horizontal divergence

hence

^ Aei = (ba, and

annular lens

Thus the intensity

obtained by multiplying that of the corre-

sponding fixed Kght by

by the annular

A = ÂŤ -^.

lens,

where

<p is

the angle subtended

and ÂŤ the horizontal semi-divergence.

198

LIGHTHOUSE ILLUMINATION.

o 2

^ J= .d ^ *^ ^ Sii

[ii

•^5 °"

O -* -* O C0_0 05 oo OS t^

-r-

»^ (M <N CO Oi CT) -*^i-<_0 CO «o'->^co"c4"

1^ CO CO O 00 oo CO 1^ O <N 1^ O CO

CO lO

O ^ OJ CO 00 t^ 1^

lO CO (N (N

CO CO t^ »o Oi -* (M Ol 00 CO -*

00 t^ OS CO I— 00 T— CO (N OS in 00 lO CO CN (

i-H

H M

CO * OS cc

-^ Pi Ph

O m

l« CO OS in CO CO

<>f od'io'co'

05 1~» rH CO CO oo r-l VO >0 iC 50 t^ kC CO

l-H

r-l

O ^

00 O CO 1— ^

OS i^

t^i^

osco-<tieo

i-it>.mco

OS T-H

.-H

i-H

VO r-( OS 00 OS I-H CO (N

-* CO cs oo CO CO OS

CO CO o^

<N t~-^ 00 OO CO OS CO CO

t—

CO 00

lO 0» 00

t^ 00 c^ 00 oo

00 1-1 >0 CO (N <N 1-^

«o CO CO (N OS lO i-H

'^ CO (M vn (M CO in 0> »0 CO (N rH

m CO OO (M oo o iM o O oo CO I— CO OO CN CO OO CO oo CO o m Ci ^ m CO -* (M tN (N I— T-H

r-l

^!t<

I-H

O (M o

t^ CO CO CO

r-l 1-1

CO ^J< -^ OS

lO t-~ >o CO 00 CO lO

'^ Om

t>.

H-1

^ s

1-1

w P4

-e-1

i5 1—1

eo-*

oom-^co ocooeo cous

000>*<M OStO-*CO (MCO

t>.-*CO<M

oco-*co (Mt-^meo

*-<)<

osm-*co r-it^m-* -*m

eoc^osip

wip

coipixios (NI>.0>i5 '^(MtNi-l

OSCO-*CO

(N-»H

^ o,

O o

1—

eo!M05>p °C^

OOOCO iJOOOOCO COM

->}i(MC^>-l

-<*(M<Mi-l

<*'i-(

ootocoos (f^l^Olis

oooi (NlH

-"^itNC^Ii-l

-Jlr-I

199

LIGHTHOUSE STATISTICS.

K) Oi <n

asi>.kO-^co

<MOit--.ur5-*

t^va

loco

CO Oi <p

200

LIGHTHOUSE ILLUMINATION. Dioptric Sjplierical MiiTor.

The

intensity

apparatus

from an ordinary fixed light

light

increased 38 per cent in the angle correspond-

ing to the miri'or.

Drum.

Increase of the Height of the

angle of 38째 instead of 30째 above the axis should

be subtended by the dioptric lens.

advantageous

In one respect

the angle farther

increase

absorption of light due to thickness of glass rings than in the lenses

sary to

take

account

the

due

effects

are produced at the entrance

lens,

and

would

for

the

greater in the

on the other hand,

but,

In the elements of the

dispersion.

which

into

;

it is

neces-

coloured

the dispersions

exit of the limiinous

ray are added because they are in the same direction, and their

sum

increases with the distance of the horizontal axis

the last elements of these lenses give very marked colorations,

the

which cause a small

catadioptric

rings,

no dispersion

reflection causes

and site

exit produce directions,

colour.

Hence

to increase

;

that

may

hand, the

internal

the refractions at entrance

emerging rays

in this respect it

much

luminous intensity.

other

two dispersions nearly

equal,

and in oppo-

have no sensible

would be disadvantageous

the vertical amplitude of the dioptric lens.

Taking into account at most 40째,

loss of

on the

all

the asj)ects of the question, 38째, or

be taken as a reasonable

limit.

Obstruction of Light hy the Burner.

In order to diminish the obstruction of luminous rays going in the direction of the lower parts of the apparatus,

each wick of the burner has been

which comes next

in passing

made

to stand above that

from the centre outwards.

LIGHTHOUSE STATISTICS.

201

Ahsorption of Liglit hy the Atmosphere. If a denote

tlie

of transparency, tliat

coefficient

proportion of light wliicli unit IcDgtli of to pass,

medium

tlie

the intensity whicli the light from the

would have

at unit distance in a

tlie

allows source

vacuum, and y the intensity medium, then the

of this light at distance x in the absorbing

law of intensity in terms of the distance will be expressed

Taking the kilometre

as unit of distance, the value of

obtained from Bouguer's experiments in clear air

'973;

while in a fog at Paris in 1861 the value of a was reduced to

(•62)^°°'',

to pass

or '62

was the

by a layer

aii*

fraction of the

whole

light allowed

1 metre thick.

Equation of Luminous Ranges.

Tor every observer there

below which the eye

-r

a limit of luminous intensity

no longer

Putting

affected.

«^

the value of x obtained shows the range of the light L.

For a person with average eyesight, X

may

be taken

= "01

of a Carcel burner. State of the Atmosphere defined hy the range of the

Unit of Light. If

value

p be the range in metres of the unit of may be found from the equation a^ = p)\ ;

and

for

„

= -973, = 8860 a = (-62)1^^5 =25

metres. „

light, its

202

LIGHTHOUSE ILLUMINATION.

Here

must be noted that

has different values for

different eyesights.

In the Table (Plate XXXIII.), the

column

lower

left

hand column con-

luminous intensity in Carcel burners

tains values of

contains

different

values

a there

corresponds

a value

the

the co-

To each of the

efQcient of transparency of the atmosphere. coefficients

;

of the

range of

the unit intensity, and these ranges are inscribed along the uj)per

column

of the table, while the oblique lines represent

The use

luminous ranges.

be easily understood, thus

of the table thus if

of a light for a given value of left for

the value of

composed will

one desires to know the range

he looks in the column to the

its intensity,

100

for

same time he looks in the lower column

example

at the

;

for the value of a,

â&#x20AC;˘871 for example, and, on ascending vertically to the horizontal line through 100, he arrives at a point of intersection

through which passes the oblique range sought for

therefore

line

marked 22

22 kilometres.

;

the

These three

numbers, 100, "871, and 22, are so connected that any one of

them may be found by means

of the other two.

"When

the point of intersection of the horizontal and vertical lines is

not traversed by an oblique called

the aid of the eye must

determine the range

in to

table given in

line,

required.

In the

Allard's M&moire, each side of the small

squares here given

subdivided into five parts, so that

closer values of the intensity

and transparency are given

the table being large enough to deal with intensities varying from '32 to 10,000,000 burners, while he shows more

extended values of

Another useful table given in M. Allard's Mimoire

may

The

first

be mentioned.

It is divided into three parts.

gives the luminous ranges corresponding to different inten-

LIGHTHOUSE STATISTICS. sities

of light in times of dense fog.

the unit Carcel

humer can

203

It sho-^ys that

penetrate only

27^

when

yards, it

would require an intensity 45 times greater than that of the strongest electric light used in France

(Gris-Nez)

penetrate twice that distance, or 54-| yards.

3270

the unit intensity can penetrate

yards,

Again, it

when

would

re-

quire an intensity 1-| time that of the Gris-Nez light to

penetrate four times that distance.

The second part

gives intensities required to carry from

â&#x20AC;˘27 to 1G2 nautic miles in states of the atmosphere called foggy, mean,

and

clear,

for

penetrate respectively from to

7630 The

sities,

yards,

which the unit intensity can

5341

7085

yards, from

7085

and 9347 yards.

third shows ranges, corresponding to different inten-

from

10,000,000 Carcel burners

three states of the atmosphere.

in the

same

CHAPTER

VIII.

SOURCES OF ILLUMINATION. The

application of the electric spark to lighthouse illumina-

tion having been dealt with in Chapter VI., the present will

he confined to the consideration of the other sources of light

employed rig.

for that purpose.

122

a section of the

Argand burner,

in wliich

t is

the cylindrical space through the centre of the wick for the ^' passage of the second air current, and I

the glass chimney.

an inch

in diameter, and the central air space is

f inch diameter.

25 (page

Fig.

60) shows the arrangement of the

Argand lamp used

((\

in the Northern

Lighthouse Service. Frcsnel Burner.

â&#x20AC;&#x201D;When

Tresnel

designed his dioptric apparatus he at

once saw the necessity for a greater

y&^

"-S

intensity of flame than could be got

from the Argand, and, with the

assist-

ance of Arago and Mathieu, he devised multiple

Fig. 122.

one of

liis

wick burners,

suggested by Eumford;

previously Figs.

123

and 124 are plan and section of

burners, with four concentric wicks, the

between them being passages which allow

air to pass

spaces

205

SOURCES OF ILLUMINATION. the inner wicks raising

;

wliicli lead oil to

of tin

C'"

and lowering the wicks

the four wicks

are the rack handles for

A B

;

\s>

one of the ducts

L, L, L, are

by which the burners are soldered

smaU

together,

and placed

so as not to liinder the free passage of air to the flame. air spaces are about \ inch wide.

the gallery

which serves

damper

B, and

fixed in

it,

cool and protect

The by

carried

surmounted by a sheet-iron cylinder,

to give it a greater length,

and has a small

capable of being turned by a handle for

regulating the current of

The chimney

plates

air.

In order to keep the burner

from the excessive heat produced, the

oil

supplied in superabundant quantity, so as to overflow the

wicks.

Fresnel used two, three, and four concentric wicks

Mr. Alan Stevenson, in 1843, introduced a Douglass has more recently adopted a

fifth;

and Mr.

sixth.

It will be observed that the leading principle of

of all other lighthouse burners introduced

since

tliis,

Argand's

time, whether for burning vegetable, animal, or mineral oUs, is

his double current.

206

LIGHTHOUSE ILLUMINATIOX.

The Argand and Fresnel burners were constructed burning animal and vegetable

oils,

for

those most employed being

sperm, lard, olive, cocoa-nut, and colza, which last was used

by the majority

of lighthouse services since the middle of the

present century oil

took

within the last few years,

till

when mineral

its place.

The advantages

be derived from the use of mineral

oil

were so apparent and important,

as a lighthouse luminant,

increased luminous intensity for equal consumption, and

viz.

many

a cost per gallon one-half that of colza, that

were made

to devise a

burner for

its

was found no easy matter,

This, however,

tion of carbon to hydrogen in mineral oils

quently the mineral

oils

require

very

oils,

large, as

and conse-

greater quantities of

combustion.

Maris Burner.

â&#x20AC;&#x201D; In

1856 M. Maris devised a

wick burner, with a disc or

current

air

into

gave good results with mineral

at the

deflector

which had the

central passage, tral

much

as the propor-

compared with that in animal or vegetable

air for their

attempts

consumption.

effect of

the oils,

single-

top of the

throwing the cen-

flame.

This

burner

and was employed by

the French Lighthouse authorities in several of their har-

bour

lights.

Doty Burner.

â&#x20AC;&#x201D;All attempts however,

burn mineral

either in the Fresnel or specially constructed multiple

burners, failed

till

so recently as 1868,

125 a

oil

parts

and by the addition of an exterior cylinder

rounding the outer wick, and a central in such a

burner.

a section and plan of a four- wick Doty burner.

happy choice of proportions in the various

his burner,

wick

when Captain Doty

solved the problem and brought out his mineral Fig.

oils,

manner

as to

disc,

throw a current of

sur-

both placed air

into

the

207

SOURCES OF ILLUMINATION. flame at the right place,

Doty succeeded

pro-

ducing single and multiple

wick hydro -carbon burners,

which carry a flame luminous

great

and

regularity.

flow of the

oil

intensity

The over-

required to keep

colza burners cool

unnecessary in these, and is

main-

constant

level,

therefore the oil

tained

being effected in the

tills

mechanical

ordinary

moderator

lamps

and an

ingenious stand pipe, de-

by Doty, and

repre-

sented in Fig. 125.

The

vised

following table gives the details

and

these

burners,

also the candle

and consumption, 1

power

as determined

Fig. 125.

by Dr. Stevenson Macadam-

LIGHTHOUSE ILLUMINATION.

208

notice of the Frencli Liglitliouse authorities, and they tried

them

December 1868

in a first-order light in

and fourth orders

all their third

converted

;

1870 they

to paraffin

in 1873 they resolved to adopt his burners in

and

;

all classes

lights.

In September 1870 the Commissioners of Northern Lights one

tried Doty's four-wick burner at Girdleness, in Scotland,

of their first-order lights; and in

their engineers, recom-

their general adoption in all classes of lights in the

mending

and

service,

March 1871 they approved

by Messrs. Stevenson,

of a report

recommendation has since been carried

this

out.

These burners have also been introduced, under

the

direction of Messrs. Stevenson, into the lighthouse services

of China,

Japan,

New

and

and Newfoundland,

Zealand,

experience has fully justified their claim to efiiciency, durability,

and simplicity

as a proof of the real importance of

;

the change from colza to mineral point of view,

may be

observed

oils

from an economical

that, in the case of the

Scotch Lighthouses, an annual saving of between ÂŁ4000 and

ÂŁ5000 has been

effected.

may

be stated that the Doty

burners are equally suitable for the consumption of colza

The Trinity House did not adopt the Doty burner their

shown

engineer,

Mr.

in Fig. 126, and

N. its

;

Douglass, introduced the action

wiU be

oil.

but one

readily under-

stood from what has been said in connection with other burners.

Mr. Douglass employs an outside jacket, a

cone-shaped perforated disc or button, and also tips for the tops of the wick cases, which can readily be taken off and

replaced

when burned

does not occur in the interior deflector,

out

;

Doty

an eventuality, however, which burners.

By means

also of

which can be put on when required, and

igniting only half the

number

of wicks (the outer three in

209

SOUECES OF ILLUMINATION. a six- wick for instance),

tlie

diminished by one-half. Douglass's

lamp

power of the burner can be

This

of single

Mr.

and double

power, for use in clear and foggy weathers respectively,

and by

claimed for gas

duced into

oil

the ''flexibility"

some extent

intro-

lamps.

Mr. Douglass claims to

have obtained even

better results with this

burner than those of

Doty is

stated above.

right to state that

Capt.

Doty claims the

Trinity burner as being Fig. 126.

the same as his own. Messrs. Farquhar, Silber, Funk, and others, have

produced burners, but their use lighthouse illumination Parcvffin Oil.

wells,

bituminous

concerned.

— Llineral

principally schist

found in a natural state in

oil is

and

in America, shale

Germany, and Great

also

on a large

distilled

scale

considered the best for the purpose.

specific gravity, oil,

which

France,

Scotch paraffin, distilled

from shale found in the county of Linlithgow, and

;

from

That employed most exten-

Britain.

sively in lighthouse illumination

luminous intensity

also

very limited so far as

generally

It gives a flame of great

quite safe, and low in price.

The

a test of the relative richness of the

should be from 0-8 to 0-82 (water

1) at 60° Fahr.,

the flashing point or temperature at which

and

begins to evolve

inflammable vapour should not be lower than 125° Fahr., nor higher than 135° Fahr.

210

LIGHTHOUSE ILLUMINATION.

Fountain Lamp. oil

— Single-wick

burners are supplied with

may

from the cistern or fountain (which

side, or

be placed at one

immediately below them) by the capillary action of

But

the wick alone. other methods

in the case of multiple

must be employed

wick burners

secure a sufficient

supply.

Mechanical and 3Ioclcrator Lamps.

—

If the cistern be placed

above the level of the top of the burner the flow of the

wick cases

effected

by the

oil to

the

direct action of gravity, regulated

by a contrivance which maintains a constant head.

If,

however,

the cistern be placed below this level, either a mechanical lamjy

employed, in which the

forced into the burner

oil is

by pumps worked by clockwork, or a moderator lamp, in which this is effected by the pressure exerted by a weighted piston descending in a cylinder forming the cistern.

Coal Gas.

minant

— Coal gas

was

first

used as a lighthouse

at Salvore, near Trieste, in 1817.

has been

used in the harbour lights of Great Britain

in the neighbourhood of gasworks,

flame

circular

Mr.

Wigham,

when

and the burners employed

were either those giving an ordinary cases,

illu-

For many years

flat flame, or,

on Argand's

principle.

of Dublin, has designed a

burner, which the Commissioners of

some

Eecently

new form

of gas

Irish Lights have adopted

in several of their lighthouses.

Mr. Wigham's compound or crocus burner consists of a group of 28 vertical tubes, each carrying an ordinary double fish-tail burner.

When

tlie

gas issuing from

ignited, the incandescent gases unite into

and by suspending, immediately above

it,

all

these jets

one large flame, a chimney of

which not only induces a powerful current of

air

talc,

but also

carbon at the top of the

acts as

flame

rendered incandescent (adding to the intensity of the

oxidiser, the excess of

SOURCES OF ILLUMINATION. flame),

and

valve,

which

cock,

additional groups,

By means

ultimately consumed.

acts both as a connection

of 20

and in

this

108

can readily be

Fig. 128-.

first,

which forms a central nucleus;

way, depending on the state of the atmosphere,

the power of the burner can be 88, or

of a mercurial

and regulating gas

each,

jets

Fig. 127.

arranged round the

211

arranged for 28

127

Fig.

jets.

jets,

the same for 108

with

jets,

of the 108-jet burner.

its

and

made

at will 28,

48, 68,

an elevation of the burner, as oxidiser above.

Fig.

129

In what he

Fig.

128

an enlarged section

calls his biform, triform,

or quadriform systems, referred to in Chap. X., Mr.

Wigham

places two, three, or four of the burners already described vertically one above the other,

or flue the products of the

are intercepted

and by means

an iron casing

combustion of the lower burners

and thrown outwards,

so as not to interfere

212

LIGHTHOUSE ILLUMINATIOX.

witli the

upper burners, while pure

them by

cylindrical openings brought through the flues.

3 5 H 5

H5 Enn

air is supplied to

S ^ ^ ^ S

Fig. 129.

Hie following

Kos. of Jets.

table gives the candle powers,

etc.'

each of

SOURCES OF ILLUMINATION.

Among proposed

the

for

many

213

other sources of light which have been

lighthouse illumination

may

be mentioned

Drummond's oxyliydrogen light Gurney's proposal to supply pure oxygen to an oil flame and Mr. Wigham's method of ;

;

introducing the electric spark, or oxyhydrogen light, into the centre of his gas flame.

CHAPTER

IX.

MISCELLANEOUS SUBJECTS CONNECTED WITH LIGHTHOUSES. Me. Alan Stevenson on the Distribution of Lights ON A Coast. "

The

considerations," says Mr. Stevenson, in 1848, " whicli

enter into the choice of the position and character of the lights

on a line of

coast, are either,

self-evident, as scarcely to

on the one hand, so simple and

admit of being stated in a general

form, without becoming mere truisms

;

or are, on the other

hand, so very numerous, and often so complicated, as scarcely to be susceptible of compression into shall not, therefore,

any general laws.

of the chief considerations selection of the sites

and

which should guide us in the characteristic appearance of the

lighthouses to be placed on a line of coast.

may

views

be most

distinct propositions " 1. first

Perhaps these

conveniently stated in the form of

The most prominent points

made on

do more than very briefly allude to a few

of a line of coast, or those

over-sea voyages, should be first lighted

;

and

the most powerful lights should be adapted to them, so that

they

may be

discovered by the mariner as long as possible

before his reaching land. " 2.

far as is consistent

with a due attention to distinc-

MISCELLANEOUS SUBJECTS.

215

some description, wliicli are necesmore powerful than fixed lights, should be employed

tion, revolving lights of

sarily

at the outposts on a line of coast. " 3. Lights of precisely identical character

should not,

if possible,

and appearance

occur within a less distance than 100

miles of each other on the same line of coast which

made

over-sea vessels. " 4.

In aU cases the distinction of colour should never be

adopted except from absolute necessity. " 5.

Fixed

lights

and others of

power may be more

less

readily adopted in narrow seas, because the rarige of the lights

such situations

generally less

than

that

open

sea-lights. " 6.

In narrow seas also the distance between

same appearance may often be lower limits than there are

many

lights of the

may

between similar Hghts

much

intended to guard vessels from

placed semvard of the danger

;

and

less separation

sufficient.

other dangers, should in every case where

" 8.

Thus

instances in which the distance separating

same character need not exceed 50 miles

" 7. Lights

much

desirable for the greater sea-lights.

peculiar cases occur in which even a

men be

lights of the

safely reduced within

itself, as it is

reefs, shoals, or

it is

practicable be

desirable that sea-

enabled to make the lights with confidence.

Views

economy

in the

first cost of a lighthouse should never be permitted to interfere with placing it in the

best possible position

;

and when funds are

generally be found that the wise course until a

sum

shall

have been obtained

of the lighthouse on the best " 9.

The elevation

deficient, it will

to delay the

work

sufficient for the erection

site,

of the lantern above the sea should not,

if possible, for sea-lights,

exceed 200 feet; and about 150 feet

21G is

LIGHTHOUSE ILLUMIXATION. under almost any circumstances,

sufficient,

range which

are subject to be frequently

wrapped in

thereby rendered useless, at times

might be perfectly

level

and indeed cannot British coast,

to give the

Lights placed on high headlands

required.

efficient.

be, strictly

fog,

when But

and are often

lights

on a lower

must

this rule

on the

followed, especially

where there are so many projecting

not,

cliffs,

which,

while they subject the lights placed on them to occasional

obscuration

from view

In such

would

fog,

cases, all that

also entnely

and permanently hide

on the lower land adjoining them.

lights j)laced

can be done

the circumstances of the locality,

weigh

carefully to

and choose

all

that site for the

lighthouse which seems to afford the greatest balance of ad-

vantage to navigation.

As might be expected

this kind, the opinions of the

in questions of

most experienced persons are

often very conflicting, according to the value

which

set

the various elements which enter into the inquiry. " 10.

The best

position for a sea-light ought rarely to be

neglected for the sake of the more immediate benefit of some

neighbouring port, however important or influential

;

and the

interests of navigation, as well as the true welfare of the port itself,

will generally be

light where

ought

much

to he,

better served

by placing the

and adding, on a smaller

scale,

sea-

such

subsidiary lights as the channel leading to the entrance of the port

may

" 11.

require. It

may

be held as a general

lights that can be

employed

maxim

that the fewer

in the illumination of a coast the

better, not

only on the score of economy but also of real

efficiency.

Every

light needlessly

erected may, in certain

circumstances, become a source of confusion to the mariner and, in the event of another light being required in the neigh-

bourhood,

becomes a deduction from the means of

distin-

MISCELLANEOUS SUBJECTS. giiislnng

from the

establishment. therefore,

means

liglits

By the

217

whicli existed previous to

needless erection of a

its

new lighthouse,

we not only expend public treasure but waste distinction among the neighbouring lights.

the

" 12. Distinctions of lights, founded upon the minute estimation of intervals of time between flashes, and especially on

the measurement of the duration of light and dark periods, are less satisfactory to the great majority of coasting seamen,

and are more

derangement by atmospheric changes, than those distinctions which are founded on what may more liable to

properly be called the characteristic appearance of the lights, in which the times for the recurrence of certain appearances differ so

detection

widely from each other as not to require for their any very minute observation in a stormy night.

Thus, for example, flashing lights of five seconds' interval, and revolving lights of half-a-minute, one minute, and two minutes, are

much more

characteristic than those which are distinguished from each other by intervals varying according to a slower series of 5", 10", 20", 40", etc. " 13.

Harbour and local

lights,

which have a circumscribed

range, should generally be fixed instead of revolving often, for the

media. into a

same

reason, be safely distinguished

many cases

also,

where they are

narrow channel, the leading

should, at the distinction

same

lights

;

and

may

by coloured

to serve as guides

which are used

time, be so arranged as to serve for a

from any neighbouring

" 14. Floating lights,

lights.

which are very expensive, and more

or less uncertain from their liability to drift from their moorings, as well as defective in power, should

never be employed

to indicate a turning-point in a navigation in

any situation where the conjunction of lights on the shore can be applied at any reasonable expense."

218

LIGHTHOUSE ILLUMIXATIOX.

Mr. Alan Stevenson's Table of Eanges of Lights. Table of Distances

which Objects can be seen at Sea,

according to their respective elevations, and the elevation of the eye of the Observer.

Heights

219

inSCELLAXEOUS SUBJECTS.

and

be capable of being lowered down to the deck to be

trimmed, and raised when required to be exhibited. plan had the advantage of giving a lantern of size,

because

enabled larger and more perfect apparatus

be introduced, as well as gearing for working a revolving Fig.

light.

this lantern,

lowing of

His

greater

encased the mast of the ship, and with this

increase of size to

much

130 shows

and the

fol-

description

is his

:â&#x20AC;&#x201D;

"The

lanterns were

so constructed as to clasp

round the masts and verse

upon them.

was

effected

structing

This

con-

them with a

tube of copper in capable

centre,

ceiving

tra-

the

re-

and

mast,

through which

the

passed.

The lanterns were

first

completely formed, and fitted

with brass flanges

were

they

then

longitudinally

;

cut

asunder,

which conveniently admitted

their

being

screwed together on the Fig. 130.

masts after

was

the

vessel

fully equipped

show

and moored

at her station.

Letters

a a

part of one of the masts, h one of the tackle-hooks for

raising and lowering the lanterns,

c c

the brass flanges with

220

LIGHTHOUSE ILLUMINATION.

by which, the body

screw-bolts,

tlieir

was ultimately put and

also at

or case of the lantern

There were holes in the bottom

together.

the top connected with the ventilation; the

and g form guards against the effects of the The letter h shows the front of the lantern, which

collar-pieces e

weather.

was glazed with

plate-glass;

one ot the glass shutters

by which the lamps were trimmed

made

groove

raised, slides into a

â&#x20AC;&#x201D; the

lower half being

for its reception

k shows

;

the range of ten agitable burners or lamps out of which the

oil

cannot be

Each lamp had a the flame."

by the

spilt

rolling

motion of the

silvered copper reflector

placed

ship.

behmd

After that important alteration, revolving catoptric apparatus

was applied

to floating

Letourneau, in 1851, j)roposed

lights in

England; and M.

employ

different

fixed

dioptric apparatus in one lantern for floating lights.

Plate

XXVI. shows

elevation and horizontal section of

one of the Hooghly Floating Lights on the dioptric principle, designed for the Indian Government by Messrs. Stevenson. It will be observed that four of the separate lights are

been constructed

for the

Government

lights

were seen

these

was reported by the

the

for a distance of

secretary of the

efficient one,

19 miles, and the

officiating

and the best and most brilHant that has ever

XXVII. shows

siren fog-signal apparatus

seas."

a section of one of the latest light-

which are on the

kindly communicated by Mr.

first

attendant master to

Government of Bengal "to be a most

ships of the Trinity House, ciple,

The Hooghly

of Japan.

been exhibited from a Kght-vessel in these Plate

always

Similar floating lights have since

visible in every azimuth.

catoptric prin-

N. Douglass.

A steam

and windlass are shown on the draw-

Account of Bell Rock Lighthouse.

Edinburgli, 1824.

MISCELLANEOUS SUBJECTS.

Mr. Douglass informs

ing.

that during

221 tlie last five

years

the Trinity House have been replacing the 6 -feet octagonal lanterns with cylindrical ones 8 feet diameter; and

21-mch

diameter reflectors have also been substituted for 12-inch,

The

Seven Stones

Trinity

lightship,

House improvements,

42 fathoms.

Her moorings

which has

moored

all

off the

the

Land's

consist of a 40-cwt.

recent

End

mushroom

anchor, and 315 fathoms of l|-inch unstudded chain cable.

The

vessel,

which was specially designed by Mr. Bernard

Waymouth, secretary to Lloyd's, is timber-built, copper-fastened throughout, sheathed with Muntz metal, and of the following dimensions, viz.

â&#x20AC;&#x201D;

length,

extreme breadth, 21

103

feet

between perpendiculars;

feet 3 inches;

depth of hold from the

strake next the timbers to the upper side of the upper deck

beams, 10 feet 3 inches. adrift (a foresail,

In the event of the vessel breakino-

most improbable occurrence), she

provided with

lug mainsail, and mizzen, the latter being frequently

used with advantage for steadying the vessel at her moorings.

The

cost of the vessel, fully equipped for sea, with illuminat-

ing and fog signal apparatus complete, was about ÂŁ9500.^

Landmaeks. Wliere there

a sunk rock which

low in the water

as to preclude the erection of a beacon, towers are sometimes

erected on the land for guiding vessels clear of the danger,

but these guides are limited to the azimuthal plane.

It is

worthy of consideration whether such indications of the place of danger might not be extended to the vertical plane, at

places where the land

suitable

and the rock

not very far

from the shore. ^

Min. of Proc. of

Inst, of Civil Engineers, vol. Ixii.

LIGHTHOUSE ILLUMINATION.

222 Fig,

131 represents in plan and section a lieadland, with

rocks lying both near the shore and farther out in the sea.

,.--iD

JfOCA"

way

Plan.

Fig. 131.

In order to mark

off the safe limits of the channel, a wall

circular in the horizontal plane,

and

land,

might be built on the head-

at its centre of curvature a tower B, of greater height

than the wall, and also three low towers, C C C, between the wall and the sea. An imaginary line joining the tower with the top of the wall would that

would keep

mark

off the

seaward limit of safety,

vessels sufficiently clear of the outer rock.

Another imaginary

line,

ACE,

joining the hottom of the

wall with the tops of the low towers would in like manner point to the verge of safety on the side next the land. sailor

would therefore never

steer so far

The

from the land as to

see the top of the high tower projecting above the top of the wall, nor so near the shore as to see the top

low towers projecting above the lottom of the wall.

any of the Thus, by

MISCELLAXEOUS SUBJECTS. keeping

tlie liigli

223

tower always out of view, and the bottom

of the wall always in view (clear of the low towers), he

would

For greater distinction when snow

never be in danger.

on the ground, the upper part of the high tower should be painted red, the wall black with white

stri^^es,

and the low

towers white.

Lanterns.

The lantern supporting a

of a lighthouse consists of a glazed framing

dome and

ventilator.

Its object is to protect

the light and apparatus from wind and rain, and yet give free ventilation inside, not only to cause the

lamp

burn

well,

but also to prevent the condensation of moisture on the inside,

and consequent clouding

The

of the panes of glass.

early form of lantern had vertical astragals,

necessarily reduced the

amount

which

of light in the particular arcs

which they subtended. Mr. Eobert Stevenson improved the dome by introducing the present inner dome, the air space between the two acting as a

bad conductor of

heat,

and preventing the rapid con-

densation of water that would otherwise take place with a single dome.

Chapter

II.,

lantern, as

lantern

Mr. Alan Stevenson, as already mentioned in mtroduced inclined or diagonal framing for the

shown on Plate

much

making the any rocking motion, but also

XI., thus not only

stronger to resist

causing the loss of light by the astragals to be spread nearly

uniformly over the whole lighted

arc.

The

astragals in the

Northern Lighthouse service are made of gun metal (having a tensile strength of 33,000 lbs. to the square inch), glazed with flat plate-glass

held in

its

place

plates riveted together,

by means

of gun-metal strips

The dome

made

and the foUowmg

the specification

screwed into the astragals.

of copper

LIGHTHOUSE ILLUiMIXATION.

224

of the glass of a first-order lantern

â&#x20AC;&#x201D;

panes are to be of

" Tlie

the best quality of polished mirror plate-glass, not less than ^

inch in thickness, nor more than

air bubbles, colour,

from to

The

inch.

-^j^

have truly parallel

to be perfectly i)olished, to

and

striae,

glass requires

faces, to

be free

and

or other blemishes,

be carefully rounded on the edges by grinding, after being

cut to the proper size."

Mr. Douglass's lantern, Plate XII., already referred also shuts out only a small

This lantern

loss equally.

made

and

helical,

the light

may

fall

of steel,

In France, and

made with

amount

cylindrical, the diagonals are

and the glass

also in America,

cur\'ed so that

The panes

radially.

are

inch thick.

the lanterns are

and are glazed with

vertical astragals,

to,

of light, and spreads the

still

glass

inch thick, protected, in some situations, from breakage by

by means

birds being driven against them,

gauze, the meshes being about

The

of brass

"vru'e

inch square.

lanterns of cqjparent lights should be well ventilated,

so as to prevent a haze

from forming on the inside of the

lantern glass.

Glazing of Lanterns.

â&#x20AC;&#x201D;Great care

of the lantern, in order

that

to water during the heaviest

amount

of risk

of the

glass

bestowed on the glazing

may be

plates

this

The ing;

arris of

certain

;

and

to prevent

as possible various precautions are adopted.

each plate

always carefuUy rounded by grind-

and grooves, about |

a good

being broken by the

shaking of the lantern during high winds

much

quite imper^'ious

There

gales.

iucli

wide, capable of holding

thickness of putty, are provided in the astragals for

receiving the glass, which

of lead or cork are inserted

\ inch thick.

Small pieces

between the frames and the

plates of glass, against wliich they

may

press,

and by which

225

MISCELLANEOUS SUBJECTS. they are completely separated from

more unyielding

tlie

material of wliicli the lantern frames are composed.

Storm Panes.

â&#x20AC;&#x201D;In the event of a breakage occurring

of the panes in the lantern,

which

any

not unfrequently the

case from birds or small stones being driven against

during

Fig. 132.

gales,

glazed copper frames, as

shown on

in the Northern Lighthouse service. astragals

Fig. 132, are

They

used

are fixed to the

from the inside by means of the screws shown in

the woodcut,

a a are the

astragals, h h are the glass panes.

LiGHTNING-CONDUCTOES.

lighthouse towers generally occupy exposed situations

on high headlands liable to

or isolated rocks, they

are

peculiarly

be struck by lightning, and the question of their

protection

demands the

special attention of the engineer, for

an efficient lightning-conductor

may be

said to be an essential

part of every properly designed lighthouse establishment.

LIGHTHOUSE ILLUMINATIOX.

226

Franklin, about the year 1749, conceived the idea that electricity, w^hich is

produced in the laboratory, and

ning were identical

and founding on

;

this,

and

light-

his previous

discovery of the action of metallic points on electricity, he

published a pampldet iu 1750, in which he states that

damage done

to buildings

clouds, could be prevented,

electricity

all

descending from the

fixing on their

summits iron

rods with sharp points, the lower ends of the rods being .

In 1752 he made his celebrated ex-

fixed in moist ground.

periment with a theories,

kite,

demonstrating the truth of his former

and the same year he erected a lightning-conductor

own

for the protection of his

house.

the present time the question of protection from light-

ning rests nearly in the same state as that in which Franklin left it

the chief improvement in construction being the sub-

;

stitution of- copper instead of iron as the material for the con-

ductor, to

effects of lightning are well

and much has been written on the

all,

still,

and although the

known

subject, science has

as in Franklin's day, to confess its inability to advance

any perfectly

satisfactory theory either of the genesis or sub-

sequent development of thunderstorms.

Although the

practical requirements of a

good conductor

were discovered and fully enunciated by Franklin, those

who

many

and

came

after

him

fell,

more harm than good It

ductors. orio-inal

1st,

2>d,

resulted from the use of their con-

be said that

till

comparatively recently, the

requirements of Franklin were in practice generally

neglected.

2d,

may

cases, into serious errors,

These requirements are

A terminal point. A good earth connection and A continuous metallic connection between

The action

;

of this

arrangement

them.

supposed to be as

227

MISCELLANEOUS SUBJECTS. follows

â&#x20AC;&#x201D;A

tliunder-cloud, inducing an equal

charge on the earth beneath tected,

and while

still

it,

and opposite

ajDproaches a building so pro-

at too great a distance for a disruptive

charge to take place between the cloud and the buildmg, or " to strike it," as it is called, the pointed rod, in virtue of the

well-known action of

points, begins to act

silently

and

continuously draining off from the earth a sufficient amount of electricity to render the cloud perfectly innocuous.

Copper being, of city, is therefore

rod,

all

metals, the best conductor of electri-

the most suitable material for a lightning

and may be used

round

rods, flat bai-s,

metal

for

tliis

purpose either in the form of

or wire rope, care being taken that the

tolerably pure, and that there

the joints, which in the case of rods

coupling boxes

;

metallic contact at

by screwed

and, though less satisfactorily, with a half

check and screws in the case of secures

is effected

The use of wire rope

bars.

more perfect continuity, and being very pHable

has

the advantage of being more easily trained and secured to the building.

The rods should be

carried about

18 inches above the

highest part of the lantern, and there terminated in two or three branches, each being finished with a sharp but not

slender point, and fitted with a platinum cap. of having

more than one point

object

The use

of platinum in

fine point is chiefly to preserve it

from oxidation.

metal of a single point being fused. forming the

The

to reduce the risk of the

The conductor

carried

down

the outside of the tower, from

the top to the bottom, as a continuous rod, and carefully put in metallic connection with all large masses of metal inside or outside the building, such as machinery, bells, stoves, pipes,

tanks, ladders, etc.

This precaution was adopted by Eobert

Stevenson in his design for the Bell Piock Lighthouse, after

228

LIGHTHOUSE ILLUiMIXATIOX.

consultation with Playfair, Leslie, and Brewster. Professor Tait

was

also recently consulted

must be

by the Commissioners

Xorthern

But besides being continuous, the conductor

Lighthouses.

of sufficient section to carry

any charge

to wliich it

may

be subjected without fusing, and in most situations a f inch rod or wire rope, as recommended by Faraday, will be

found quite

The rod should be attached

sufficient.

masonry by copper

bats, at every

to the

10 feet apart; and in towers

exposed to the stroke of the sea the bar should be sunk in a raglet in the tower, to prevent its being disturbed

by the

waves.

reaching the ground

should be carried in a trench

to a spot 20 or 30 feet from the foundation of the tower,

and

there divided into two or three branches, to each of which a

copper plate should be soldered, and these in turn buried in the earth or sunk in water.

The

depends on the nature of the but 20 X 12" X Y\ is the

size

ISTorthern Lighthouse servdce.

size of these

soil in

copper plates

which they are placed,

generally adopted in the "

good earth terminal

" is of

the very greatest importance, moist earth or large bodies of

water being the most favourable, and

it is

at this point that

the lighthouse engineer, especially at exposed rock stations,

meets with the greatest practical

difficulty.

For although

nothing can be better than the sea as the earth coniiection, yet on a rocky face, exposed to the rise and

and the

full action of

the waves,

fall of

the tide

must be admitted

in certain situations hardly practicable to provide that even for a single winter's storms the termination of the conductor,

however well secured, insure

its

of the sea

shall

remain so undisturbed as

constant immersion in the water during

and

all states

tide.

A tower, furnished with a lightning rod as described above,

229

MISCELLANEOUS SUBJECTS.

constructed with ordinary 'care and occasionally inspected ocularly, will be quite safe

But

damage.

damage

would

from demolition and even serious

prevent the possibility of

even

slight

be necessary periodically to test electrically This can

the whole rod from the upper point to the earth.

be effected by employing an apparatus, several varieties of wliich have recently been brought out, the principle involved in all of

them being

measure the resistance of the con-

ductor to the passage of a current of electricity generated in a small portable battery, coils

by means

of a series of resistance

and a galvanometer.

Testing, however, as described

come

into general use in this country,

above, has not as yet

although employed to some extent in Germany.

Eadial Masking Screens. ¥igs.

masking

Fig.

133 and 134 show in elevation and

yAaii radial

Balfour, for

screens, designed

by the

133.— Elevation.

late

Mr.

Fig.

134.—Plan.

cutting off sharply, in a given direction, the light proceeding These screens, which are in use at from any apparatus. several of the Northern Lighthouses where the fairway for ships is narrow, intercept all rays of light wliich have too

great divergence, and prevent

them from passing beyond the

230

LIGHTHOUSE ILLUMINATION.

line in whicli the light has to be cut

the parallel rays to pass throngh, as h c

and

off,

shown

while they permit

where

in Fig, 134,

d are the rays which are intercepted, a

the focal

and

point, h a reflecting prism, while the rays h e

f pass

freely tlirough.

Machinery for Eevolving Lights. The machines employed

for causing the

frame to revolve

When the

were originally of comparatively light clockwork. dio^Dtric

system was introduced a somewhat heavier construc-

was adopted; and when the holophotal

tion of machinery

arrangement was introduced, the with

its

much

heavier armature, required a

and powerful

class of

greater

still

wheels and pinions.

rollers required for dioptric lights

have

mass of

glass,

more substantial

The

steel friction

also, in like

manner,

been increased in number from 12 in the original aj)paratus to 16,

which

number adopted

the

Condensing Intermittent

light.

for the

Mull

Galloway

Mr. David Stevenson, in 1849,

enlarged the driving-wheel at ISTorth Eonaldshay holophotal apparatus, which at

feet

first

had an irregular staggering motion,

8f inches diameter, which has been found to produce Instead of an outside driving-

a smooth and steady revolution.

wheel an internal wheel was adopted, which miglit be regarded as a circular rack, 5 feet in diameter,

pinion; so that, as the teeth

worked

worked by an internal

internally, nothing could

accidentally be brought in contact with the gearing, the outer

periphery of the driving-wheel having a smooth surface, and the pinion working inside instead of outside.

For small

class

apparatus for Japan, he had also covered the peripheries of the wheels with bands of indiarubber, which, simply by their friction,

secured a

much smoother and

that produced by toothed gearing.

steadier motion than

Mr. Douglass, in 1871,

231

MISCELLANEOUS SUBJECTS.

enlarged the diameter of the driving wheel to that of the revolving frame.

AsEisMATic Arrangement.

A less

common

dealt with, to

source of danger than any of those already

which a lighthouse may be

exjDosed, is that of

earthquakes, which are believed to be the result of a sudden

blow delivered

to the crust of the earth

by the collapse of

internal cavities, the generation of steam, or the sudden frac-

ture of large masses of rock previously strained.

From

the

locus of this blow a series of concentric waves of vibration is

propagated through the crust of the earth to the buildings

and the direction of the motion there

situated on the surface,

produced will of course depend on the direction in which the

wave

when

travelling

The motion

arrives there.

will therefore be truly vertical at a place on

the earth's surface immediately above the seat of the disturbance, but will acquire a greater

ponent, as the place

farther

and greater horizontal com-

and farther removed from the

points on the earth's surface where the motion

The

action to

sudden movement direction,

is vertical.

which buildings are thus subjected of the earth

on which they

rest in a certain

and then a sudden reversal of that movement,

Mr.

E. Mallet, the author of a well-known work on this subject, giving an elaborate account of the Neapolitan earthquake of

1857, found that the

amount

maximum

amplitude of the wave, or

of " shove " in a horizontal du^ection, did not exceed

three or four inches.

The observed

effects, as is

well known, vary from a mere

tremor to such a violent disturbance as to result in the over-

throw of whole

cities

;

and, as

may be

readily imagined, even

in comparatively slight shocks the delicate apparatus in a

232

LIGHTHOUSE ILLUMINATION.

lighthouse

peculiarly liable to derangement, and instances

are recorded in

which the lamp

glasses

have been shaken

off

and the lamps extinguished. In 1867, the Board of Trade asked Messrs. Stevenson

advise as to establishing a lightliouse system on the coasts of

Japan, which being a country peculiarly subject to earthquakes,

they requested them specially to consider in what

way

anticipated injury to the apparatus could be mitigated.

meet the

difficulty thus raised,

the

Mr. David Stevenson devised

an Aseismatic-joint in the table supporting the apparatus and

Fig. 135.

machine, so contrived as to admit of a certain amount of horizontal

movement taking

affecting the table

place in the building without

on wliich the apparatus

rests,

the idea

being that in some horizontal plane the connection between the apparatus and the building should be cut through and separated, so that the sudden motion of the lower part should

not be directly communicated to the superincumbent portion. Fig.

135

a section in which the upper table bearing the

mSCELLANEOUS SUBJECTS. holophotal reflectors rests

on spherical

bell

233

sliowu in hatched lines.

metal

balls,

This table

contained in cups formed

in the upper and lower tables, and Fig. 136

a plan of the

lower table, showing the three points of support.

The same

Fig. 136.

arrangement, on a larger

scale,

was proposed

for the

founda-

tions of the towers, where these were intended to be constructed

of iron.

The effect of the aseismatic arrangement, which adds

about ÂŁ90 to the cost of the apparatus, was tested in the manufacturer's shop,

truck,

by placing one

and driving

of the tables on a

against buffers

weights allowed to swing against

it.

by the impact The

effect

movable of

heavy

produced on

a lighthouse lamp placed on the aseismatically arranged table

was imperceptible, but when the aseismatic arrangement was clamped, the glass chimney was thrown off and the apparatus disarranged.

The experimental

trials

having thus proved

234

LIGHTHOUSE ILLUMINATION.

entirely satisfactory, several aseismatic tables were constructed

and sent out

however, complained of venience

But

Those in charge of the apparatus

to Japan.

this, as

sensitiveness, causing

its

when engaged

them incon-

in cleaning or trimming the lamps.

was pointed out in a report

to the

Japanese

Government, could be easily cured by locking the tables while the lamps were being cleaned or trimmed.

Focusing of Lights. Paraholic Reficdor. parabolic reflectors

â&#x20AC;&#x201D;

was found by experiment that

might be dipped

about 2i degrees

below the horizontal w^ithout diminishing the power of the light sent to the horizon.

that no advantage

about 1 degree.

practice, however, it appears

gained by dipping them more than

This amount, indeed, sends the central rays

to within three-quarters of a mile of the foot of the

tower

when its height is 100 feet. AU paraffin placed 14 â&#x201E;˘^^ below the focus, which brings the brightest part Argand burners are

of the flame into the focus.

In Scotland the mode of testing the accuracy of the position of the burner

by an instrument having three arms

each other at the centre points in the paraboloid.

;

Another arm, which represents the

position of the axis of the parabola, betwixt the focus, passes

arms.

pushed in

rod,

and when so as to

made on

the paraboloid

the pointed end of the central

touch this point, the other end

arm

is at

the

plummet hangs from the focus end of this which passes down through the burner, and on the thread

focus.

apex and the

through the point of intersection of the three

small centre punch-mark

at its apex,

crossing

the extremities of these arms are

fixed a

light

round ivory plug, on the surface of which, at the

proper distance under the focus, a circle

engraved.

The top

235

mSCELLANEOUS SUBJECTS. of the burner itself

set exactly at the level of this line, the

plug

hanging at the same time exactly in the centre of the

Where

inner tube.

the reflectors have been holophotalised

the same instrument

the

applies,

only being

bar

axial

lengthened to go back to the centre of the spherical mirror.

— The

Dioptric Lights.

brightest horizontal sections of the

flames of the different orders of lamps have been found by short exposed photographs to be as follows One-wick lamp 14 ™°^ above the top of burner. :

Two-wick lamp 19"™ Three- wick

lamp 23 °™

Four- wick lamp 25 ™°^

do.

These points are placed in the sea horizon focus of the

The upper

central or refracting portions of the apparatus.

prisms are ground and set so as to bring the sea horizon to a

30™^ above the burner,

focus for four- wick burners at a point

and 9™°^ lehind the

and the lower prisms

axis,

18™°^ above the burner, and

way the

38™™

to a point

lefore the axis

;

in this

brightest sections of the flame are sent to the horizon,

and the bulk of the

between the

light is spread over the sea

horizon and the lighthouse.

burners are 29, 12, 17, and

These figures

for

three -wick

23 respectively.

Mr. Alan Stevenson suggested the dipping of dioptric lights

below the normal

December

to a

plumb

is,

Eeport of

10th, 1839, to the Commissioners of Northern Light-

houses, in the following passage "

line, in his

A more

serious inconvenience in using catadioptric zones

that in very high towers, where

some correction of the

position of the apparatus becomes necessary, so as to direct

the rays to the horizon, the means of regulating the zones in

a manner similar to that used for the mirrors "

The adoption

inapplicable.

of a high point in the flame for the focus

236

LIGHTHOUSE ILLUMINATION.

of these zones, however, affords a considerable compensation for this defect

and

;

might even be entirely obviated by

constructing each set of zones of the form suited to the

known

height of each tower and the required range of each light,

such a correction were found to be of sufficient importance to

warrant

its

application."

But though the precaution to the

light

sea horizon

Stevenson in high towers, the year 1860,

when Mr.

of dipping the strongest of the

was followed out by Mr. Alan was not always attended

J. F.

to till

Campbell, the Secretary of the

Eoyal Commission on Lighthouses, independently brought the subject prominently forward, and suggested the internal of adjustment.

Since then the strongest

beam has been

mode

invari-

ably dipped to the horizon. It must, however, be

remembered that when the weather

becomes even in the least degree thick or hazy, not to say foggy, the range of the light

absorption and refraction

;

greatly curtailed

which

by atmospheric

last produces,

during fogs,

irregular diffusion of the light in every plane, so that at high

towers, it is

where the beam

pointed to a very distant horizon,

obvious that the strongest light

the sea where

the weather

exceptionally clear.

directed to a part of

cannot be seen with certainty unless

beam should be dipped lower than Mr. and as is now everywhere adopted.

that the strongest

Stevenson proposed,

The best

of the light should certainly be directed to the

place where the safety of shipping most requires

may of

when

It has lately appeared to

most cases be

any vessel

laid

down

it.

I^ow,

as axiomatic, that the peril

inversely proportional to her distance from

the danger, whether

tliat

danger be a lee-shore or an insulated

rock.

Confining ourselves to this one view of the subject,

would

follow that the strongest of the light should, to suit

237

MISCELLANEOUS SUBJECTS.

hazy

be thrown as near to the shore

states of tlie atmospliere,

or the rock as -would admit of vessels keeping clear of the

But such a

danger. greatly

ing

impab the

restriction as this would, if

usefulness of the light by unduly curtail-

range in clear states of the atmosphere

its

cccteris

paribus, the

farther

a sailor

off

approach to the shore the better and

due

loss

ratio,

permanent,

;

and of

course,

warned of

his

Besides, the

safer.

to atmospheric absorption increases in a geometric

and

as the rays diverge in cones

the power of the light

further decreased in the inverse

ratio of the squares of the distances

from the shore.

course well kno-s\Ti that the sun itseK

and we cannot expect

from the apparatus

It is of

extinguished by fog,

compete with that luminary.

But

seeing there are endless variations in the density of fogs, and in the transparency of the air so called,

of doing light,

when

always appeared to

we ought

so,

there

that,

no fog properly

had we an easy way

to increase temjjorarily the dip of the

and thus, during haze and

fogs, to direct the strongest

beam to a point much nearer the shore than the sea horizon. At present we dhect our strongest light, not only in clear weather, when it can be seen, but also during fogs, when it cannot possibly be seen, to a part of the sea to

shipping

in most situations the smallest

to the detriment of that region where, even is

hazy, there

and

to a part

at least

some chance

of the sea

;

ivliere

and

when

the

danger

this is

done

the weather

of the light being visible,

where the danger

to shipping

unquestionably the greatest.

The simplest mode would be

to raise the

plane of the lens. inexpedient.

of depressing the light temporarily

lamp

But

this

itself in is,

The proper adjustment

focus, so as to secure its

bemg

relation to the focal

for several reasons,

very

of the apparatus to the

situate in the section of

maxi-

238

LIGHTHOUSE ILLUMINATION.

mum

luminosity of the flame,

and ought,

if possible,

necessary for changing the lamp. of the

somewhat

lamp would depress the the opposite

effect

Moreover, while the raising light

which passes through

the refracting portion of the apx^aratus, cisely

delicate one,

not to be disturbed oftener than

would have pre-

upon the portions which pass

through the totally reflecting prisms placed above and below the refracting part, which would then throw the rays upwards to the sky,

where they would be

But any desired

useless.

change could be effected by surrounding the flame with prisms spheric on their inner faces, and concentric with the foci of the different parts of the apparatus, so as to depress

the rays before they

upon the main apparatus.

fall

prisms which subtend the lens would hare lowest,

and those subtending the

Those

their thicker ends

would have

reflectors

their

thicker ends uppermost.

From

made with two kinds

a series of observations

photometer by Messrs. Stevenson, in 1865, on the penetrative

power it

of light from a first-order lens

appears that for an

plane of

maximum

and cylindric

angle of 0째 30' in altitude

intensity,

and

for 0째 30'

refractor,

above the

below that plane,

the power of the light does not vary more than at greatest

from 4 to 6 per to the horizon,

cent,

and

that, if the strongest part

about one half of the whole

to the skies.

Povjer of Lens in the Vertical Plane. Mean

of four sets

of observations.

maximum

-90

30'

do.

-94

20'

do.

-97

10'

do.

0째 40'

ABOVE

tlie

level of

maximum power

be sent

sent uselessly

-98

1-00

MISCELLAXEOUS SUBJECTS.

239

Mean

of four sets of observations.

0° 10'

Note.

BELOW

the level of

maximiim

'99

20'

do.

-97

30'

do.

-96

—These

results,

which

are the

mean

of foui- sets of observatious

did not extend fui'ther in the vertical plane.

BesuU of Dipping Light as proposed, present System. Present System.

contrasted luith

240

LIGHTHOUSE ILLUMINATION,

divergence, and shore,

the only agent for giving light near the

only should be dipped so as to throw as few of the

rays as possible uselessly on the skies, while the reflecting

which have much

prisms,

distribution of the light from the lens

the strongest

remain

less divergence, will

present throwing their rays to the horizon.

beams from the lens

the horizon, which causes a

as at

this different

and the prisms, by which are dipped 0° 20' below

loss there of 3 per cent of lens

power, yet the loss on the wliole light coming from both lens

and prisms, taken

70 and 30 respectively,

be reduced

"will

to only about 2 per cent, wliile the sea near the shore will

be more powerfully illuminated than at present.

ought not to be dipped to 0° increase the

power near the

30', as tliis

shore,

would

that,

keeper, fall

still

beam

further

and would only depreciate

the light at the horizon by about 5"8 per cent.

remember

may,

however, be fairly questioned whether the strongest

It is well to

should the flame, through the neglect of the

any time below the standard height, such

a defect would operate most injuriously on the light falling

near the shore, horizon.

Now

vessel

cases is

and not so much on that sent an

with such

ofling

safer position

than

in a far

she were within a mile or two of the

and hence the propriety of increasing the

the shore, so long as

reduce

the

20 miles, which

the sea-range due to 300 feet of elevation,

shore,

there can be no question that in all ordinary

we do

light near

not to any appreciable extent

at the horizon.

Optical Pkotractoes. Mr. Balfour's Protractor.

—The

the description of his protractor,

late

Mr.

which was the

Balfour, in first

of the

241

MISCELLANEOUS SUBJECTS. kind

Eoyal

Trans.

(vol.

method described

in Dr.

sopliy, of finding the

Scot.

Soc.

Arts),

refers

Young's Lectures on Natural

Pliilo-

path of a refracted ray, and on that

principle he constructed an instrument for tracing totally reflecting prisms.

Dr. Young's method

tlie

form of

as follows

ABC (Fig. 137) is

a circle touching

the

refracting

sur-

face at A, if another

D E F

circle

drawn on the same centre, having its dia-

meter

to that of the

as the

first

sine

^jg- ^37.

the angle of incidence circle,

which

tion as the second

to that of refraction,

is less

than the

greater

;

and

first

in the

tliird

the direction of the in-

K A, be continued to D, and common centre 0, cutting G H I in

cident ray,

the

and a

same propor-

D be drawn from A G will be the

direction of the refracted ray.

In the figure

a ruler, turning on a centre 0.

the point where the ruler cuts the circle

I'O.

I'D,

C, is

marked

the index of refraction for lighthouse

glass.

To use the instrument, draw a perpendicidar of the

medium from

at the point

in this line,

Turn round the

ruler

till

making the distance

OA = OM.

the division on the outer scale corre-

sponding to the index of refraction incident ray produced, and division on the inner scale.

mark shows

to the surface

the point of incidence A, insert a needle

(1'5 in the figure) cuts

make a mark

the

at the corresponding

A liae drawn from A through this

the direction of the ray through the

medium.

242 The

LIGHTHOUSE ILLUMIXATIOX. principle of this protractor

applied to the construction

of totally reflecting prisms in the following

M for

iST

(Fig. 138) is the ruler

an index of refraction of

1'5.

manner

marked, as already described,

F C is

the direction of a ray

from the flame which has to be bent into

the direction

The

the

sides

Q and C P

prism C

must have such

di-

rections that the ray

r C

falling

side

C P at C will be

point

on the the re-

fracted into the di-

rection

Q, and

must be such that Fi;C.

the ray

13S.

falling

on C Q, be refracted in the direction C P.

The dotted

found by the instrument shown.

C L

bisects the angle

from the point C on the the end of the ruler

while the mark

and the point

line

C Q and C P then form is

L, until

through

moved

to or

observed that

The

lines

the two refracting sides of the prism all

the rays falling upon

are reflected in directions parallel to the line

Mr. Brehners Optical Protractor.

This

line

with the line

drawn, so that

shown on Plate XXV.,

traced

M N" just touches the line F C produced,

1"5 coincides

the reflecting side it

E C

backward must, after

consists of

â&#x20AC;&#x201D; This

two parts

instrument, as the lower limb

G, which forms a complete circle, and the upper limb.

243

MISCELLANEOUS SUBJECTS.

move

M'liicli

easily

The lower limb

a semicircle.

made

roimd a centre on the upper limb, which

per-

forated and filled in with glass,

and two

drawn on the

side of this centre part rests

on the

glass.

and

table,

The under

is fitted

axial cross lines are

with three points which penetrate

the table and keep the upper limb from turning

lower one, which

the lower limb

divided into degrees, and

;

while the

One half of numbered from

next the table, turns freely. is

the middle towards each side, and also from each side towards

the middle

;

the opposite half

sines of the opposing angles

so divided as to

have a

make

the

ratio of 1"51 to 1.

For finding the path of a ray through a piece of glass Let

represent the surface of a piece of glass, and

placed out of position in the plate) any ray falling upon

L C

Piead off the angle from the perpendicular

it.

mark

off the

XXV.

the ray

same reading from

towards

which cuts the limb

N",

and

In Plate

at 19JÂ°, will pass

through the glass in the direction C P, which

For constructing a Totally -reflecting Prism.

19iÂ° from X,

â&#x20AC;&#x201D; Suppose

required to construct a prism which shall reflect into a

parallel beam, in a required direction, the angle of light

F C

emanating from a flame F.

The point

prism C, and the required direction

C T

C F

F, the apex of the

being fixed, place the

centre of the instrument on C, with the axis of the upper

limb

coinciding with the line of the emergent ray

and keep the upper limb in while the instrument

this position

being used.

lower limb, bringing the perpendicular

C, until the angle

plement (which

marked above

between

L and

the

Xow move

round the

C L towards the ray ray F C and its com-

to save performing the deduction

it)

slight pressure,

mentally

reads the same as the angle from

A towards

244

LIGHTHOUSE ILLUMINATIOX.

K, and the angle from

which

70|°,

prism, and the angle

To determine the

j\I,

and C

;

and

then

draw the dotted

side

line

Bisect the

M "W perpendicular

meets the medial bisecting line

P C H, and from

refracted so as to fall on the side

the point

to the bisecting line

p 0^ M, and

from the focus

;

parallel to

the

will be

0^ p, at such an angle as

]\I

and will consequently be refracted

p AV

all

to ensure their being reflected in a direction parallel to

emerge from

E C

cuts the lower limb at X,

the same reading at Y.

p "W perpendicular

upon the

rays falling

^°

the two sides of the

are the tangents to the curve

]\1

in the

complement of

S coincides with 70

fix

M draw the

then bisect the angle line

observe where

to the bisecting line, until

draw the

For example,

TCP will be equal to the angle F C H.

and mark off C H, and from

N towards

reflecting side,

viz, 10f°,

angle

cuts at 19J°, the

and in no other position of the two limbs

G H

can this occur.

parallel to

towards

and 19|° from

A towards K,

from

F C which

figure the ray

at the side

C T and p

M C,

p, so as to

Photometers.

Many which are

photometers have been proposed, the simplest of

the comparison of the shadows of two lights which

moved

sities are

until the

shadows are equal.

of course ascertained

The

by the law

relative inten-

of inverse squares

of the distance.

In Plate in

1860.

XXIV.

Fig. 3

are

liquid photometer, in

rubber bag,

filled

shown two

forms,

which

I proj)Osed

shows, in cross section and elevation, a

which A B

a compressible india-

with diluted ink, and in order to

length of fluid passed through by the light there

alter the

a milled

245

MISCELLANEOUS SUBJECTS. Leaded screw, E,

compressing the bag, in wliich there are

for

two pieces of

fitted

Eig,

glass.

4 shows a photometer on

a different principle, which I described in 1860, in which rays

through a small hole

are admitted

of light

allowed to diverge into the tube

which the

light diverges

which the

sliding tube

By is

this

h g

made

and

the distance through

measured by the distance

mode, as the light

c b

ÂŤ,

has been drawn out.

divergent, its intensity

reduced in the inverse ratio of the squares of the distances.

shows another instrument on the same

Fig. 5

which a lens

principle, in

substituted for the small hole in the diaphragm

of the photometer last described.

which I have adopted

that

The

shown

latest

m Fig.

arrangement

139.

It is a

box with openings in two adjacent through which the rays pass

sides,

from the illuminant L, used

lEkvafiffn

for the

purpose of comparison, and from L'

The

the light to be compared.

through these open-

after passing ings,

on two planes

fall

<^.

rays,

the JPIart

points

d and

beneath

the

which

are directly

eyepiece

vation.

rays

through

which the operator makes

his obser-

The planes on which the

fall are

horizontal,

inclined at 45Â° to the

and 90Â°

planes of the rays.

to the vertical

The opening

through which the ray from

L passes Fig. 139.

of constant area,

and the illumi-

nant should be placed at a fixed distance from that opening.

The opening

through which the ray from L' passes can be

enlarged or contracted by means of two opaque plates moving

246

LIGHTHOUSE ILLUMINATION.

diaG;onally

and attached

the pLites there

When

light admitted. light, a candle, is

to the double screw

one of

which shows the proportion of the

a scale it

wanted

to find the

known

Carcel burner, or other

power of a

unit of light

placed at a fixed distance from the fixed opening; the

variable opening h

then brought round

judged of shines directly through turned

till

it,

till

the light to be

the screw

both lights appear of equal intensity,

number on the

scale gives the

power

then

when

the

of the light.

Time of Lighting and Extinguishing Lighthouses.

The lamps

are

lit

in most lighthouses at sunset and ex-

tinguished at sunrise, but in northern latitudes, where there are long periods of twilight, this

quite unnecessary,

woid-d occasion a needless expenditure of

Stevenson, with

the

assistance

the

oil.

late

and

Mr. Eobert distinguished

Professor Henderson, Astronomer Eoyal of Scotland, deter-

mined the mean duration

of the twilight in different latitudes,

from which tables were calculated

The Scotch lighthouses

away

lights at

weather be

earlier period.

and extinguishing lightroom.

for

the

day in the year.

are therefore lighted from the going

of daylight in the evening

ing, unless the

for every

dull,

till its

when

return in the morn-

the keepers exhibit the

Tables of the times of lighting

each day of the year are hung in each adoption of this plan of lighting an

annual saAdng resulted for the Scotch

lights, in

1861, of

£1600, representing, at 5 per cent, a capital sum of £32,000

— (Eoyal Commission Eeport, 1861,

vol.

p. 189.)

CHAPTER

FOG -SIGNALS. Fog-Signals are of two kinds,

(1)

the luminous, wliicli are

produced during fogs by accessions to the light which usually shown in clear weather

;

and

(2) those wdiich

are

produced by sound.

LUMINOUS FOG -SIGNALS. Different

modes have been proposed

creasing the power of a light

I know, the

first

of the kind

The

Ayrshire, in 1826.

method

Sir

when

far as

was by Mr. Wilson of Troon, in

effect

was produced by the simple

David Brewster,

optical apparatus

for temporarily in-

fog comes on.

of increasing the supply of gas

burners.

action

when

and the number of

1827,

proposed special

which could occasionally be brought into

the state of the atmosphere required

it.

The

exhibition of Bengal and other lights was also proposed by him

and

others.

Mr.

Wigham

several places in Ireland

has very successfully introduced at fog-signals

produced by separate

optical apparatus, placed one above another, each of

lighted

by a burner

of large size.

The

which

excellent effect pro-

duced has been proved on the authority of Professor Tyndall, as well as

by captains

of vessels

which regularly pass along

the coasts where these lights have been established.

TyndaU has found

that

when

Professor

other lights were invisible the

LIGHTHOUSE ILLUMINATION.

248

illumination of the fog was not only sufficient to attract the attention of the sailor, but that even the variations in the

volume of the flame were

distinctly visible in the haze. Fig.

of Mr.

140 represents that form

Wigham's apparatus which

he terms the quadriform arrangement,

which

four

separate

burners and lenses are employed.

The burners have been already described in Chapter VIII.

The

optical objection to such

arrangements parent, for

sufficiently ap-

the only rays from

each burner that

are

correctly

which

utilised are those

the lens which has

its

fall

focus in

that burner. This objection must,

however, be regarded as of less

importance than the advantage of having, at immediate disposal,

when required, so large an amount of light. In every case the practical

must certainly

result

be looked on as more important

than the merely economic. occurred to Fig. 140.

more than one

in 1875 to design

apparatus which could be applied flame, so as to carry out Mr.

Wigham's pro-

posal without any loss of light, and although such an arrange-

ment may be not very suitable

an ordinary flame,

quite applicable to the electric light. flames,

Fig.

would be

141 //' are two

the upper and L' the lower lenses or cylindric re-

249

FOG -SIGNALS.

The rays escaping below the

fractors.

lenses JJ of the lower

apparatus are returned again to the focus/' by pieces of spherical mirror

m, while at the top of the lower lenses or re-

fractors there are placed "

Back Prisms

(Chap. III.)

These

Fig. 141.

prisms

will, in this case,

have conjugate

foci situate in the

upper and lower flames (/and/'), so that the light escaping above the lenses or refractors of the lower flame f' and below those of the upper flame

The rays escaping above the

and in a

ii,

the conjugate

be situate respectively in the upper flame,

point/''' equidistant above the prisms P.

converging towards this upper focus a parabolic conoid

not as hitherto

utilised.

lenses or refractors of the upper

flame /will be intercepted by similar prisms p foci of wliich will

and

will be intercepted

The rays

will be intercepted

of novel form, presenting its convex,

its concave,

surface to the rays.

and

This com-

250

LIGHTHOUSE ILLUMINATION.

pound arrangement

for the

upper rays might be avoided by

the ordinary cupola of reflecting prisms described, though less perfect,

property of this instrument rays of light which

is to

upon

fall

;

but what has been

more compact.

The

optical

render parallel the converging

In this way the separate

it.

flames and agents will be optically combined, so as to utilise all

the light.

Mr. Douglass's burner, of single and double power,

for use

in thick weather, has already been described in Chap. VIII. J/r. Douglass's

at high levels, is

Movable Lantern.

when

â&#x20AC;&#x201D;The prevalence

of fogs

places at lower levels are free from

it,

well known, and in order to meet this evil Mr. Douglass

introduced at the South Stack Lighthouse, Island of Anglesea,

a portable lantern, which moves

during

fogs, to

give

The arrangement

down

a railway, so as,

light close to the level of high water.

its

shown on

Fig. 1, Plate

XXXII.

SOUND FOG -SIGNALS.

The various modes

of producing

sound

for fog-signals are

very briefly given in the following statements,

many

which

are taken from IMr. Beazeley's paper in the Transactions of

the Institution of Civil Engineers Bells.

â&#x20AC;&#x201D;In 1811, two

bells

were supported on standards on

Eock Lighthouse, for the purpose of They are tolled by the same round.

the balcony of the Bell diffusing

sound

all

machine which causes the lighting apparatus to revolve. Similar bells were used at the South Eock Island in 1812,

and since that date they have been

api)lied at

many

rock

stations.

In 1811, Mr. E. Stevenson designed a beacon

for the Carr

251

FOG -SIGNALS. Eock, already referred

which was surmounted by a

to,

bell

rung by a train of wheels, and actuated not by the waves, but by the rising and falling

The

tide.

was ad-

tide water

mitted through a small aperture, 3 inches in diameter, perforated in the solid

masonry

communicating with a

of the building, in which a float tide.

working model on

and kept in motion

was

itself

chamber in the centre and

to rise

with the

fall

was constructed

this principle

for several

months, but unfortunately, as

already described, on account of

beacon

low water, and

at the level of

close cylindrical

its

exposed situation the

In 1817, when

was never completed.

was

resolved to substitute an iron for a stone beacon, Mr. E.

Stevenson found that he could not apply a

some experiments with a whistle blown by the rise and

fall of

the tide

;

sufficiently satisfactory to

bell,

and

tried

compressed by

air

but he did not consider the result

warrant

thought of employing a phosphoric

its

adoption.

light,

but no

also

trial of it

seems to have been made.

For the purpose of

distinction,

Mr. Douglass, at the Wolf,

in 1870, employed a bell struck three times in quick succes-

At Dhu Heartach,

sion every 15 seconds.

Stevenson introduced another distinction,

in 1872, Messrs.

a bell rapidly

viz.

rung by rotating hammers, producing a continuous

rattle for

10 seconds, the intervals of silence being 30 seconds

1875 Messrs. Stevenson

also

employed

Kempock

;

and in

Point, on

the Clyde, two bells of different notes, struck immediately one after the other, to distinguish

station

Kempock from

remarked

in passing, are

a neighbouring bells, it

may be

worked by gas-engines. Bells

are also

where there was a single

bell.

These

frequently fixed on buoys, which are intended to be rung

the motion of the waves

;

but owing to the necessarily heavy

construction of these buoys they do not ring imless the sea

252

LIGHTHOUSE illumixatiox.

To obviate

greatly agitated.

pend under the water, and sinker,

tliis,

I proposed, in 1873, to sus-

below the buoy, a heavy

which arrangement virtually forms a shorter pendulum,

thus increasing the number of oscillations in a given time. Bells used as fog-signals vary from 3 cwt. to 2 tons in weight.

Gongs, struck

Ijy

hand with padded hammers,

are largely

used, especially on lightships, but they produce very feeble

sounds, and are rapidly being superseded by more powerful apparatus.

Guns, fired at intervals, are largely employed as fogsignals,

but they are not very

efficient,

not only on ac-

count of the long interval which must necessarily intervene

between the

shots,

leeward, makes is

but the report, although very effective to

little or

no headway against the wind.

This

in accordance with the ascertained fact that the best carry-

ing sounds are those which are prolonged or sustained for

some seconds, such

as are

and the report of a gun being short and sharp.

produced by whistles or

sirens,

of course defective in this respect,

The guns used

12 or 18 pounders, with 3

lbs,

for this

purpose are

charges of powder, fired at

intervals of from 15 to 20 minutes.

Mr. Wigham has with

considerable success used a charge of gas instead of powder. Bockets.

â&#x20AC;&#x201D;Experiments were made in 1877 by the Trinity

House on the use rockets

oz.

powder

gun

of explosive rockets as fog signals.

to 8

oz.),

cotton,

which are made of

These

either cotton

on reaching a certain elevation, ex-

plode with a loud report, and the results, as reported by the

Elder Brethren and Professor Tyndall, were very encouraging, finding, as they did, that the rockets

were better heard than

the gun, though less effective than the siren.

At rock

Light-

FOG -SIGNALS.

253

house stations the Trinity House thought they might be used

with advantage in place of be safely stored and

bells,

fired.

provided the material could

The great space wliich

re-

quired for the ordinary class of fog -signals, besides other objections, precludes the possibility of

rock stations

;

rockets,

employing them

on the contrary, can be stored in any

lighthouse tower.

Wliistles.

â&#x20AC;&#x201D;The

late

Mr. A. Gordon in 1845 suggested the

combination of a steam whistle with a

reflector.

proposed to place a wliistle sounded by

air or

Bordier Marcet parabolic

reflector, or in the focus

reflectors

also

steam in a

common

to three

grouped together, and revolving on a

vertical axis.

Mr. Daboll in 1850 de^dsed a whistle sounded by compressed

air,

which was

tried at

Beaver Tail Point with ex-

cellent results.

In 1856, Mr.

W. Smith

patented a combination of four

whistles, with their sounding edges straight in plan instead

of circular.

In 1860, Mr. E. A. Cowper designed a fog-signal in which he proposed to employ a whistle 15 inches in diameter, placed in the focus of a parabolic

trumpet 25

feet long.

also

suggested the propriety of making the intervals and character of fog-signals to correspond with the character of the light to

which

it is

auxiliary.

In 1860, Mr. Beazeley, in his paper (Min. stated that whistles might produce a lower

This proposal was

alternately.

carried

Instit. Civ.

Eng.)

and higher note

out at

the Cloch

Lighthouse on the Clyde in 1874, on the recommendation of Messrs. Stevenson, and

kind which

now

believed to be the only signal of the

in use.

LIGHTHOUSE ILLUMINATION.

254

In 1861, Mr. Barker proposed sounding a fog-signal,

for

Partridge Island,

New

to use superheated

wliicli

now

steam

operation

Brunswick, and other places.

Whistles, varying in diameter from 2| to

18'''

in diameter,

were experimented on in America, with the result that those of 10'' to 12'' were

recommended

best results were obtained

proportions

— diameter of

for

with whistles of the following

bell equal to

|ds of

its

and cup from i

length,

the vertical distance of the lower edge above the to

The

ordinary use.

of the diameter, for a pressure of from 50 to 60 lbs. of

steam.

At is

Partridge Island,

New

Brunswick, the steam-whistle

between 5" and 6" diameter, with a pressure of 100

per square inch. diameter,

At Aberdeen harbour

and the pressure 60

lbs.

Professor Tyndall's experiments, to be to,

the largest whistle used was

inches high, with a pressure of 70

the whistle the

inch;

lbs.

and in

afterwards referred

10 inches diameter, 18

lbs. to

the inch.

There have been at different times various suggestions

made

for increasing the

power of fog-whistles and

for

work-

ing the apparatus automatically, but the whistle has in a great measure been superseded

by the more powerful

siren

and reed trumpet. Courtenays Automatic Whistling Buoys.

— This

excellent

device has been already described in Chapter V.

Reed Trumpet.

—In

this

instrument the sound

produced

by compressed air escaping past the edges of a vibrating metal tongue or reed placed in the small end of a trumpet.

1844 Admiral Tayler suggested reed pipes sounded by condensed air to produce the sound for fog-signals. Mr. Daboll, in 1851, designed a trumpet

having a metallic reed, also

255

FOG -SIGNALS. sounded by compressed

air,

Tail Point in America.

one of

wliicli

was placed

Beaver

have been distinctly

It is said to

heard at 6 miles distance against a light breeze, and during a dense fog

;

and Captain Walden

at 2 1 miles distance, in a rough

states

sea

that he heard

Mr. Daboll also patented, in 1860, the

cross wind.

and with a strong aj^plica-

tion of condensed air to the soimding of whistles or horns,

with machinery to produce the revolution of the horn whilst

The Daboll horn has been introduced

sounding.

at several

lighthouse stations in America and in this country.

In 1863, Professor Holmes devised a trumpet, having a reed of

German

sounded by compressed

silver

air,

with an

automatic arrangement whereby the trumpet can be placed at a distance from the air compressor and engines. is

constructed to vibrate in unison with

This trumpet

its reed,

been successfully established at several lighthouse

and

has

stations.

In Professor Tyndall's experiments the reed was 11 inches

3y X

long

long,

and ^ inch

and

I'',

thick.

pressure reed 51'^

Pladda

it is

If inch broad, 8| inches

Mr. Beazeley recommends for a high

x ^" X jV''

^^'^

^*^^'

^ -^^^ pressure reed

6rx2rxxV. Siren.

pressed

â&#x20AC;&#x201D;This

air,

instrument

sounded by steam or com-

escaping in a series of puffs through small

slits

in a disc or cylinder caused to revolve with great rapidity,

and

fixed in the small end of a trumpet.

In 1867, Mr. Brown of

New York made

experiments with

the siren, and Professor Tyndall subsequently experimented

with

this

House

instrument

;

that the " siren

ful fog-signal is specially

and in 1874 he reported is,

which has hitherto been

powerful

when

to the Trinity

beyond question, the most powertried in England.

local noises, such as those of

mnd,

LIGHTHOUSE ILLUMINATION.

256

rigging, breaking-waves, sliore-surf,

have

to be overcome.

tration, render

and the

dominant over such noises

signal sounds have succumbed."

be affirmed that in almost steamers with paddles going,

upon

cases

all

may be

may with the

relied

In the gTeat majority of cases

at a distance of 3 miles,

and peneother

after all

Professor Tyndall, in his

report, already referred to, states that it "

of 2 miles.

rattle of pebbles,

Its density, quality, pitch,

siren,

certainty

even on

on at a distance it

may

be relied

and in the majority of cases

a distance greater than 3 miles."

In Tyndall's experiments the diameter of the disc was

making sometimes 2400 revolutions per minute

inches, at St.

Abb's Head and Sanda the diameter

making 1464 revolutions per minute. The trumpets, which have been referred with reeds and

12^

;

4i-

and

inches,

to as being

used

vary from about 8 feet long, 2 feet

sirens,

diameter at mouth, and 2 inches at small end, to about 17 feet long, 3 feet

diameter at mouth, and 3 inches at small end.

The pressures with which they from 5

lbs.

are sounded vary as

per square inch

;

about 20

lbs. is,

much

however,

a very effective pressure.

Motors.

窶認or the purposes of compressing the

trumpets or etc.,

sirens,

and

for actuating the

air for reed

hammers

of bells,

various motive powers have been proposed or adopted,

which the following (1.)

summary

Clockwork driven by the descent of weights or the action of springs.

(2.)

Water-power applied

(3.)

Eise and

to wheels, turbines,

and water-

enrines of various kinds. fall of

the tide

;

advantage being taken of

the hydrostatic pressure thereby produced, or of

257

FOG -SIGNALS. the natural tidal currents, or of

currents

artificial

produced by impounding tide water

;

or lastly, of

the motion of waves. (4.)

(5.)

Steam-engines of various types.

Hot air engines, either as designed by Professor Holmes on Stirling and Ericsson's principle, or as constructed by Messrs. Brown of New York, in which the loorldng air is heated not by mere contact

with a metallic surface, but by being

passed through the pressure), (G.)

(which

and

for ringing bells

;

gas, Keith's mineral oil

air gas.

power

of these being of small

first

principally used

the second and third are applicable only

where the necessary

in situations

therefore under

and supports combustion.

Gas engines worked by coal gas,

The

fire

itself

present themselves

;

and

]3hysical

characteristics

the fourth requires a large volvune

of water, not always to be got at lighthouse stations,

\s,

liable

to explosion in unskilful hands, and takes a long time to start,

which

a very objectionable feature, as fogs frequently

come on suddenly. This last objection also applies to hot air engines, which require no water, and are otherwise unobjectionable objection,

water.

it,

;

but, while the gas engine

It will thus be seen that

which form must be considered on

ralise as to

of motor its

own

from

this

impossible to gene-

the best, but each case

The horse -power

merits.

necessary at a fog -signal station depends duration,

free

on the other hand, requires a small supply of

and frequency with which the

well as the range or distance at which

sound should be heard.

It varies

on the pressure,

blast is,

emitted, as

desirable that the

from 8 to 20 horse-power.

258 The

LIGHTHOUSE ILLUMINATION. investigations

Tyndall,

and

which show

that,

Henry,

Professors

Stokes, have elicited very important facts,

independent of the efficiency of the generating apparatns, there are disturbing causes in the atmosphere

which

-itself,

certain times modify the effects of fog -signals of every kind.

Although the findings of Henry and Tyndall may be

re-

garded as to a large extent the same, the causes which influence the unequal transmission of sound are differently accounted for

by as

these philosophers.

Professor

Henry

explains the effects

due to the co-existence of winds blowing in

different direc-

upper and lower strata of the atmosphere

tions in the

;

while

Professor Tyndall, whose extremely interesting narrative of

the investigations

made by him

for

well worthy of attentive perusal,

the Trinity House

attributes

the unequal

transmission of sound in different directions to what he has

termed flocculence, or want of homogeneity in the density of the atmosphere.

Professor Stokes has proved the important fact that the

inequality and the velocity of the wind as

we ascend

atmosj)here have necessarily the

of disturbing

effect

regularity of the transmission in spherical shells.

into the

the

states

that " near the earth, in a direction contrary to the wind, the

sound continually tends to be propagated upwards, and consequently there

a continual tendency for an observer in

that direction to be left in a sort of sound shadow. at a sufficient distance, the

enfeebled

;

sound ought

be very

Hence,

much

but near the source .of disturbance this cause has

not yet had time to operate, and therefore the ^\dnd produces

no sensible

effect

except what arises from the augmentation

on the radius of the sound- wave, and perceptible."

this is too small to

This result should be kept in view in dealing

with places at high levels above the

sea.

259

FOG -SIGNALS.

phenomena cannot

these atmospheric

of course be

overcome, being entirely external to the apparatus which

produces the sound,

manual with

in this

ever, of a

The

will not be necessary to

details of the results,

occupy space

which

are,

how-

very interesting character.

results arrived at

by Professor Henry

will be found

in the Eeports of the American Lighthouse Board and the

Smithsonian Institution. vestigations undertaken

Trinity

House

The important narrative of the mby Professor Tyndall in 1873 for the

given in his Pteports to that Board.

Professor Henry, in his Report of 1874, speaks

somewhat

disparagingly of reflectors for condensing sound-waves, though

he states that better results might be expected were the

made

flector

reflector

to intercept

with which he made his experiments. if

sufficient size to suit the

ampHtude

rigid

reflect

There can be

such instruments could be made of

no doubt, however, that

enough

re-

waves of sound than the

of the

waves of sound, and

and not absorb them, they would

enormously increase the intensity in the direction of their

The experiments about to be described, with a small sized reflector at Inchkeith, showed a greatly increased effectiveness at moderate distances from the instrument, and there axes.

seems every reason to expect that the intensity of sound

would increase with the dimensions In 1866 I suggested the plan all

of the reflector.

ot a reflector for utilising

the sound waves (as in the case of light for illuminating

apparatus for lighthouses), under the

which

name

of the Holoplione,

thus described in Eoy. Scot. Soc. Arts. Trans. (12th

February 1866)

"Fig. 142 represents the front elevation of an arrangement for sound over 180째, and Fig. 144 that for condensing the

diffusing the

sound into one beam.

The

action of these instruments will be easily

260

LIGHTHOUSE ILLUMINATION.

Fiff.

Fig. 143.

142.

understood on looking at the diagrams.

142 and 143 are elevation and

Figs.

section of reflectors, which, parallelise

the

in the

rays

'BCD

vertical

plane only

are paraboloidal strips for act-

ing on the waves of sound produced

by the anterior half

of the whistle, or

other radiant placed in the focus.

F G ing

are similar

the

strips for parallelis-

waves proceeding from the

posterior half of the radiant.

H H

are hollow cones for reflecting forward

the rays parallelised by " Fig.

which and

reflects the

44 represents the holophone

sound in every plane,

are the side reflectors,

which

in this case, instead of being conical are plane, Fig. 144.

and placed

an angle of

45째 with the lateral beams of sound.

261

FOG -SIGNALS.

The

the whistle, bell, siren, or whatever radiant

whistle

may

to be

shown

either be placed vertically, as

In order

or horizontally, should that position be found preferable.

reduce the itself,

loss

sound passing

off

employed.

in the dramngs, to

vibrations of the metal

by the

the backs of the reflectors should probably be lined with plaster

of Paris

some other

practically useful,

suitable

To render

material

the reflector

should of course be mounted on an axis

(as

has been done in the large horn recently constructed for the Trinity

House

London by Mr.

Daboll), so

admit of being turned

round, and thus, like a revolving light, to reach successively every azimuth.

It will

be observed that in these designs a hemispherical

been adopted for returning the vibrations through the focus, as they might to a great extent be destroyed by impinging on By the arrangement the whistle, or other instrument placed there. reflector has not

shown the sound waves

are reflected in sucb a

manner

as to

meet with

no interruption.

By directions of the Commissioners of Northern Lighthouses, and with the approbation of the Board of Trade, a holophone 8 feet *'

by 16 feet, was constructed of malleable iron about one-sixteenth of an inch thick. " The result of experiments tried at Inchkeith, which were made with a low power of steam was, that, while at moderate distances of about 1^ mUe, the holophone was very much, louder than a naked between the

whistle supplied from the same boiler, the difference

maximum

distances at

which

have been expected from the

each,

could be heard was less than might

results obtained at lesser distances.

while at the distance of 2 miles the holophone was very the naked whistle died out at 3

3 and 4 miles.

must be kept

miles,

far

efficient

Fogs in but often

use of fog-

for that the

holophone

about 6 hours in duration,

20 hours, and the longest recorded in the

Northern Lighthouse service

number

and

than the simple whistle."

this country average

last

Thus, louder,

and the holophone between

in view that the great

signals is for comparatively short distances,

was

much

of hours of fog per

70 hours

annum

;

while the total

about 400.

262

LIGHTHOUSE ILLUMINATION.

MEANS OF DISTINGUISHING VESSEL'S FOG-SIGNALS FEOM THOSE AT LAND STATIONS. seems highly desirable that there should be a radical

distinction

them

between the fog-signals used by vessels

of their approach to each other,

and those

warn

em-

signals

ployed to mark headlands which vessels are to avoid; and so long as no

which are

law

to be

sible to insure

for the regulation of the signals

exists

employed on board of

ships, it is

not pos-

such a distinction as to make their sound

clearly distinguishable in all cases

from a fog-signal at a

land station. Messrs. Stevenson, in a Report to the Northern Lights

Board, suggested that this difficulty might be overcome were

some

legislative

enactment passed that the signals made by

steamers afloat should in

all cases

be steam whistles of a

shrill

character, like that of a locomotive, while those used at light-

house stations should be, as at present, the deeper sound of the fog-horn or siren;

employed they

or if whistles are

should give two different musical sounds, as at the Cloch

Lighthouse on the Clyde.

MR. A. CUNINGHAM's OBSERVATIONS OF THE RELATIVE VISIBILITY OF DIFFERENT COLOURS IN

Mr. Cuningham,

late Secretary to the

house Board, instituted a

series of

FOG.

Northern Light-

experiments on this sub-

ject at 11 of the Scotch Lighthouse stations,

by means

of a

horizontally placed spar of wood, coloured black, white,

and

red,

and the distances

visible results

which the colours ceased

were observed by the lightkeepers, with the following :

FOG -SIGNALS.

263

APPENDIX. MATHEMATICAL FOEMUL^. The

object which I kept in view in the

to explain the general

preceding pages was

on which every lighthouse

principles

â&#x20AC;&#x201D;the

optical agents which have from time and the combination of those agents, so as to form arrangements by which the optical problems of the lightIt would be obviously out of house engineer have been solved.

apparatus

designed

to time been invented,

place, in a practical

handbook such

as this, to give the different

steps in the mathematical theory of those oj^tical instruments.

All that appears necessary for the engineer

to be put into

more important formulae, which are accordingly appended. For those who wish to satisfy themselves as to the steps which lead to these formulae, reference must be made possession of the

some of the following

authorities.

Augustin Fresnel, in addition to his invaluable inventions,

which have already been

fully described in

Chapter

II.,

investi-

gated mathematically the principles on which they depend.

Alan Stevenson, besides many improvements which he originand to which reference has already been made, gave, in the "Notes on Lighthouse Illumination," which accompany his "Account of the Skerryvore Lighthouse" in 1848, the first published account of the formulae of Fresnel, to which he added mathematical investigation and tables of the calculated elements ated,

In his of totally reflecting prisms for lights of the first order. " Eudimentary Treatise on Lighthouses," published by Weale in 1850, these were again published. Professor Swan, in his

memoir

On New Forms

of Lighthouse

Apparatus," in the Transactions of the Eoyal Scottish Society of

265

APPENDIX. Arts 1868, besides his independent invention of (Chap.

II.),

and

his proposal of the

tlie

back prisms

method of sending through

the interspaces of apparatus, light coming from other apparatus placed behind (Chap.

II.)

given

moreover,

has,

has also devised several other dioptric

II.),

lighthouse agents (Chap.

very

complete

mathematical

analysis of the formation of a great variety of lighthouse prisms,

including in the more important cases the consideration of excentric rays, so that

due the

first

light

may

escape reflection.

To him

also is

mathematical investigation of the double reflecting

prisms employed in the dioptric spherical mirror, as well as of the " back prisms," (Chap. II.) of his work, he largely than

may

Looking to the extent and character

be regarded as having contributed more

any other writer since Fresnel to the mathematics

of lighthouse illumination.

Mr.

J. T.

Chance published other modes of investigations re-

lating to the instruments which were then

known,

in the

Minutes

of Proceedings of the Institution of Civil Engineers in 1867, (vol. xxvi.)

Allard^s formulae for lighthouse statistics have already been

Besides these, reference may be made to Reynaucl " Sur I'eclairage et les Balisage des Cotes de France," published in 1864; Corwar/Zia, " Sulla Verificazi one degli Appa-

given in Chapter VII.

recchi Lenticolari per Fari," published in Nehls's

"Anliang"

to the

German

Lighthouse Illumination, 1871. lished at

Hanover

1874; and Chridopher

translation of

Nehls's translation

book on was pub-

1878.

Abstract of Professor Swan's Paper " On New Forms OF Lighthouse Apparatus," in the Transactions of the Royal Society of Arts, 1867;

furnished by

the

Author. Totally reflecting lighthouse prisms, in order to distinguish

them from other kinds

of dioptric lighthouse apparatus, have

266

LIGHTHOUSE ILUMINATION.

hitherto been denominated

For

word

have

l^roposed to substitute esoptrlc (from eo-oTrrpov, a mirror), as

suffi-

cafadiojjtric.

this

ciently indicating the distinctive property of a reflecting or mirror-

prism; and also to denote prisms as monesoptric, esoptric,

diesoptric, tri-

according as they reflect internally the intromitted rays

of light, one, two, or three times.

General formulae for the construction of Monesoptric Prisms are

first

investigated, for which,

and

for mathematical demonstra-

tions generally as not falling within the treatise, the reader is referred to the

scope of the present

original

memoir.

These

formulae suffice for the construction of any monesoptric prism

whatever, of which the following examples are here given

ÂŤL

_iL

Fig. 145.

In the diagram,

the luminous origin

of the prism perpendicular to

its

faces

;

and

AHB in the

a section

same plane

with 0, and D, the extreme rays of the pencil of light A, incident on the face A C. The index of refraction being given it

required to determine the form of the section

where

that the rays

X are given, flection at

E 0', whose

are straight lines,

O A,

shall

and

AH B

AHB

a circular arc, so

Avhose inclinations to the horizontal axis

be transmitted, after refraction at

B, and refraction at

C, in the directions

inclinations to the horizontal are also given.

C, re-

0',

267

APPENDIX.

In the following formulae the notation (that of the original

memoir, slightly altered for the sake of simplicity) DOa-

= a^ AOx = ttg ,

EOV = Ug

BO'a;'

as follows

= a^

;

ACB = i,BAC = A, ABC = B, LAO - e angles of refraction at D and A, and of internal incidence ;

and the at

and B, which

shown

are not

in the diagram, r^

respectively

BC = «, AC = 5, AB=:c, AO = d, AD=/, BE = A,

AHB = p

and the radius of curvature of

BE' =

FremeTs Prisms. (Chap.

II. p. 65.)

In Fresnel's prisms the extreme ray

BD,

the intromitted rays AE,

From

the sides AC, BC.

these conditions

sin J

from which

i is

(ttj

—

incident at C,

and

are respectively coincident with

= /*

cos

shown that

it is

most readily obtained by

trial

and the dimenby the follow-

sions of the prism are then completely determined

ing formulae

;

where

has been assumed that the prism, as in

Fresnel's original construction, emits the rays BO', EO', horizontally.

sin i\

sin ^

a^)

sin ?2

sin 1

- 2 a^

A = 90°-i*-hi(ri-g, B=90°-Ji-i(r,-r,), C=i; sin (ttg — a^) sec \ {i — a^ h = a = b sin A cosec B c = & sin C cosec B - r^) = I c cosec i

;

(?-j

In this

case,

and

in that also of the

referred to the original

memoir

back prisms, the reader

for formulae for the co-ordinates

268

LIGHTHOUSE ILLUMINATION.

employed able to

in their manufacture and it lias been thought advisadd here numerical examples of both cases. :

Examjyle. Computed.

Given.

= 34° = 38°

A= B=

53', 47',

a3=180°, a4=180°,

= 30,

/>i=l-54

(z;i(/e

note, p. 270).

0",

33° 15' 32°

38",

114° 35' 23",

= 2-7378, = 2-6579, c=4-5405,

, *

= 238-7960.

Back Prisms. (Chap.

11. p.

91.)

The form of these prisms is determined on the principle that among all the rays of the intromitted pencil, LA, which is incident least advantageously for total reflection, shall nevertheless be totally reflected at the " critical angle " ^, whose sine is equal

;

where Mr

the index of refraction for the least refran-

It is also

gible "rays of light.

are equally inclined to the faces

The flame,

distance

and the magnitude of the flame

accuracy.

If,

LAO =

being both supposed

itself,

can be found with sufficient

then,

be a subsidiary angle assumed, so that tan K

it is

of the prism.

of the prism from the assumed focus in the

to be given, the angle

and

assumed that the rays AO, EO'

AC BC

shown, that

on AC, we

(j>

shall

{(f)

tan i

denote the angle of incidence of the ray

have

e)=

7 cos k

269

APPENDIX. a relation from whicli,

and

being known,

may be

<\>

readily obtained

found that

lastly, it is

i^TT

— Olg — 2^,

ttg

In the case of a prism emitting focal rays horizontally, a^ = a^ = 180° ; and the formulae which completely determine its various dimensions are as follow

D = e+^ + a2tan K

(<^

——

sin

?-j

[i-^

cos k sin |

= a2 ~

^*^

= isin

— a^ — %),

1 sin J (2 a^

sinr2

tan \

sin

90°,

| ("2-^),

A = 90°-ii-i(r2-^'i), '

B = 90°-ii + i(r2-r,), C = i. /=(Z

=/ cosec ^ (2 + i = c sin B cosec C z

—

A-

(2aj— ag— *'),

sin (ttg— a^) sec |

c sin

sin c

A cosec 1

rg)

cos r^

3 r^

r^ sec

i <<}\-r^

sin

Light which would otherwise fall uselessly between D and C is intercepted by an adjacent prism or other optical agent, and superfluous glass

may be

but better by a

line joining

extreme ex-focal ray so that

would

none of the

cut off

may

sin r\ li

sin

}

line joining

after reflection at

light

usefully send out inclined

then BE' or

by a

with the point

=i i

lost,

(

and E,

which the

meets the side

;

which otherwise the prism

downwards.

sin

E', at

+ r^- 1\ +

4/2) sec r\

270

LIGHTHOUSE ILLUMIXATIOX. Example. Given.

Cotnputed.

A =70°

114°,

0,3=180°,

a^=180°, e

a=10-SG59, &=10-9759, c=7-1182,

= 6°,

d= /Xp

4:0,

42",

B=71°49']0", C = 38° 2' 8",

«2=120°,

/= 5-1031,

l-50,

/x=1-.j4*

5-8691,

= 243-5763.

Besides the neAV monesoptric prism here described, I have

devised other forms in order to obtain deviations of light up to 180°, two diesoptric and one triesoptric.

—

When

Fresnel's or the

new back prisms

are emplo3'ed for de-

viations exceeding 90°, a portion of the light will side of the prism

and be wasted, unless that

fall

on a wrong

side be screened or

hidden from the luminous origin by an adjacent prism or other agent, as already noticed. for designing

ties

In order to obtain additional

facili-

lighthouse apparatus, I have proposed two

forms of monesoptric prism, which admit of being freely exposed to the light without

being in any

way masked

or hidden,

property which I have sought to exj^ress by the word

had long ago occurred

acryptic.

by an appropriate arrangemight be left through which to project back rays at ome in their proper directions, and of such arrangements I have described and figured two designs. For details generally regarding these and various It

ment

that

of the front prisms or lenticular zones, interspaces

other matters, the reader

referred to the original memoir.

Dioptric Spherical Mirror.

(Chap.

II. p.

87.)

Professor Swan's formulae for this instrument have already

been given. * St.

Gobain glass

now

1-53

;

Messrs. Chance's I'Sl.

271

APPENDIX.

Fresnel's Lenticular Apparatus.

(Chap.

The

following

formulae

II. p. 63.)

the

for

construction of Fresnel's

Annular Lens and Fixed Light Apparatus, as furnished by Professor

Swan,

are, in

accordance with the plan of this work, here

presented without demonstration

The

figure

and the other

D, whose

sides, of

which

side,

A B

perpendicular to

an arc of a

circle, x,

are

-F

Fig. 140.

straight lines, represents the section of a refi-acting zone, which,

according as vertical,

revolves about the horizontal axis,

will generate

or the

a ring of an Annular Lens, or a

refracting zone of a Fixed Light Apparatus.

Rays tion at

B, from the luminous origin at 0, after refrac-

and B, proceed

refraction parallel to

and

in the directions

emerge

A, (

B C

;

and E,

after

B, let

i/,

d be the angles of incidence on

the corresponding angles of refraction, S the

angle of internal incidence at C, and

gence at

in the directions

For the rays the face

and

ÂŤ, /3

the angles of emer-

272

LIGHTHOUSE ILLUMIXATIOK

Drawing

parallel,

and

M B=

perpendicular to

let

M a= C

M=f,

_ -2

tan

sin 9

P R=

the co-

and

?/,

— b)

sin 8

= 7,

C-1

cos

sni

tan ^

cos f

{a -13) cos

cos a

sin B

,* /x

(a sin

a;=/+ ^ -p The

n /5 '

{

tan

D, = p

jc,

t= i_

sin

e-1

/icos

tan^ = -^,

sin v

sin v'

we have then

= -^,

tan>/ sin

the centre of curvature

the index of refraction,

E, the radius of curvature of the arc

ordinates of

tan a

-/3 2/

2 8)

= ^~Psina.

radius of curvature, and co-ordinates of the centre of

ascertained, its profile

If the ray

of the prism

A B CD

being thus

completely determined.

M, and A with B, then become B L H, which,

coincide with

A B C D

the generating profile

according as

curvature, of the face

will

revolves about

will be that of the

central disc of Fresnel's lens, or the middle zone of his Fixed

Light Apparatus.

In this case the preceding formulae at once reduce to the following

,„^—

tan

sui

^ i

* Subsidiary angles k

tan K

raa}', if it

tan a = ,

—

•

sin

sni 6

tan

and X being assumed, ,

and tan X

be preferred, compute a and sin K sin e

sin (k — e)

j. ,

sin 6

Bn =

tan

sin 6 j3

cos C

so tliat

;

by the formulae

o sin X sin ?. f /3= sin

(\-

273

APPENDIX.

b-d +

tan C,

x-f+t-pcos/S, In

—

= -. sm p = o.

D, B H, will, by which the C D, may be cemented

construction here adopted, the lines

tlie

in revolving, generate the surfaces of the joints lenticular zone,

whose section

A B

to the adjoining portions of the apparatus original design

;

these joints were generated

while in Fresnel's

by the

lines

but the formulae serve for either construction.

;

According to Fresnel's method, as given by Mr. Alan Stevenson in the notes appended to his "Account of the Skerry vore Lighthouse " (p. 250), the radius of curvature of the surface L

of the lens, or middle refracting zone,

obtained as follows

near the point H, in the surface

First, for rays

L, there

found an expression for the radius of curvature equivalent to that already given. Next, for rays near the axis L, the radius is

obtained by the ordinary method for a small pencil incident

directly shall

on a

Denoting these two

lens.

radii

and

then have

compromise between these values

then made by assum-

ing, for the radius of the lens,

P=^i{Pi Example.

— For a

Pi)-

Fresnel's Fourth Order Apparatus,

we may

assume, in millimetres,

/=150, From /«.

d = iO,

= 66,

these data, for glass whose

1*54, the following dimensions

preceding formulae

audits.

index of refraction

have been computed by the

For the surface C D,

p=U0-\4:, For the surface /3j

= 91-01,

= 60-46,

y=— 10-62.

= 82-05,

= 86-53, T

x=66-47,

y=0.

LIGHTHOUSE ILLUMINATION.

274

Differential Mirror. (Chap. III.

104.)

Solution of Prohlem hj Professor Tait.

The complete example

solution of your problem

of a beautiful

easily given as

W. Suppose

method invented by

Sir

source of light, line

E. Hamilton.

be the

A B a vertical

from which the reflected

rays are to diverge horizontally.

Then

after reflection

they diverge surfaces of axis.

Suppose

ray reflected Fig. 147.

direction

be produced backwards, it meets P (suppose), and our condition is

P Q = constant. Hence = constant, i.e., the surface

which

cylindrical

AB P P

the

to be a

Then

in if

the

A B at right angles in S + P Q = constant. But also

S P +

the locus of

such that S

P-

such that the distance of each

from the axis A B diff'ers from its distance from by a constant quantity. and A B is thus The section by a vertical plane through that made by a horizontal as focus evidently a parabola with are the foci. a hyperbola of which A and plane through

point in

the luminous source

One

particular case of this hyperbola gives a horizontal straight

line bisecting

one surface

at right angles

fulfilling

â&#x20AC;&#x201D;

so that a parabolic cylinder

the conditions.

The general equation above may be written

from which we get the parabolic section by putting y = hyperbolic section by putting ÂŤ

and the

APPENDIX.

giving z various values

made by

get the sections of the surface

various horizontal planes

to y those

made by

275

;

and by giving various values

vertical planes parallel to the central parabolic

section.

But

may more

easily be constructed

by the

intersection of

the spheres X-

+ if + z^ =

with the circular cylinders (a

- xf +

+ cf,

and in various other ways, which might be given. The values of a, h, and c, depend upon the diameter of the front lens, its distance from the light, and the amount of divergence required. [Professor Tait has given anotlier tion in the Proc. K.S.E., 1870-71.]

mode

of dealing with this ques-

EEFEKENCE TO PLATES. CHAPTER

DESIGN AND CONSTRUCTION OF LIGHTHOUSE TOWERS. PLATES. I.

Sections on the same scale of the most remarkable light-

house towers, II., III.

modes

Various

p. 12.

the stones of lighthouse

of joggling

towers.

IV. Temporary barrack of timber used at the Bell Rock and Skerryvore, during the erection of the towers.

V. Temporary barrack of malleable

iron,

used at

Dhu

Heartach during the erection of the tower. VI. Section of Fastnet Rock, Ireland, illustrative of the the contour of the rock on the rise of the

effect of

waves,

p. 4.

VII. Iron lighthouse, exposed to floes of ice at Craighills

Channel, America.

Sections of the

work have not

been published. VIII. Stone tower, exposed to floes of ice at Spectacle Reef,

America.

Sections

the

work have not

been

published. IX., X.

Land

lighthouse

station

Ardnamurchan, Argyll-

shire.

XXI. Screw

pile light, erected at

Haneda, Japan.

CHAPTER

II.

LIGHTHOUSE ILLUMINATION. XXIII. Original drawing of the of the

first

order,

North Ronaldsay,

first

holophotal revolving light

designed

October

in Orkney, p. 89.

1850, for

277

APPENDIX.

CHAPTER

III.

AZIMUTHAL CONDENSING SYSTEM, PLATES.

XIII. Intermittent condensing light for light periods of 20° and dark periods of 40° and 10°, p. 121.

XIV. Intermittent condensing and dark periods of

light for light periods of 20°

80°, p. 122.

XV., XVI. Different forms of condensing

mirrors of

sj)herical

In Plate XV. the light which would

unequal areas.

otherwise be lost on the mainland

sent seaward

by a dioptric mirror. But to illuminate the sea up to the shore-line betAveen A and F, the mirror is

partially cut

arcs

AC, CD,

down

to an extent in each of the

inversely proportional to the

etc.,

lengths of the different landward ranges.

XXXII. The

mirrors

and C

hyperbola

common

axis,

of the flame.

(having the same

one of the

The

by and a

are formed respectively

the revolution of parts of an ellipse

foci

foci)

round their

being in the centre

rays falling on

A B are reflected

in the direction of the other focus, but being in-

tercepted by C D, are returned through the flame and go to strengthen the arc opposite C D, while the arc subtended by A B is correspondingly

weakened.

XXXIV.

Intermittent condensing light, with siDherical mirrors for light periods

24°,

and dark periods of

48°, p. 124.

XXXVI.

Fixed condensing light of uniform

intensity,

changing

from red to Avhite without dark intervals, p. 124. XVII. Condensing light for ships, shown in side elevation and middle horizontal section, in which A a A is 180° of fixed light apparatus, ])p, straight condensing prisms, H, spherical mirror. XVIII. Front elevation of above, in which a is 180° of fixed

278

LIGHTHOUSE ILLUMINATION.

PLATES.

light apparatus,

the straight condensing prisms,

the gymbals in which the apparatus

and hung,

117.

XX. Fixed condensing

octant,

used at Buddonness,

leading lights at River Tay, p. 109.

XXXVII. Condensing

Fig. 1 represents an which shows a fixed white

revolving light.

apparatus

existing

light over the greater part of the horizon, with a

red sector over a danger is

arc,

while a larger sector

entirely lost towards the shore.

In order to

strengthen to the greatest extent the light over

the danger arc (which has to be seen at a distance of 9 miles), there should be 3 sets of condensing

prisms P, each of which spreads the light (which at present lost) over the danger arc of 20°.

Owing

want of room in the lightroom we cannot and spread over the danger arc more

intercept

than 83°, while 52° are This lost light

following expedient spherical mirror

and the

still lost

may however :

—

in the dark arc.

be utilised by the

If a portion of a dioptric

be placed between the flame

cylindric refractor of the

main apparatus

opposite the danger arc, the rays incident on will

be sent back through the flame, so as to

and partly by But this arrangement manifestly greatly weaken the power of the

strengthen that

arc, partly directly

the condensing prisms. will

main apparatus

in the seaward direction

restore uniformity of strength over this arc, the unutilised part of

P',

the other seaward arc

light, less absorption, etc.,

the light going land-

wards must be deflected by twin (Chap. condensing prisms

weakened

III.)

spread

and over

The strength of the

over the danger arc will

thus be increased about five times by the con-

densing prisms P, and

twice

by the

spherical

279

APPENDIX. mirror, while that over the seaward arc will

still

be maintained of uniform strength by the prisms Finally,

P'.

B C

danger arc white

light,

by the

(Chap. will

substitution of

moving screens

IV.), for the red shades, the

made

show an intermittent

and thus be distinguished

from the rest of the apparatus.

in character

using white

instead of red, as at present, the power will be

increased about 4 times, so that by this means,

and the condensing prisms P, and the spherical mirror

D, the danger arc will be strengthened

(less absorption, etc.)

Fig. 2

about 28 times.

shows a mode of

utilising the

light

which is not intercepted by the spherical mirror, and which would be lost by passing landwards at stations where only a certain seaward sector has to be illuminated.

which

represents

EPLB

marked

revolving

light,

the

while those

revolve,

are stationary.

In the drawing,

parts

marked

a portion of a holo-

phote which projects upwards, in an angle inclined to the vertical, the rays which it intercepts. These rays

fall

on fixed expanding prisms F, at the top

of the apparatus, which cause

through

them

spread

half of the circle, in azimuth.

CHAPTER

IV.

DISTINCTIVE CHARACTERISTICS OF LIGHTS. PLATES.

XXII. Dr. Hopkinson's group flashing light, p. 136. XXII. Figs. 1 and 2. Drawing in illustration of ments on coloured lights, p. 140.

XXIV.

XXXV.

Fig.

Mr. p.

Reversing

Brebner's 136.

light, p.

form

experi-

129.

group -flashing

apparatus,

280

LIGHTHOUSE ILLUMINATION.

CHAPTER

ILLUraNATION OF BEACONS AND BUOYS. PLATES.

XXIV. XXVIII.

Pintsch's floating buoy, lighted with gas, p. 169. Different forms of buoys suitable for different localities, p.

XXIX.

177.

Courtenay's automatic signal buoy,

173.

XXX.

Malleable -iron beacon, generally employed in Scot-

XXXI.

Cast-iron beacon employed in Scotland at rocks ex-

land, p. 177,

posed to heavy

XXXII. Dipping p.

seas, p. 177.

lights for covering shoals

and sunk

rocks,

153.

CHAPTER

VI.

ELECTRIC LIGHT.

XIX. Mr. Brebner's Double /j

Lens.

â&#x20AC;&#x201D;In

the upper diagrams

the focus for parallel horizontal rays for the

lower part, and f^ the same for the upper part of the lens; the flame has been moved forward from/j to the position shown, in order to give the required

horizontal divergence; at the same time,

away and lowering the upper part

by cutting

of the lens to

an extent f^ /g, it is ensured that the rays of greatest vertical divergence from the upper half of the unaltered lens are sent to the horizon

the same as modified. half

The

light

from the lower

strongest in the direction of the horizon,

and becomes gradually Aveaker approaching the lighthouse for the light

;

an observer

the converse holds

from the upper part of the

that in the plane of the middle

lens, so

vertical section

of the modified lens, and approximately in the

281

APPENDIX. PLATES.

vertical planes at either side of this,

intense light

directed to

all

equally-

points on the sea

between the horizon and the inner boundary of the illuminated surface.

connection with

this

Mr. Brebner

lens

designed to give such a form to the upper and

lower totally reflecting prisms as would strengthen the light sent to the horizon or any other part of the sea where desired. Differential Lens, p. 187.

â&#x20AC;&#x201D;The lower diagrams show

elevation and horizontal section a differential lens for the

electric

In vertical section the

light.

inner surface of the lens

straight

;

in horizontal

section the centre of curvature of the inner surface is

taken so as to give to the light the required

horizontal divergence.

XXXII.

Fig. 4.

Lens

for electric light, as designed for the

Trinity House

by Mr. Chance.

CHAPTER

VII.

LIGHTHOUSE STATISTICS.

XXXIII. Graphic

Table, deduced from

M. AUard's work,

for

the ranges of lights, p. 202.

CHAPTER

IX.

MISCELLANEOUS SUBJECTS CONNECTED WITH LIGHTHOUSES. XI., XII. Mr. Alan Stevenson's diagonal lantern of bronze, and

Mr. Douglass's

helical lantern of steel, pp.

223.

XXIV.

Difi"erent

forms of photometers,

244.

75 and

282

LIGHTHOUSE ILLUMINATION.

PLATES.

XXV. XXVI.

Mr. Brebner's refraction protractor, Dioptric

floating

Hooghly,

XXVII.

light

apparatus

242. the

for

220.

Trinity House floating light vessel,

CHAPTER

220.

POG-SIGNALS.

XXXII.

Fig.

Mr. Douglass's movable lantern,

250.

River

INDEX. Absorption, atmospheric, 99, 201. by glass and metal, 49, 50, 194. â&#x20AC;˘

Adie, J., 86, 89, 94. Airy, Sir G. B., 60. Alguada Lighthouse, 24. AUard, M., 50, 124, 192, 265. Alternating red and white lights, 131. Apparatus, focusing of, 59, 234. ranges of, 201. relative intensities 194, 196.

of different,

statistics of, 192.

Apparent

light, 156.

designs for increasing density of

beam of light for, 160. modes of reducing divergence

of,

155.

Arago, M., 55, 66, 204. Ardrossan Harbour Light, 128, 149. Argand, M., 55, 204. lamp. See Lamps. Aseismatic arrangements, 231. Atmospheric absorption, 99, 201. Automatic gas meter for producing intermittent or occulting lights, 170. signal buoy, 173.

Azimuthal condensing system of lights,

Beacons and Buoys, illumination by gas, 167.

Apparent light for, 156. Automatic gas meter for producing intermittent lights, 170. Bell buoys, 251. Courtenay's automatic signal buoy, 173, 254. Designs for beacons and buoys, ^ 177. Dipping light, 153. Gymbals for preserving horizontality of buoy lights, 171. Landmarks, 221. Pintsch's gas-lighted buoy, 169. Sound -warnings during fogs, 176. uniform system of, 171. Beazeley, Mr., 239, 250, 253. Bedford, Admiral E. J., 172. Bell Eock Lighthouse, force of sea at,

7,8. Tower, 15. See Coloured Lights,

also

Bells,

etc.

Bells, 250.

Biot, M., 176. Bishop Rock Lighthouse, 4, 20. Borda, M., 55. Bound Skerry Lighthouse, 5.

Brebner, Alan,

97.

of,

4"4,

91,

136, 140, 188,

242.

Babbage,

Alexander, 21.

Professor, 132, 146, 148. Bain, A., 176. Balfour, J. M., 115, 229, 240.

Barbier and Fenestre,

Messrs.,

Brewster, Sir D., 71, 160, 161, 247. New York, patentee of

Brown, Mr.,

121,

Barker, Mr., 254. Barlow, W. F. 62. Basses Lighthouse, 22. Beacons, cement rubble, 43. Beacons and Buoys, illumination ,

152, 175.

electricity, 163.

siren, 255, 257.

Buffon, M., 63, 64. Burners. See Lamps. Buoys. See Beacons.

145.

of,

Campbell, J. F., 172, 236. Cape Ailly Light, 181. Carcel, M., 55.

Carr Rock Beacon, 36, 177, 250.

284

INDEX. Condensing Fixed apparatus

Catadioptric Lights, 67.

of unequal range, which constantly illuminate the whole horizon,

arrangement of Sir D. Brewster, 71.

liolophotal revolving apparatus,

117.

Fixed apparatus for steamers, 117. White light, changing to fixed

83.

liolophote, 80. reflector of A.

Gordon, 76.

coloured, 124. Intermittent, which condense the light into one sector, 118. „ of equal periods, 120, 134. „ of unequal periods, 121, 134.

Catoptric system, 54.

Fixed

light, 58.

Intermittent light, 131.

Revolving light, 58. Condensing spherical mirror, 105.

Cement rubble

or concrete towers, 42. beacons, 43. Chance, J. T., 55, 87, 89, 93, 94, 111, 116, 178, 186, 187, 265. Messrs., 79, 87, 92, 110, 160, 178.

ors, 122. ,

Chickens Lighthouse Tower, 22. Cialdi, M., 4. Clyde fog-signals, 251, 253, 262.

Colour-blindness, 145.

Coloured Lights, 124, 126, 131, 132, See also Condensing System and Distinctions. M. Reynaud's experiments on, 137.

138.

Messrs. Stevenson's experiments on, 140, 141. of producing, 143. Silvered glass reflector for, 145. Condensing System, 97.

mode

—

differential lens, 103.

difi"erential

back prisms,

refractor single agency, 104.

showing

different colours alternately, 124.

light of changing colour, 132.

produced by automatic

gas meter, 135, 170. Eevolving, which condense the light into one sector, 118. „ of tinequal range, which do not illumi7iate the whole horizon, 119. „ which periodically illuminate the whole horizon, but which vary the strength of th^ flashes in passing over certain sectors, 120.

Coal-fire lights, 52.

Optical agents

xoith differential refract-

illuminating the whole horizon, but showing revolving or intermittent light over small arcs, 278.

91.

straight prisms, 102. Condorcet, M. 63, 64. right - angled expanding Coode, Sir John, 42. prisms, 102. Cookson, Messrs., 73. catoptric spherical mir- Cordouan Lighthouse Tower, 23. ,, parabolic mirrors used at, 55. ror, 105. Cornaglia, M., 265. parabolic conoid, 249. ,, spherical mirror of un- Courtenay's Automatic Signal Buoy, ,, equal area, 106. 173 254. twin prisms, 103. Cowper, E.'a., 253. ,, Fixed apixindus for single sector, Craighill Channel Light, 24, 43. Cuningham, A., 172, 262. 106. octant, 108 quadrant, Cunningham, D. , 171. ,, 108; for 30", 111; twin prism apparatus, as Daboll, Mr., 253, 254, 255. used at Lamlash, 112. Degrande, M., 85. ,,

, ,

;

Fixed apparatus for more than one sector of unequal range, 112, 116.

Dhu

Heartach Lighthouse,

21, 37, 38,

39, 44, 131, 251.

force of sea at, 37, 38, 39.

285

INDEX. Dynamometer,

Differential lens, 103.

flush, 8.

for electric light, 187.

Eaethquake Shocks,

refractor, 104, 122.

reflector of single agency, 104. Dioptric system, 63, 72, Lights, focusing of, 235, Dipxjing Lights, 153.

Eddystone Lighthouse,

Distinctions for Lights, 125. Alternating red and white, 131.

Electric Light,

Babbage's

intermittent

light,

132, 146.

Changing coloured light, 126. Coloured lights, 124, 125, 137, 143.

Double, iixed or revolving, 125, 126.

Fixed, 125, 126. Fixed, varied by flashes, 128. Fixed, illuminating the whole horizon, but showing revolving or intermittent light over small arcs, 130, 278.

Flashing, 125, 128. Group revolving and flashing, 126. Brebner's, ,, ,, 136. ,,

Hopkin-

son's, 136.

Group

flashing, "Wigham's, 135. Intermittent of equal periods, 126 cntoptric, 132.

of unequal 126, 132.

periods,

condensing, of changing colour, 132. dioptric condensing,

strengthened, 133.

â&#x20AC;&#x17E;

Felly's, 134.

134.

Morse system, 146. Reversing, 129. Ptevolving, 125, 128. Reciprocating, 129. Separating, 126, 137. Doty, Captain, 206. Double Lights, 125, 126. Douglass, J. N., 20, 22, 36, 75, 111, 178, 179, 181, 190, 205, 208, 220, 250, 251. Douglass, W., 22.

Dunbar Harbour, 8, etc. Dynamometer, for measuring the sea,

178. dioptric apparatus for, 187.

experiments at South Foreland on different machines for producing, 180.

on dioptric apparatus for, at Edinburgh, 187. focal compactness of, 191. merits of, 182. experiments

Facets, Mirror

of, 54.

Faraday, Professoi-, 178. Fastnet Lighthouse, 4, 19.

Fixed

catoptric apparatus, 58. catadioptric apparatus of

force of

formula

for testing springs, 6.

forces of

waves indicated by,

Fres-

nel, 67.

condensing apparatus, 98, 107. dioptric apparatus of Fresnel, 67.

Mr. Alan Stevenson's improvements on, ,,

73-74. dioptric

apparatus,

varied

70 of Leof Tabouret, 79.

flashes, of Fresnel,

231.

Rudyerd's, 13, 32, 37. Smeaton's, 14, 34, 37. Winstanley's, 12, 32, 36.

;

tourueau, 78 ; holophotal apparatus, 80. apparatus varied by ,, flashes, 89.

with masks, showing revolving or intermittent lights over small arcs, 130. apparatus, intensities of, 48, 196. Floating Lights, 218. Flames. See Lamps.

compactness of, 191. height of, 192. intensities of, 193-207. focal

transpa7-ency

of, 193. 192, 193. Focusing of lights, 234. Fog, observations on relative visibility of diff"erent colours in, 262. Fog-Signals. See Beacons. luminous, 247. movable lantern, 250. rockets, 252.

volume

of,

by sound

â&#x20AC;&#x201D;

251 Courtenay 254 gongs, 252

bells,

buoy, 173,

;

286

INDEX.

Fog-Signals, guns, 252 reed trumpet, 254 siren, 255 whistles, 253. liolophone, or sound reflector, ;

;

259.

motors for producing sound

for,

256. distinctions for ships' signals from those at land stations, 262.

Henderson, Professor, 246. Henry, Professor, 258-259. Herschell, Sir

proved do.,

Formula

for constructing lighthouse apparatus, 264. Franklin, B., 226. Fraser, Captain, 24, 35. Fresnel, A., 53, 72, 83, 204, 264. lens, optical agents invented by cylindric refractor, 64 63 totally reflecting prisms, 65

80.

dioptric holophote with spherical mirror, 85. dioptric spherical mirror, 86, 93. double reflecting prisms, 84. ellipsoidal mirror, 162. fixed apparatus, varied by flashes,

;

straight refracting, 66. concentric wick-lamp, 55, 66. catadioptric fixed apparatus

89. of,

67.

dioptric

of,

67.

varied by

paraboloidal holophote, 161. revolving light, 88. — totally reflecting prisms, 83. See also Condensing System. Holmes, Professor, 164, 178, 185, 255, 257.

flashes, 70.

revolving light, 68. Fresnel, Leonor, 72, 75, 84. improved revolving apparatus

90.

catadioptric revolving apparatus,

—

;

J., 99.

Holmgren, Professor, 146. Holophoue, or sound reflector, 259. Holophotal System, 78-80. back prisms, 91. Imcatadioptric holophote, 80.

Hooghly floating lights, 220. Hook, Dr. P., 54. of,

77.

Hopkinson, Dr., 116, 117, 122,135, 136. Horsburgh Lighthouse, first revolving with totally reflecting prisms, 89.

Hot air engines, 257. Galley Head Light, 135. Gas automatic meter for producing an Hutchinson, Mr., 54. intermittent light, 135, 170. design for apparatus lighted by, 248. engines, 257. flames, focal compactness of, 191, 210. group flashing light, 135. illumination of beacons and buoys by, 167. Pintsch's buoy lighted by, 169,

176-177.

Wigham's

burners, 210.

Gordon, A., 76, 253. Gongs, 252. flashing and revolving lights, 126. Mr. Brebner's, 136. Dr. Hopkinson's, 136. Mr. Wigham's, 135.

Ice

floes, 24, 43.

— coal

52 candles, gas, 135, 178 oil, 53, 167, 170, 191, 210, 247 paraffin, 206, 209. 191, 194 focal compactness of different, 191. Inchkeith Lighthouse, first dioptric revolving apparatus in Britain, 72. Intermittent Lights. See Beacons and Buoys, Condensing System, Distinctions, Holophotal System. Iron lighthouse towers, 43.

lUuminants 53

;

electric

fire,

light,

;

May Light, coal fire, 52. dioptric fixed light, first in Britain,

Isle of

Group

72. Isle

Oronsay,

first

condensing appara-

tus, 112.

Guns, 252.

Japanese Lights, ment for, 231.

Haklutt's Voyages, 53. Hart, Mr., 164. Henderson, D., 94.

Jenkin, Professor, 177.

Keith's mineral

aseismatic arrange-

oil gas,

257.

INDEX.

Lamlash Lighthouse, 112. La Corbiere Lighthouse, 42. Lamps, history

of iuvention

Mackintosh, Mr., 176. of,

54-55,

66.

Argand,

54, 204. burners, dimensions of, 192. concentric wick, 66, 204.

Doty

paraffin,

207. focusing fountain

206

of, 59,

;

statistics of,

234.

and mechanical,

66, 210.

Maris paraffin, 206. Trinity House, 208.

Landmarks, 221, Lange, M., 55.

Lanterns,

287

Mallet, R., 231. Marcet, Bordier, 60, 61. Maris, M., 206. Masking screens, 229. Masonry. See Lighthouses. Masselin, M., 94. Mathieu, M., 66, 204. Metallic reflection, 48. Mills, B, 137. Milne, Messrs., 170. Mitchell, Mr., 43. Morris, Mr., 150. Morse system, 146. Mull of Galloway Lighthouse, 122.

diagonal, 75, 223.

Nehls, C, 265. North Ronaldsay

helical, 75, 224.

glazing

of,

224.

storm panes

for,

225.

Lens, annular, 63.

Lighthouse,

first

Holophotal revolving apparatus of first

order, 89.

differential, for oil flame, 103. for electric light, 187.

double, for electric light, 188, 280. early use of, 53. intensity of, 193. Lepaute, M., 77, 78, 84. Letourneau, M., 78, 85, 89, 220. Light, ranges of, 201, 218. Lighthouse construction, 1. illumination, general principles of, 45.

Lighthouse Toweks,

12.

general principles which regulate design of, 24. application of general principles, 32.

centre of gravity of, 39. cement rubble or concrete, 42. force of waves on, 2-41. of wind on, 41. ,, iron, 43. of, modes of uniting, 44. Mitchell's screen pile, 43. relative levels of the solid at different, 38. Lighthouses, distribution of, on the coast, 214. time of lighting and extinguishing, in Scotland, 246. Lightning-conductors, 225. Lochindaal Lighthouse, 92, 116.

masonry

Oil. See Lamps. Optical protractors, 240. Ormiston, Mr., 24.

Parabolic Paraffin

Reflectors.

oil,

Reflectors.

See.

209.

Parkes, Mr., 24. Peclet, M., 55. Pelly, Sir H., 134. Petit, M., 190.

Photometers, 244. Pintsch's Gas Buoy, 169, 176, 177. Port-Glasgow, first light with submarine gas pipes, 168. Portland Lighthouse, 53. Potter, Professor, 50, 194.

Prongs Lighthouse,

â&#x20AC;&#x201D;

24.

Protractors Balfour's, 240 242. Prisms, back, 91.

;

Brebner's,

condensing straight, 102. double reflecting, 84. fixed reflecting, 75.

revolving holophotal, 81, 83. right-angled expanding, 102. Professor Swan's new forms of, 94. straight refracting, 66. totally reflecting, 65.

twin, 103.

Radial masking

screens, 229.

Machinery

Ranges of

Macadam,

Rankine, Professor, 11. Reciprocating apparatus, 129.

for revolving lights, 230. for fog-signals, 256.

Dr., 207.

lights, 201, 218.

288

INDEX.

Eed Sea Lighthouses, 24. Eed Lights. See Colour. Eeflector Facet, 54.

Scoresby, Dr., 4. Screw-pile lighthouses, 43, 276. Seamen, " colours test " for, 146. Separating lights, 137. Seven Stones floating light, 221.

—

Pai'abolic, 55.

defects of, 57. properties of, 55.

convex

Sheringham, Admiral, 167.

joaraboloid, 249.

useful divergence

of,

Ships' fog-signals.

56.

Mr. Stevenson's focusing arrangements, 59.

focusing of, 234. ,, Barlow's, 62. Gordon's, 76. Marcet's, 60, 61. Catadioptric Holophote, 80. Differential of single agency,104. Keflection, metallic, 48.

See Fog-signals.

lights, 117, 171.

Siemens, Dr., 164, 179. Siren fog-signal, 255. Skerryvore Lighthouse, 7, 17, 75. Smeaton, J. See Eddystone. Smith, Captain, 129.

Smith, Thomas, 54. Smith, William, 253. Soleil, M., 75. South Eock Lighthouse, 24, 250. total by glass, 49. Spectacle Reef Lighthouse, 24, 43. application to fixed Spherical mirror first ,, Dioptric, 93 Dioplights, 67. tric of unequal area, 106 condensing application to revolving first Catoptric, 105. Sec Condensing lights, 83, 88. Lights Holophotal System. loss of light due to, 194. Steamers' side lights, 117, 171. Stevenson, Alan, 18 etjMssim. Eefraction by glass, 49. Mr. Alan Stevenson, David, 21 et passim. Eefractor, cylindric, 64 Stevenson's improvements on, Stevenson, D. A., 21, 38-39. Stevenson, E., 15 et 2}a,ssim. 73, 74. Difl"erential condensing, of single Stevenson, E. L., 133.

—

;

, ,

;

agency, 104, 122.

Eennie,

Stokes, Professor, 258.

J., l7.

Eepeating light, 119. Eeversing lights, 129, 130. Eevolving and ilxed lights, difference in power of, 48.

Eevolving Apparatus,

Stornoway Apparent Light, 117, 157. Storm panes, 225. Submarine conduction of light-producing agency, 163.

Swan, Professor,

86, 87, 92, 94, 164, 165, 264. new optical agents, 94.

catoptric, 58.

Dioptric of A. Fresnel, 66. Holophotal, 88. ,,

mathematical formulse

for constructing lighthouse apparatus, 265.

L. Fresnel's improved, 77. catadioptric appaholophotal ratus, 83.

Tabouret, M.,

Lepaute'-s, 77.

79, 83, 84. Tait, Professor, 105, 165, 228, 274.

machinery for, 230. Eeynaud's design, 80. Skerryvore design, 75. See also Condensing System.

Thomson, Thomson,

Eeynaud, M., 80, 84, 138, 265. Eochon, Abbe, 63. Eogers, Thomas, 71. Eudyerd, J., 13.

Sang,

—

55, 144, 204.

E., 75.

Sautter, Lemmouier, 121, 181.

and

Cie.,

J. T., 24, 129. Sir William, 134, 135, 146,

147, 148, 149. Total reflection. See Eeflection. Towers. See Lighthouses. Trumpets fog-signal, 254. Tyndall, Professor, 179, 190, 247, 252, 254, 255, 256, 258, 259.

Eussell, J. Scott, 11.

Eumford, Count,

Tayler, Admiral, 254. Telford, T., 17. Teulere, M., 55.

MM., Unst

Lighthouse,

4, 42.

289

INDEX. Vakley, Mr,,

Waves, modifying

179.

influence of rocks

on breaking waves,

Walden,

Waymouth,

Captain, 255.

Walker, James, 20.

Waves, height

Law

of.

of increase as

square roots of distance from

windward

Wick Breakwater, 5, Wigham, J. K., 135,

shore, 2.

force of, 4, 7, 8. ,,

dynamometers

for

mea-

suring, 5, 9. force of, at different levels, 8.

â&#x20AC;&#x17E;

summer and winter

gales, 7.

force of, horizontal 10.

back draught,

greatest height of, in the ocean, 3. relation between height of and depth of water, 11.

Printed by R.

36.

221. Wheatstone, Professor, 179. Whistles, 253. Wicks, dimensions of, 192. 13.,

11.

210, 247, 248. Wilde, Air., 164, 176, 181, 187. Wilson, J., 135, 247. Wilson, Dr. George, 146. AViud, force of, on lighthouse towers, 41, relative force of currents of wind and water of equal velocity, 41,

Winstanley, H., 12. Rock Lighthouse,

Wolf

251.

Wright, Dr. StrethiU, 164.

R. Clark, Edinburgh.

20,

142,

PLATE BELL ROCK

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UNITED STPvTES.

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