Aircraft Communications and Navigation Systems
Introducing the principles of communications and navigation systems, this book is written for anyone pursuing a career in aircraft maintenance engineering or a related aerospace engineering discipline, and in particular will be suitable for those studying for licensed aircraft maintenance engineer status. It systematically addresses the relevant sections (Air Transport Association of America chapters 23/34) of modules 11 and 13 of Part-66 of the European Aviation Safety Agency (EASA) syllabus and is ideal for anyone studying as part of an EASA and FAR-147-approved course in aerospace engineering.
• Delivers the essential principles and knowledge base required by Airframe and Propulsion (A&P) Mechanics for Modules 11 and 13 of the EASA Part-66 syllabus and BTEC National awards in aerospace engineering
• Supports mechanics, technicians and engineers studying for a Part-66 qualification
• Comprehensive and accessible, with self-test questions, exercises and multiple choice questions to enhance learning for both independent and tutor-assisted study
• Additional resources and interactive materials are available at the book’s companion website at www.66web.co.uk
This new and updated third edition provides readers with an overview of the latest key technologies that underpin the functioning of safety-critical systems such as those used in flight management, reporting, navigation, and air traffic control.
Mike Tooley has over 30 years’ experience of teaching electrical principles, electronics and avionics to engineers and technicians, previously as Head of the Department of Engineering and Vice Principal at Brooklands College, Surrey, UK. He currently works as a consultant and freelance technical author.
David Wyatt has over 45 years’ experience in the aviation industry. After a technician apprenticeship in BOAC he progressed as a development engineer at BA. After a period as an avionics engineering lecturer in further education, David went on to become Head of Airworthiness at Gama Aviation. David is the author of several popular books in the Routledge Aircraft Engineering Series.
Aircraft Communications and Navigation Systems
Third edition
Mike Tooley and David Wyatt
Cover image: Mike Tooley and David Wyatt
Third edition published 2024 by Routledge
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Routledge is an imprint of the Taylor & Francis Group, an informa business © 2024 Mike Tooley and David Wyatt
The right of Mike Tooley and David Wyatt to be identified as authors of this work has been asserted in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988.
All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers.
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First published 2007
Previous edition published 2017
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library
Library of Congress Cataloging-in-Publication Data
Names: Tooley, Michael H., author. | Wyatt, David, 1968- author.
Title: Aircraft communications and navigation systems / Mike Tooley and David Wyatt.
Description: Third edition. | New York, NY : Routledge, 2024. | Includes bibliographical references and index.
Identifiers: LCCN 2023028817 | ISBN 9781032534152 (hardback) | ISBN 9781032518084 (paperback) | ISBN 9781003411932 (ebook) | ISBN 9781032626130 (ebook other)
Subjects: LCSH: Airplanes--Electronic equipment. | Avionics. | Aeronautics--Communication systems.
Classification: LCC TL694 .T664 2024 | DDC 629.135/1--dc23/eng/20230630
LC record available at https://lccn.loc.gov/2023028817
ISBN: 978-1-032-53415-2 (hbk)
ISBN: 978-1-032-51808-4 (pbk)
ISBN: 978-1-003-41193-2 (ebk)
ISBN: 978-1-032-62613-0 (eBook+)
DOI: 10.1201/9781003411932
Typeset in Times New Roman by SPi Technologies India Pvt Ltd (Straive)
Preface
The first edition of Aircraft Communications and Navigation Systems was published in 2007 as part of a series designed for both independent and tutor-assisted studies. This third edition has been updated with reference to the latest version of EASA’s Part 66 syllabus as well as technology updates and requests from readers/ reviewers. The book forms part of a series of titles:
• Aircraft Engineering Principles (AEP)
• Aircraft Communications and Navigation Systems (ACNS)
• Aircraft Flight Instruments and Guidance Systems (AFIGS)
• Aircraft Electrical and Electronic Systems (AEES)
ACNS is designed to cover the essential knowledge base required by certifying mechanics, technicians and engineers engaged in engineering maintenance activities on commercial aircraft. In addition, this book should appeal to members of the armed forces and others attending training and educational establishments engaged in aircraft maintenance and related aeronautical engineering programmes (including Foundation Degree courses, BTEC National and Higher National units as well as City and Guilds and NVQ courses).
The book introduces the principles, operation and maintenance of aircraft communications and navigation systems. The aim has been to make the subject material accessible and presented in a form that can be readily assimilated. The book provides syllabus coverage of the communications and navigation section of Module 13 (ATA 23/34). The book assumes a basic understanding of aircraft flight controls as well as an appreciation of electricity and electronics (broadly equivalent to Modules 3 and 4 of the EASA Part-66 syllabus).
It is important to be aware that this book is not designed to replace aircraft maintenance manuals. Nor does it attempt to provide the level of detail required by those engaged in the
maintenance of specific aircraft types. Any maintenance statements made in this book are for training/educational purposes only. Always refer to the approved aircraft data and applicable safety instructions.
Chapter 1 sets the scene by providing an explanation of electromagnetic wave propagation and the radio frequency spectrum. The chapter also describes the various mechanisms by which radio waves propagate together with a detailed description of the behaviour of the ionosphere and its effect on radio signals.
Antennas are introduced in Chapter 2. This chapter explains the principles of isotropic and directional radiating elements and introduces several important concepts including radiation resistance, antenna impedance, radiated power, gain and efficiency. Several practical forms of antenna are described including dipoles, Yagi beam antennas, quarter wave (Marconi) antennas, corner reflectors, horn and parabolic dish radiators. Chapter 2 also provides an introduction to feeders (including coaxial cable and open-wire types), connectors and standing wave ratio (SWR). The chapter concludes with a brief introduction to waveguide systems.
Radio transmitters and receivers are the subject of Chapter 3. This chapter provides readers with an introduction to the operating principles of AM and FM transmitters as well as tuned radio frequency (TRF) and supersonicheterodyne (superhet) receivers. Selectivity, image channel rejection and automatic gain control (AGC) are important requirements of a modern radio receiver, and these topics are introduced before moving on to describe more complex receiving equipment. Modern aircraft radio equipment is increasingly based on the use of digital frequency synthesis, and the basic principles of phase-locked loops and digital synthesisers are described and explained.
Very high frequency (VHF) radio has long been the primary means of communication between aircraft and the ground. Chapter 4 describes the principles of VHF communications (both voice and data). The chapter also
provides an introduction to the Aircraft Communications Addressing and Reporting System (ACARS).
High frequency (HF) radio provides aircraft with an effective means of communicating over long distance oceanic and trans-polar routes. In addition, global data communication has recently been made possible using strategically located HF datalink (HFDL) ground stations. Chapter 5 describes the principles of HF radio communication as well as the equipment and technology used.
As well as communication with ground stations, modern passenger aircraft require facilities for local communication within the aircraft. Chapter 6 describes flight-deck audio systems including the interphone system and all-important cockpit voice recorder (CVR) which captures audio signals so that they can be later analysed in the event of a serious malfunction of the aircraft or of any of its systems.
The detection and location of the site of an air crash is vitally important to the search and rescue (SAR) teams and potential survivors. Chapter 7 describes the construction and operation of emergency locator transmitters (ELT) fitted to modern passenger aircraft. The chapter also provides a brief introduction to satellitebased location techniques.
Chapter 8 introduces the subject of aircraft navigation; this sets the scene for the remaining chapters of the book. Navigation is the science of conducting journeys over land and/or sea. This chapter reviews some basic features of the earth’s geometry as it relates to navigation, and introduces some basic aircraft navigation terminology, e.g., latitude, longitude, dead reckoning, etc. The chapter concludes by reviewing a range of navigation systems used on modern transport and military aircraft. Many aircraft navigation systems utilise radio frequency methods to determine a position fix; this links very well into the previous chapters of the book describing fundamental principles of radio transmitters, receivers and antennas.
Radio waves have directional characteristics as described in the early chapters of the book. This is the basis of the automatic direction finder (ADF), one of earliest forms of radio navigation that is still in use today. ADF is a short/medium-range (200 nm) navigation system providing directional information.
Chapter 9 looks at the historical background to radio navigation, reviews some typical ADF hardware that is fitted to modern commercial transport aircraft, and concludes with some practical aspects associated with the operational use of ADF.
During the late 1940s, it was evident to the aviation world that an accurate and reliable short-range navigation system was needed. Since radio communication systems based on VHF were being successfully deployed, a decision was made to develop a radio navigation system based on VHF. This system became the VHF omni range (VOR) system and is described in Chapter 10. This system is in widespread use throughout the world today. VOR is the basis of the current network of ‘airways’ that are used in navigation charts.
Chapter 11 develops this theme with a system for measuring distance to a navigation aid. The advent of radar in the 1940s led to the development of a number of navigation aids including distance measuring equipment (DME). This is a short/medium-range navigation system, often used in conjunction with the VOR system to provide accurate navigation fixes. The system is based on secondary radar principles.
ADF, VOR and DME navigation aids are installed at airfields to assist with approaches to those airfields. These navigation aids cannot however be used for precision approaches and landings. The standard approach and landing system installed at airfields around the world is the instrument landing system (ILS). Chapter 12 describes how the ILS can be used for approach through to Autoland. The ILS uses a combination of VHF and UHF radio waves and has been in operation since 1946.
The advent of computers, in particular the increasing capabilities of integrated circuits using digital techniques, has led to several important advances in aircraft navigation. One example of this is the area navigation system (RNAV); this is described in Chapter 14. Area navigation is a means of combining, or filtering, inputs from one or more navigation sensors and defining positions that are not necessarily colocated with ground-based navigation aids.
A major advance in aircraft navigation came with the introduction of the inertial navigation system (INS); this is the subject of Chapter 15 The inertial navigation system is an autonomous
dead reckoning system, i.e., it requires no external inputs or references from ground stations. The system was developed in the 1950s for use by the US military and subsequently the space programmes. Inertial navigation systems (INS) were introduced into commercial aircraft service during the early 1970s. The system is able to compute navigation data such as present position, distance to waypoint, heading, ground speed, wind speed, wind direction, etc. The system does not need radio navigation inputs, and it does not transmit radio frequencies. Being self-contained, the system can be used for long distance navigation over oceans and undeveloped areas of the globe.
Navigation by reference to the stars and planets has been employed since ancient times; aircraft navigators have utilised periscopes to take celestial fixes for long distance navigation. An artificial constellation of navigation aids was initiated in 1973 and referred to as Navstar (navigation system with timing and ranging). This global positioning system (GPS) was developed for use by the US military; it is now widely available for use in many applications including aircraft navigation. Chapter 16 looks at GPS and other global navigation satellite systems that are in use or planned for future deployment.
The term ‘navigation’ can be applied in both the lateral and vertical senses for aircraft applications. Vertical navigation is concerned with optimising the performance of the aircraft to reduce operating costs; this is the subject of Chapter 17. During the 1980s, lateral navigation and performance management functions were combined into a single system known as the flight management system (FMS). Various tasks previously routinely performed by the crew can now be automated with the intention of reducing crew workload.
Chapter 18 reviews how the planned journey from A to B could be affected by adverse weather conditions. Radar was introduced onto passenger aircraft during the 1950s to allow pilots to identify weather conditions and subsequently reroute around these conditions for the safety and comfort of passengers. A secondary use of weather radar is the terrain-mapping mode that allows the pilot to identify features of the ground, e.g. rivers, coastlines and mountains.
Increasing traffic density, particularly around airports, means that we need a method of air
traffic control (ATC) to manage the flow of traffic and maintain safe separation of aircraft. The ATC system is based on secondary surveillance radar (SSR). Ground controllers use the system to address individual aircraft. An emerging ATC technology is ADS-B; this is also covered in Chapter 19.
With ever-increasing air traffic congestion, and the subsequent demands on ATC resources, the risk of a mid-air collision increases. The need for improved traffic flow led to the introduction of the traffic alert and collision avoidance system (TCAS); this is the subject of Chapter 20. TCAS is an automatic surveillance system that helps aircrews and ATC to maintain safe separation of aircraft. TCAS is an airborne system based on secondary radar that interrogates and replies directly with aircraft via a high-integrity datalink. The system is functionally independent of ground stations and alerts the crew if another aircraft comes within a predetermined time to a potential collision.
It is an unfortunate fact of life that the operation of virtually all items of electronic equipment can potentially disturb the operation of other nearby items of electronic equipment. This final chapter deals with electromagnetic compatibility (EMC) and begins by describing the electromagnetic environment within a modern aircraft and its potential for impacting on its increasingly complex avionic systems. The classification, potential sources and effects of electromagnetic interference (EMI) are discussed together with frequency spectral analysis of some typical sources of EMI. Various means of reducing EMI, such as wiring and cabling, grounding, bonding, shielding, and filtering are described. The chapter includes a case study of the risk to an aircraft’s altimeter system from interference caused by radiation from 5G masts. This topic has generated considerable debate and controversy and further underlines the need to be fully aware of the electromagnetic environment in which all modern aircraft operate.
The book concludes with four useful appendices, including a comprehensive list of abbreviations and acronyms used with aircraft communications and navigation systems.
The review questions at the end of each chapter are typical of these used in CAA and other examinations. Further examination practice can be gained from the four revision papers given in
Appendix 2. Other features that will be particularly useful if you are an independent learner are the ‘Key Points’ and ‘Test your understanding’ questions interspersed throughout the text.
Preface to the second edition
When producing the second edition of the book the authors considered advances in technology as well as suggestions and feedback from readers (students and lecturers) and a review by the National Aerospace Library (NAL), one of the most prestigious aerospace and aeronautical library collections in the world. New content developed specifically for the second edition included:
• Satellite communications (SATCOM). SATCOM is becoming increasingly important in its ability to facilitate global communication between aircraft and the ground for both crew and passenger use. Chapter 1 was updated accordingly.
• Space weather. The subject of space weather was not included in the first edition but has been given some attention in recent EASA and CAA publications. The UK CAA (CAP 1428) recommends the aviation industry should “initiate educational programmes that provide staff with a greater understanding of the impact of severe space weather events on their operations and to ensure that the risk of extreme space weather is captured in their Safety Management System (SMS)”. Conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere can influence the performance and reliability of aircraft avionic systems. Space weather can impact on several of the systems included in this book, e.g., HF communications, GNSS and satellite communications. Introductory notes have therefore been included in Chapter 1.
• GPS. Although GPS operates in the same was as it always has, there are more aircraft flying now using GPS for approaches and landings. Chapter 16 was updated accordingly.
• Electronic instruments/displays and Integrated systems. (e.g., GA combined nav/com/GPS). Chapter 17 was updated to reflect these changes.
• Weather radar. Chapter 18 was updated to incorporate additional technologies used on General Aviation (GA) aircraft: lightning detection, and satellite datalink weather services.
• Air traffic control (ATC). ADS-B is currently being introduced in Europe, North America, and other areas worldwide. Chapter 21 has therefore been expanded to include Single European Sky and Future Navigation Systems (FANS).
• Traffic alert and collision avoidance systems. The low-cost FLARM® traffic awareness system for GA is becoming increasingly popular. A new section on FLARM® was added to Chapter 20
In addition to this important new material the authors attempted to respond to requests and suggestions from readers. For example, a review via the National Aerospace Library noted that the low range radio altimeter (LRRA) was “buried” within Chapter 12 (Instrument landing systems). It was not the intention of the authors to diminish the purpose or value of an LRRA. Although the LRRA is an integral part of ILS/Autoland, the authors acknowledge that radio altimeters are often used as stand-alone systems, e.g., on rotorcraft for low-level operations. Accordingly, the section describing LRRA has been relocated to Chapter 8.
New EASA rules for aircraft maintenance for GA were introduced via EASA 208598 leaflet 03; Flying in the EU’. This leaflet described simpler Part-66 licences for GA aircraft mechanics (B2L and L licences). It also described CSSTAN; this has been introduced for standard changes and standard repairs. These can be incorporated on the aircraft by a mechanic immediately, i.e., there is no need to have it approved by the EASA or by a design organisation. Examples include installation of VHF voice communication systems and FLARM® equipment (as mentioned earlier, the latter has been added as a new section in Chapter 20.)
A new section was added into Chapter 8 for minimum navigation performance specifications
(MNPS) required for remote areas, e.g., in the North Atlantic. This new section also references reduced vertical separation minimums (RVSM); this allows aircraft to fly with a vertical separation of 1000 feet between FL290 and FL410 inclusive. Further information on RVSM is given in another title in this book series, AFIGS. Chapter 8 also includes a new section on electronic flight bags (EFB); these are replacing traditional paper-based documents, e.g., navigation charts.
Several high-profile accidents have occurred since the first edition was published, particularly flight AF447 (June 2009) and flight MH370 (March 2014). These accidents have highlighted shortcomings in the way that flights are currently tracked. The subsequent investigations into these two accidents is beyond the scope of this book, and indeed the book series; however, the subjects covered by the book series are very topical in the context of the accident investigations:
• Air Traffic Control (Chapter 19)
• Emergency Locator Transmitters (Chapter 7)
• Flight Data Recorders (covered in AEES)
• Satellite Communications (Chapter 1)
• Satellite Navigation (Chapter 16).
Aeroplanes flying over land with a high density of population are permanently tracked by air traffic control (ATC) systems, as described in Chapter 19. Aeroplanes flying over remote regions, e.g., oceans, polar routes, etc., will have limited, if any, ATC surveillance. The frequency of position reports by pilots or aircraft to ATC in remote and oceanic airspace is not systematic. Instead, it varies at certain intervals, depending on the density of the airspace and the procedures in place. In order to improve the positioning of aeroplanes in remote areas, the International Civil Aviation Organization (ICAO) has adopted international standards for the:
• location of aircraft and recorders using underwater beacons
• tracking of aircraft
• location system for aircraft in distress
• fast recovery of data from flight recorders.
One technology that has become both an opportunity and a threat is that of remotely piloted aircraft systems (RPAS), unmanned aerial vehicles (UAV), or ‘drones’. A recent EASA report, ‘Drone Collision’ Task Force Final Report (04/10/16), illustrates the increasing number of RPAS occurrences per year between 2010 (virtually zero) and 2015 (circa 500). The report discusses airborne conflict (defined as a potential collision between a drone and an aircraft in the air) being the most common type of occurrence. There are increasing commercial and military requirements for using RPAS in controlled airspace. This subject is not specifically addressed in this new edition. However, some of the technology described in this book could be adapted in the future to account for RPAS. The reader is encouraged to follow industry media on this subject, e.g., mandates and rulemaking.
A recent article published by the RAeS (Aerospace, November 2016) discusses the subject of “remote control tower” technology, i.e., enabling air traffic controllers to manage flights from remote airports without the need for a manned control tower. The authors believe that this will not affect the on-board systems, e.g., VHF communications, transponders, etc., and so the technology is not described in any detail in this second edition. At the time of publishing this second edition, trials are still being conducted; the reader is encouraged to monitor developments via the industry press, e.g., mandates and rulemaking.
Unscheduled maintenance for any aircraft system is more expensive than scheduled maintenance. Furthermore, an avionic unit that is removed, but subsequently tested and found to be serviceable is deemed No Fault Found. The cost of NFFs, i.e., cost of time to remove, logistics costs for returning it to a repair shop, costly test equipment, higher inventories etc. can all be reduced when the maintenance technician has a thorough understanding of systems and equipment. NFFs may also cause expensive flight delays or cancellations. Although NFFs are common with avionic equipment, it is in everyone’s interest to minimise occurrences.
Importantly, this second edition introduced “Key Points” covering very basic system testing to give additional understanding of systems. Troubleshooting starts (where possible) with a
debriefing from the pilots and asking the right questions. The latter can be greatly enhanced by having a thorough understanding of how the system works, theory of operation, etc.
Finally, please note that, in order to be consistent with the EASA definitions, the book adopts the following terminology:
• ‘Aeroplane’ means an engine-driven, fixed-wing aircraft heavier than air that is supported in flight by the dynamic reaction of the air against its wings.
• ‘Rotorcraft’ means a heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors.
• ‘Helicopter’ means a rotorcraft that, for its horizontal motion, depends principally on its engine-driven rotors.
• ‘Aircraft’ means a machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the air against the earth’s surface. In this book, ‘aircraft’ applies to both aeroplanes and rotorcraft.
Preface to the third edition
With global availability of the internet it is hardly surprising that aircraft manufacturers and operators have looked at adapting the Aircraft Communications and Reporting System (ACARS) for use with Internet Protocol (IP) and, whilst ACARS is introduced in our companion title, Aircraft Digital Electronics and Computer Systems, we have now included an introduction in Chapter 1 together with some examples of ACARS message.
Vector network analysis (VNA) is increasingly used to analyse the performance of antennas and transmission lines as well as a wide variety of RF components. Chapter 2 now includes an introduction to VNA, including the use of Smith Charts for impedance measurement. The chapter also provides an introduction to the use of j-notation for expressing complex impedances. Several practical examples are included.
Software defined radio (SDR) technology is finding its way into the latest avionic equipment
where signal processing (including filtering, modulation/demodulation and encoding/decoding) is performed by software rather than hardware. Accordingly, we have included an introduction to SDR in Chapter 3.
Microwave landing systems (MLS) are still in the EASA Part 66 syllabus, but the authors have decided to take it out of the 3rd edition because MLS is now superseded by satellite-based augmentation systems (SBAS). The MLS content of the previous edition of ACNS will be archived in the author’s website http://www.key2study. com/66web
Very low frequency (VLF) navigation is deleted from the EASA Part 66 syllabus and is now considered redundant for commercial aircraft; this chapter has been removed from the 3rd edition. The VLF navigation content of the previous edition of ACNS will be archived in the author’s website.
Doppler navigation is deleted from the EASA Part 66 syllabus; however, the authors decided it should be kept in the 3rd edition because the system is still being used, albeit in specialised applications.
Radio altimeter systems still perform a critical function in an aircraft’s operation, providing a direct measurement of the aircraft’s clearance over the terrain. At the time of publishing the 3rd edition, radio altimeters were being affected by the fifth generation (5G) telecommunications standard for broadband cellular networks. (5G telecommunications began deploying worldwide in 2019.) Chapter 21 of this book gives a description of how 5G can cause interference with avionic systems in general. The radio altimeter is used as a sensor in many aircraft systems, including the instrument landing system (ILS), terrain awareness and collision avoidance (TCAS) and terrain awareness system (TAWS). The ILS and TCAS are described in more detail in this book; TAWS is described in another book title in the series, Aircraft electrical and electronic systems (AEES). The proposed solution for this problem is to incorporate filters in the radio altimeter equipment. Chapter 21 gives more details of 5G interference in the form of a case study. Readers are encouraged to monitor industry developments.
Precision area navigation (P-RNAV) is introduced into the area navigation chapter of the
3rd edition. P-RNAV is the European terminal airspace application evolved from B-RNAV. P-RNAV applications are intended primarily for terminal airspace where there is higher traffic density, terrain clearance considerations etc. These considerations require more accuracy than en-route B-RNAV navigation. The area navigation chapter has also been updated with a description of actual navigation performance (ANP) where the aircraft’s navigation system monitors its actual navigation performance during flight.
The global navigation satellite system (GNSS) chapter is updated with a description of how loss of the GNSS sensor can be mitigated by using other navigation systems, e.g., distance measuring equipment (DME). This subject continues in the flight management system chapter with automatic DME tuning and scanning.
Four-dimensional (4D) navigation is introduced in the flight management system (FMS) chapter. This is a key air traffic control function performed by the flight management system. 4D navigation is a fundamental component of the air traffic control (ATC) evolution being undertaken via several programmes (SESAR in Europe and NextGen in the USA); these subjects are both discussed further in the ATC chapter of this book.
The weather radar chapter is updated with a description of solid-state weather radar antennas based on phased array, or electronically steered antenna (ESA) technology. Aside from no moving parts and reduced weight, phased array weather radar systems incorporate several advanced features, e.g., faster scanning, automatic threat analysis that adjusts the antenna’s “scan and tilt” patterns to accurately profile weather cells, predictive windshear etc.
The air traffic control (ATC) chapter is updated with numerous subjects and new technology. Automatic dependent surveillancecontract (ADS-C) is a development of ADS-B, where the data exchanges are via an open contract between the air navigation service provider (ANSP) and a specific aircraft. Remote airfield control towers are now being deployed, enabling air traffic controllers to manage flights without the need for staff to be physically present in the airfield control tower. The chapter gives updates to the Single European Sky (SES) initiative; the
FAA’s Next Generation Air Transportation System (NextGen); future air navigation system (FANS).
There is an increasing use of unmanned aerial vehicle/systems (UAV/UAS), also known as “drones” for applications in agriculture, surveying, delivery of food, medicines etc. Work continues with integrating drones into national airspace systems. Along with commercial aircraft, drones need to provide tracking and identification whilst in flight. Although drone collision avoidance technology is outside the scope of this book, a brief introduction is given to this subject.
With the increasing use of radio and wireless and networked devices electromagnetic compatibility is becoming increasingly important in the design, operation and maintenance of all items of avionic equipment that must safely co-exist in a modern aircraft environment. Chapter 21 provides an overview of the avionic electromagnetic environment before discussing the effect and sources of electromagnetic interference (EMI). Methods of reducing EMI are discussed and the chapter ends with a useful Case Study based on the introduction of C-band 5G networks and their potential for interference with aircraft radio altimeters.
Acknowledgements
Thanks to Chris Brady for access to his website www.b737.org.uk with its comprehensive content on aircraft systems, in particular the flight management system.
Online resources
Additional resources and interactive materials are available from the book’s companion website at www.66web.co.uk
1 Introduction
Maxwell first suggested the existence of electromagnetic waves in 1864. Later, Heinrich Rudolf Hertz used an arrangement of rudimentary resonators to demonstrate the existence of electromagnetic waves. Hertz’s apparatus was extremely simple and comprised two resonant loops, one for transmitting and the other for receiving. Each loop acted both as a tuned circuit and as a resonant antenna (or ‘aerial’).
Hertz’s transmitting loop was excited by means of an induction coil and battery. Some of the energy radiated by the transmitting loop was intercepted by the receiving loop, and the received energy was conveyed to a spark gap where it could be released as an arc. The energy radiated by the transmitting loop was in the form of an electromagnetic wave—a wave that has both electric and magnetic field components and travels at the speed of light.
In 1894, Marconi demonstrated the commercial potential of the phenomenon that Maxwell predicted and Hertz actually used in his apparatus. It was also Marconi that made radio a reality by pioneering the development of telegraphy without wires (i.e. ‘wireless’). Marconi was able to demonstrate very effectively that information could be exchanged between distant locations without the need for a ‘landline’.
Marconi’s system of wireless telegraphy proved to be invaluable for maritime communications (ship to ship and ship to shore) and was to be instrumental in saving many lives. The military applications of radio were first exploited during the First World War (1914 to 1918) and, during that period, radio was first used in aircraft.
This first chapter has been designed to set the scene and to provide you with an introduction to the principles of radio communication systems. The various topics are developed more fully in the later chapters, but the information provided here is designed to provide you with a starting point for the theory that follows.
1.1 The radio frequency spectrum
Radio frequency signals are generally understood to occupy a frequency range that extends from a few tens of kilohertz (kHz) to several hundred gigahertz (GHz). The lowest part of the radio frequency range that is of practical use (below 30 kHz) is only suitable for narrow-band communication. At this frequency, signals propagate as ground waves (following the curvature of the earth) over very long distances. At the other extreme, the highest frequency range that is of practical importance extends above 30 GHz. At these microwave frequencies, considerable bandwidths are available (sufficient to transmit many television channels using pointto-point links or to permit very high definition radar systems), and signals tend to propagate strictly along line-of-sight paths.
At other frequencies signals may propagate by various means including reflection from ionised layers in the ionosphere. At frequencies between 3 MHz and 30 MHz ionospheric propagation regularly permits intercontinental broadcasting and communications.
For convenience, the radio frequency spectrum is divided into a number of bands (see Table 1.1), each spanning a decade of frequency. The use to which each frequency range is put depends upon a number of factors, paramount among which is the propagation characteristics within the band concerned. In addition, various bands are set aside for radar use, and these are designated by letters (see Table 1.2). The use to which each band is put depends largely on how radio waves propagate within the band in question and also on the working range and target resolution that can be achieved.
Other factors that need to be taken into account include the efficiency of practical aerial systems in the range concerned and the bandwidth available. It is also worth noting that, although it may appear from Figure 1.1 that a
Table 1.1
Frequency bands
Frequency range Wavelength
300 Hz to 3 kHz 1000 km to 100 km
3 kHz to 30 kHz 100 km to 10 km
30 kHz to 300 kHz 10 km to 1 km
300 kHz to 3 MHz 1 km to 100 m
3 MHz to 30 MHz 100 m to 10 m
30 MHz to 300 MHz 10 m to 1 m
300 MHz to 3 GHz 1 m to 10 cm
3 GHz to 30 GHz 10 cm to 1 cm
Designation
Extremely low frequency (ELF)
Very low frequency (VLF)
Low frequency (LF)
Medium frequency (MF)
High frequency (HF)
Very high frequency (VHF)
Ultra high frequency (UHF)
Super high frequency (SHF)
Figure 1.1 Some examples of frequency allocations within the radio frequency spectrum
Table
1.2 Radar bands
Band
L
S
C 4 GHz to 8 GHz 7.5 cm to 3.75 cm
X 8 GHz to 12 GHz 3.75 cm to 2.5 cm
K u 12 GHz to 18 GHz 2.5 cm to 1.67 cm
K 18 GHz to 26 GHz 1.67 cm to 1.11 cm
K a 26 GHz to 40 GHz 1.11 cm to 7.5 mm
great deal of the radio frequency spectrum is not used, it should be stressed that competition for frequency space is fierce and there is, in fact, little vacant space! Frequency allocations are, therefore, ratified by international agreement and the various user services carefully safeguard their own areas of the spectrum.
1.2 Electromagnetic waves
As with light, radio waves propagate outwards from a source of energy (transmitter) and comprise electric (E) and magnetic (H) fields at right angles to one another. These two components, the E-field and the H-field, are inseparable. The resulting wave travels away from the source with the E and H lines mutually at right angles to the direction of propagation, as shown in Figure 1.2
Typical aircraft radar application
Distance measuring equipment (DME)
Airport surveillance radar (ASR)
Microwave landing systems (MLS)
Weather radar (WXR)
Military combat aircraft
Radio waves are said to be polarised in the plane of the electric (E) field. Thus, if the E-field is vertical, the signal is said to be vertically polarised, whereas, if the E-field is horizontal, the signal is said to be horizontally polarised.
Figure 1.3 shows the electric E-field lines in the space between a transmitter and a receiver. The transmitter aerial (a simple dipole, see page 28) is supplied with a high frequency alternating current. This gives rise to an alternating electric field between the ends of the aerial and an alternating magnetic field around (and at right angles to) it.
The direction of the E-field lines is reversed on each cycle of the signal as the wavefront moves outwards from the source. The receiving aerial intercepts the moving field and voltage and current is induced in it as a consequence.
Figure 1.2 An electromagnetic wave
Figure 1.3 Electric field pattern in the near field region between a transmitter and a receiver (the magnetic field has not been shown but is perpendicular to the electric field)
This equation can be arranged to make f or λ the subject, as follows:
×× = = 31088 310 Hzandm f f
As an example, a signal at a frequency of 1 MHz will have a wavelength of 300 m, whereas a signal at a frequency of 10 MHz will have a wavelength of 30 m.
When a radio wave travels in a cable (rather than in air or ‘free space’) it usually travels at a speed that is between 60% and 80% of that of the speed of light.
Example 1.1
Determine the frequency of a radio signal that has a wavelength of 15 m.
Here we will use the formula λ × = 3108 Hz f
Putting λ = 15 m gives:
×× ===× 86 6 31030010 2010Hzor20MHz 1515 f
This voltage and current is similar (but of smaller amplitude) to that produced by the transmitter.
Note that in Figure 1.3 (where the transmitter and receiver are close together) the field is shown spreading out in a spherical pattern (this is known more correctly as the near field). In practice there will be some considerable distance between the transmitter and the receiver, and so the wave that reaches the receiving antenna will have a plane wavefront. In this far field region the angular field distribution is essentially independent of the distance from the transmitting antenna.
1.3 Frequency and wavelength
Radio waves propagate in air (or space) at the speed of light (300 million meters per second). The velocity of propagation, v, wavelength, λ, and frequency, f of a radio wave are related by the equation:
==× 8 310m/s vf
Example 1.2
Determine the wavelength of a radio signal that has a frequency of 150 MHz.
In this case we will use λ × = 3108 m f
Putting f = 150 MHz gives:
××× ==== ×× 886 66 31031030010 2m 1501015010 f
Example 1.3
If the wavelength of a 30 MHz signal in a cable is 8 m, determine the velocity of propagation of the wave in the cable.
Using the formula where v is the velocity of propagation in the cable, v = fλ, gives: λ ==××=×=×668 30108m240102.410m/s vf
Test your understanding 1.1
An HF communications signal has a frequency of 25.674 MHz. Determine the wavelength of the signal.
Test your understanding 1.2
A VHF communications link operates at a wavelength of 1.2 m. Determine the frequency at which the link operates.
1.4 The atmosphere
The earth’s atmosphere (see Figure 1.4) can be divided into five concentric regions having boundaries that are not clearly defined. These layers, starting with the layer nearest the earth’s surface, are known as the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The boundary between the troposphere and the stratosphere is known as the tropopause and this region varies in height above the earth’s surface from about 7.5 km at the poles to 18 km at the equator. An average value for the height of the tropopause is around 11 km or 36,000 feet (about the same as the cruising height for most international passenger aircraft).
The thermosphere and the upper parts of the mesosphere are often referred to as the ionosphere and it is this region that has a major role to play in the long distance propagation of radio waves, as we shall see later.
The lowest part of the earth’s atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer.
1.5 Radio wave propagation
Depending on a number of complex factors, radio waves can propagate through the atmosphere in various ways, as shown in Figure 1.5. These include:
• ground waves
• ionospheric waves
• space waves
• tropospheric waves.
As their name suggests, ground waves (or surface waves) travel close to the surface of the earth and propagate for relatively short distances at HF and VHF but for much greater distances at MF and LF. For example, at 100 kHz the range of a ground wave might be in excess of 500 km whilst at 1 MHz (using the same radiated power) the range might be no more than 150 km and at 10 MHz no more than about 15 km. Ground waves have two basic components; a direct wave and a ground reflected wave (as shown in Figure 1.6). The direct path is that which exists on a line-of-sight (LOS) basis between the transmitter and receiver. An example of the use of a direct path is that which is used by terrestrial microwave repeater stations which are typically spaced 20 to 30 km apart on a LOS basis. Another example of the direct path is that used for satellite TV reception. In order to receive signals from the satellite the receiving antenna must be able to ‘see’ the satellite. In this case, and since the wave travels largely undeviated through the atmosphere, the direct wave is often referred to as a space wave. Such waves travel over LOS paths at VHF, UHF and beyond.
As shown in Figure 1.6, signals can arrive at a receiving antenna by both the direct path and by means of reflection from the ground. Ground reflection depends very much on the quality of the ground, with sandy soils being a poor reflector of radio signals and flat marshy ground
Figure 1.4 Zones of the atmosphere
being an excellent reflecting surface. Note that a proportion of the incident radio signal is absorbed into the ground and not all of it is usefully reflected. An example of the use of a mixture of direct path and ground (or building) reflected radio signals is the reception of FM broadcast signals in a car. It is also worth mentioning that, in many cases, the reflected signals can be stronger than the direct path (or the direct path may not exist at all if the car happens to be in a heavily built-up area).
Ionospheric waves (or sky waves) can travel for long distances at MF, HF and exceptionally also at VHF under certain conditions. Such waves are predominant at frequencies below VHF, and we shall examine this phenomenon in greater detail a little later, but before we do it is worth describing what can happen when waves meet
certain types of discontinuity in the atmosphere or when they encounter a physical obstruction. In both cases, the direction of travel can be significantly affected according to the nature and size of the obstruction or discontinuity. Four different effects can occur (see Figure 1.7) and they are known as:
• reflection
• refraction
• diffraction
• scattering.
Reflection occurs when a plane wave meets a plane object that is large relative to the wavelength of the signal. In such cases the wave is reflected back with minimal distortion and without any change in velocity. The effect is similar to the reflection of a beam of light when it arrives at a mirrored surface.
Refraction occurs when a wave moves from one medium into another in which it travels at a different speed. For example, when moving from a more dense to a less dense medium the wave is bent away from the normal (i.e. an imaginary line constructed at right angles to the boundary). Conversely, when moving from a less dense
Figure 1.6 Constituents of a ground wave
Figure 1.5 Radio wave propagation through the atmosphere
to a more dense medium, a wave will bend towards the normal. The effect is similar to that experienced by a beam of light when it encounters a glass prism.
Diffraction occurs when a wave meets an edge (i.e. a sudden impenetrable surface discontinuity) which has dimensions that are large relative to the wavelength of the signal. In such cases the wave is bent so that it follows the profile of the discontinuity. Diffraction occurs more readily at lower frequencies (typically VHF and below). An example of diffraction is the bending experienced by VHF broadcast signals when they encounter a sharply defined mountain ridge. Such signals can be received at some distance beyond the ‘knife edge’ even though they are well beyond the normal LOS range.
Scattering occurs when a wave encounters one or more objects in its path having a size that is a fraction of the wavelength of the signal. When a wave encounters an obstruction of this type it will become fragmented and re-radiated over a wide angle. Scattering occurs more readily at higher frequencies (typically VHF and above) and regularly occurs in the troposphere at UHF and EHF.
Radio signals can also be directed upwards (by suitable choice of antenna) so that signals enter the troposphere or ionosphere. In the former case, signals can be become scattered (i.e. partially dispersed) in the troposphere so that a small proportion arrives back at the ground. Tropospheric scatter requires high-power transmitting equipment and high-gain antennas but is regularly used for transmission beyond the horizon particularly where conditions in the troposphere (i.e. rapid changes of temperature and humidity with height) can support this mode of communication. Tropospheric scatter of radio waves is analogous to the scattering of a light beam (e.g. a torch or car headlights) when shone into a heavy fog or mist.
In addition to tropospheric scatter there is also tropospheric ducting (not shown in Figure 1.7) in which radio signals can become trapped as a result of the change of refractive index at a boundary between air masses having different temperature and humidity. Ducting usually occurs when a large mass of cold air is overrun by warm air (this is referred to as a temperature inversion). Although this condition may occur frequently in certain parts of the
Figure 1.7 Various propagation effects
world, this mode of propagation is not very predictable and is therefore not used for any practical applications.
1.6 The ionosphere
In 1924, Sir Edward Appleton was one of the first to demonstrate the existence of a reflecting layer at a height of about 100 km (now called the E-layer). This was soon followed by the discovery of another layer at around 250 km (now called the F-layer). This was achieved by broadcasting a continuous signal from one site and receiving the signal at a second site several miles away. By measuring the time difference between the signal received along the ground and the signal reflected from the atmosphere (and knowing the velocity at which the radio wave propagates) it was possible to calculate the height of the atmospheric reflecting layer. Today, the standard technique for detecting the presence of ionised layers (and determining their height above the surface of the earth) is to transmit a very
short pulse directed upwards into space and accurately measure the amplitude and time delay before the arrival back on earth of the reflected pulses. This ionospheric sounding is carried out over a range of frequencies.
The ionosphere provides us with a reasonably predictable means of communicating over long distances using HF radio signals. Much of the short and long distance communications below 30 MHz depend on the bending or refraction of the transmitted wave in the earth’s ionosphere which are regions of ionisation caused by the sun’s ultraviolet radiation and lying about 60 to 200 miles above the earth’s surface.
The useful regions of ionisation are the E-layer (at about 70 miles in height for maximum ionisation) and the F-layer (lying at about 175 miles in height at night). During the daylight hours, the F-layer splits into two distinguishable parts: F1 (lying at a height of about 140 miles) and F2 (lying at a height of about 200 miles). After sunset the F1 and F2 layers recombine into a single F-layer (see Figures 1.8 and
Figure 1.8 Typical variation of electron density versus height (note the use of logarithmic scales for both height and electron density)
1.10). During daylight, a lower layer of ionisation known as the D-layer exists in proportion to the sun’s height, peaking at local noon and largely dissipating after sunset. This lower layer primarily acts to absorb energy in the low end of the high frequency (HF) band. The F-layer
ionisation regions are primarily responsible for long distance communication using sky waves at distances of up to several thousand km (greatly in excess of those distances that can be achieved using VHF direct wave communication, see Figure 1.9). The characteristics of the ionised
Figure 1.9 Effect of ionised layers on radio signals at various frequencies
Figure 1.10 Position of ionised layers at day and night
Table 1.3
Ionospheric layers
Layer Height (km)
D 50 to 95 km
Characteristics
Develops shortly after sunrise and disappears shortly after sunset. Reaches maximum ionisation when the sun is at its highest point in the sky
E 95 to 150 km
ES 80 to 120 km
F 250 to 450 km
F1
150 to 200 km
Develops shortly after sunrise and disappears a few hours after sunset. The maximum ionisation of this layer occurs at around midday
An intense region of ionisation that sometimes appears in the summer months (peaking in June and July). Usually lasts for only a few hours (often in the late morning and recurring in the early evening of the same day)
Appears a few hours after sunset, when the F1- and F2-layers (see below) merge to form a single layer
Occurs during daylight hours with maximum ionisation reached at around midday. The F1-layer merges with the F2-layer shortly after sunset
Effect on radio waves
Responsible for the absorption of radio waves at lower frequencies (e.g. below 4 MHz) during daylight hours
Reflects waves having frequencies less than 5 MHz but tends to absorb radio signals above this frequency
Highly reflective at frequencies above 30 MHz and up to 300 MHz on some occasions. Of no practical use other than as a means of long-distance VHF communication for radio amateurs
Reflects radio waves up to 20 MHz and occasionally up to 25 MHz
Reflects radio waves in the low HF spectrum up to about 10 MHz
F2 250 to 450 km
Develops just before sunrise as the F-layer begins to divide. Maximum ionisation of the F2-layer is usually reached one hour after sunrise and it typically remains at this level until shortly after sunset. The intensity of ionisation varies greatly according to the time of day and season and is also greatly affected by solar activity
Capable of reflecting radio waves in the upper HF spectrum with frequencies of up to 30 MHz and beyond during periods of intense solar activity (i.e. at the peak of each 11-year sunspot cycle)
layers are summarised in Table 1.3 together with their effect on radio waves.
1.7 MUF and LUF
The maximum usable frequency (MUF) is the highest frequency that will allow communication over a given path at a particular time and on a particular date. MUF varies considerably with the amount of solar activity and is basically a function of the height and intensity of the F-layer. During a period of intense solar activity the MUF can exceed 30 MHz during daylight hours but is often around 16 to 20 MHz by day and around 8 to 10 MHz by night.
The variation of MUF over a 24-hour period for the London to New York path is shown in Figure 1.11. A similar plot for the summer months would be flatter with a more gradual increase in MUF at dawn and a more gradual decline at dusk.
The reason for the significant variation of MUF over any 24-hour period is that the intensity of ionisation in the upper atmosphere is significantly reduced at night and, as a consequence, lower frequencies have to be used to produce the same amount of refractive bending and also to give the same critical angle and skip distance as by day. Fortunately, the attenuation experienced by lower frequencies travelling in the
ionosphere is much reduced at night and this makes it possible to use the lower frequencies required for effective communication. The important fact to remember from this is simply that, for a given path, the frequency used at night is about half that used for daytime communication.
The lowest usable frequency (LUF) is the lowest frequency that will support communication over a given path at a particular time and on a particular date. LUF is dependent on the amount of absorption experienced by a radio wave. This absorption is worse when the D-layer is most intense (i.e. during daylight). Hence, as with MUF, the LUF rises during the day and falls during the night. A typical value of LUF is 4 to 6 MHz during the day, falling rapidly at sunset to 2 MHz.
The frequency chosen for HF communication must therefore be somewhere above the LUF and below the MUF for a given path, day and time. A typical example might be a working frequency of 5 MHz at a time when the MUF is 10 MHz and the LUF is 2 MHz.
Figure 1.12 shows the typical MUF for various angles of attack together with the corresponding working ranges. This diagram assumes a critical frequency of 5 MHz. This is the lowest frequency that would be returned from the
ionosphere using a path of vertical incidence (see ionospheric sounding on page 8).
The relationship between the critical frequency, fcrit., and electron density, N, is given by: =×× 3 crit 910 fN
where N is the electron density expressed in electrons per cm3
The angle of attack, α, is the angle of the transmitted wave relative to the horizon.
The relationship between the MUF, fm.u.f., the critical frequency, fcrit., and the angle of attack, α, is given by:
Example 1.4
Given that the electron density in the ionosphere is 5 × 105 electrons per cm3, determine the critical frequency and the MUF for an angle of attack of 15°.
Now using the relationship =×× 3 crit 910 fN gives:
Figure 1.11 Variation of MUF with time for London–New York on 16th October 2006
1.12 Effect of angle of attack on range and MUF
The MUF can now be calculated using:
6.3646.364 24.57MHz
Test your understanding 1.3
Determine the electron density in the ionosphere when the MUF is 18 MHz for a critical angle of 20°.
1.8 Silent zone and skip distance
The silent zone is simply the region that exists between the extent of the coverage of the ground wave signal and the point at which the sky wave returns to earth (see Figure 1.13). Note also that, depending on local topography and soil characteristics, when a signal returns to earth from the ionosphere it is sometimes possible for it to experience a reflection from the ground, as shown in Figure 1.13. The onward reflected signal will suffer attenuation but in some circumstances may be sufficient to provide a further hop and an approximate doubling of the working range. The condition is known as multi hop propagation.
The skip distance is simply the distance between the point at which the sky wave is radiated and the point at which it returns to earth (see Figure 1.14).
Note that where signals are received simultaneously by ground wave and sky wave paths, the signals will combine both constructively and destructively due to the different path lengths and this, in turn, will produce an effect known as fading. This effect can often be heard during the early evening on medium wave radio signals as the D-layer weakens and sky waves first begin to appear in Table 1.4
Test your understanding 1.4
Table 1.3 shows corresponding values of time and maximum usable frequency (MUF) for London to Lisbon on 28th August 2006. Plot a graph showing the variation of MUF with time and explain the shape of the graph.
1.9 Space weather
Conditions on the Sun and in the solar wind, magnetosphere, ionosphere and thermosphere
Figure
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crooked for want of cutting, dressing, combing, and ordering; and more change than this the words or sense do not bear. 3. There was no other change, but what was by natural growth; for the Hebrew word הבר doth properly signifie multus fuit, succrevit in multitudinem: so that the hairs were increased naturally in multitude and length, and the nails in magnitude and length, and so there was no essential change at all, but only an excessive augmentation of them both, he having lost the use of reason, whereby he could not use means to cut, cleanse, and order them. So that they did but grow squalid and ill-favour’d for want of using means to order and make them comely, even as many that have been lost, or left in Desarts, and desolate places, have after some length of time been found to be overgrown with hairs and ugly nails, that they have scarce been taken for men, but have appeared as savage and feral Monsters.
Argum. 3. Dan. 4. 34, 36.
Avenar. Diction. pag. 313. Palon. in loc.
3. His restauration doth plainly testifie what kind of change it was; for that which was restored unto him, did bring him into the same condition that he was in, before this transformation; and that was his knowledge or understanding. Now therefore if his knowledge or understanding did reduce him to the right use of reason, and brought those conditions and qualities that he had before: Then it is most plain, that it was only his knowledge or understanding that was taken away or changed; and so there was no other transformation, but what was internal in the mind, judgment, or imagination, by altering his will, desires, cogitations, condition, and qualities, and so no essential transformation at all, nor no change of his external shape, but what grew naturally in regard of his hair and nails or skin, for want of due ordering and decent dressing. And that this is an unanswerable truth, the words in the Text do sufficiently testifie, which are in our English: And mine understanding returned unto me, and at the same time my reason returned unto me; therefore it was only his understanding and reason, that had for a time been turned from him, and at his restauration they returned, or came again. Tremellius renders the former Verse: Et mente meâ ad me reversâ Excelso benedixi. And in the latter: Mente meâ reversâ in me. In both Verses Arias Montanus renders it: Cognitio mea super me reversa est; for
the Hebrew word there used עדי scivit, restituit, cognovit, agnovit, propriè est mentis & intellectûs, as Avenarius saith. And the Septuagint in both the Verses do agree with the Hebrew, αἱ φρένες μου ἐπ’ ἐμὲ ἐπεσρόφησαν. And to this purpose doth the French, Italian, and Luthers Translation render it, only the vulgar Latine gives it by the word sensus, & figura mea reversa est, which is altogether vicious. So that from hence we may safely conclude, that this transformation was only internal and mental, and no essential change at all: of which a most learned Divine tells us thus much: Sunt nonnulli, inter quos est Johannes Bodinus, qui putant humanam figuram reverà fuisse ei ademptam. Ac sanè Deus pro sua omnipotentia miraculum hoc in rege isto impio facere, & humanam ejus naturam in bruti animalis essentiam mutare potuit: sed verisimilius est regem alienatum mente, vel etiam maniacum factum, ademptâ ei divinitùs mente, ut patet, ex sequente vers. 34. & in furorem versum, sive per iram, sive per dolorem, ob acceptam ignominiam, quòd regiâ dignitate esset orbatus. Sic Ericus Rex Sueciæ in furorem est actus per iram & dolorem, quòd regno esset dejectus, Anno 1568.
Obser. medic. p. 57, 58.
History 2.
Sennert. de Hydrop. pag. 417.
History 3.
Obser. medic. l. 10. p. 440.
Argum. 4.
4. That this was only a mental and internal transformation, as are many sorts of Melancholy, especially that which Physicians call Lycanthropia, or Melancholia lupina, Rabies canina, and the like, is most manifest by comparing it with some of these that we have named; of which (though we have related some before) we shall give some few, from Authors of credit and veracity. 1. And first concerning the effects of that Madness caused by the biting of a mad Dog, we have a most sad and deplorable story recited by Philip Salmuth, that experienced Physician of Anhalt, which we shall here give in English: “Many (he
History 1.
De Hydrop. l. 1. c. 12. p. 99.
De medend. morb. c. 9. p. 97.
History 4. Vid. Polan. Rolloc. & alios in Dan. c. 4. v. 16.
The World surveyed, part. 2. c. 19. p. 270. History.
saith) do verily think that the force of this poyson will break out, and appear within a few months or years. But experience doth altogether testifie the contrary. As certain learned Authors do commemorate, that it hath laid hid in some the space of seven years, but in others it hath broke forth in the twelfth. Guainerius also mentioneth a certain person, to whom the Hydrophobia did happen the 18. year after he was bitten by the mad Dog. Moreover (he continueth) a most Noble person of Hagen hath told me, that a certain Noble man was bitten in the face by a little pretty Dog, which he much delighted in, and that the seeds of that poyson, as it were nourished in his bosom for a long time, at the last did suddenly break forth. For after that for some years feeling no molestation nor trouble from that bite, he addressing himself to a Virgin did marry. And the nuptial Supper being ended, and the Bride brought to the Marriage-bed, her Kinsfolks a little after do hear her complaining and lamenting. At which they laughed and jested, thinking it but to be the Venereal sport. But that howling continuing late, they by force do break the barred doors of the Chamber, and enter, and find that the Bridegroom had bitten with his teeth, plainly after the manner of a Dog, the face of the Bride, and also the shoulders and arms, and the fleshy places, and still did not give over the same sort of biting. Being much astonished with this sad spectacle and cruel wickedness, they with an ireful and provoked mind do forthwith slay him: and the new Bride also died the same day.” Though this he had but by relation, yet it was from a person of great quality; and if he had not been reasonably assured of the truth of it, he would never have writ it down amongst his Medical Observations. But this is also attested by other Authors of sufficient credit, of divers of this sort of persons, that have both barked and bitten like Dogs, and this is testified by Scribonius Largus and Rhases, as Baptista Codronchus hath cited them; and learned Sennertus tells us this: “That some (if bitten with Dogs) do bark like Dogs, and flye at whomsoever they meet, and that against or besides their will. For (he saith) Gentilis relateth in his Comment upon Avicen, that a certain young man troubled with this rabiousness, did exhort his Mother, that she should not come near him, for he could not contain himself but bite those that came near him.” 2. As concerning Wolf-melancholy, we shall only give a short relation or two, the first from Donatus ab alto mari, who confesseth that he had seen two: of the one of which he saith: “This person (he
saith) having formerly known me, did one day meet me when he was holden with this distemper; but I truly fearing went aside, and he looking at me a little went away. There was with him a multitude of men, and he did bear upon his shoulders a whole thigh and a leg of a dead man: At last being cured he was well, who afterwards when he met me again, did ask me, if I had not been afraid, when he found me in such a place when he was mad: by which it is manifest, that in him the memory was not vitiated.” Another take from that able Physician of Delfe Petrus Forestus in English thus: “A certain Countryman was in the Spring-time seen at Alemaria with an horrid look, and mad, to stay about the Church-yard, and after to enter the Church, and did leap upon a Seat or Plank (as we have seen him) only climbing upwards, and another while downwards with great fury, and never resting in one place. He carried a long staff in his hand, but did strike no body, but did with it beat off the Dogs; for he had his thighs and legs black and ulcered with black cruds or scurff by the biting of Dogs. His whole body did appear squalid, very black, and melancholick, but pale in the face, and his eyes exceeding hollow. From the foresaid signs (he saith) I did judge the man affected with the Lycanthropia or wolfish Melancholy. He never used any Physician that I know of.” And this both this Author, Schenckius and Sennertus do sufficiently confirm from Paulus, Aetius, Avicen, and the like. From all which it is clear and manifest, that Nebuchadnezzars distemper was but as some kind of Melancholy, whereby the imagination was corrupted, and the use of reason and right understanding for the time taken quite away, as saith the Text: Let his heart be changed from mans, and let a beasts heart be given unto him. That is, let his thoughts, desires, and affections be made brutish; for by the heart in Scriptures the cogitations, will, and affections are understood, as, my son give me thy heart, that is, the love and affections of thy soul and heart. So that when it is said, Let a beasts heart be given him, that is, let his mind, thoughts, and affections be made bestial; and so there was a change of the conditions and qualities of his mind and heart, but no real or essential change of his natural heart at all. And in this sense Tremellius doth take it, saying: obbrutescat, nihil humanum sapiat, and so doth Polanus, Rollock, and others understand it; for Polanus saith: Debuisse animum ejus prorsus obbrutescere, & mentem judiciúmq; animi humani amittere: non enim intelligendum hoc de
metamorphosi aliqua in corpore facta, sed de animo tantùm obbrutescente. So that from these examples it appeareth, that many persons, by reason of Melancholy in its several kinds, have been mentally and internally (as they thought, being depraved in their imaginations) changed into Wolves and other kind of Creatures, and have acted their parts, as though they had been really so, when the change was only in the qualities and conditions of the mind, and not otherwise. And so only was the change of Nebuchadnezzar, which notwithstanding Bodinus, the Popish Writers, and Witchmongers have falsely and ignorantly taken it to be a real transubstantiation, when it was only mental: so apt are men to mistake and urge things amiss, when it lyes for their own gain or interest. But if these persons that thought themselves really changed into Wolves, had been covered with a Wolfes skin fitted to their bodies, and gone upon all four, and so have acted the parts of Wolves, then it might in all likelihood have more strongly induced them to have believed a real transmutation indeed, though that way neither had there been any change of substance, but only a counterfeit and cunning disguisement: of which we shall here insert (for diversion sake) a pleasant story from the Pen of Vincent le Blanc of Marseilles, and leave it to be judged of according to the credit of the Author, which runs thus: “As concerning the Anthropolychi, I have not heard (he saith) of any thing so strange, as that the Governor of Bagaris, related once to me. He told me, that going with some of his Company from Lionac to Montpelier, they overtook an old man with a Sack on his shoulders, going a great pace towards the same Town, a Gentleman of the Company out of charity told him, if he would, one of his Servants, to ease him, should carry his burden for him: at first he seemed unwilling to be troublesom; but at length accepted the offer, and a Servant of the Commanders Chamber called Nicholas took the burden, and being late, every one doubled his pace, that they might get in in good time, telling the good old man, they would go before, and he should find them at the White Horse. The Servant of the Chamber coming in with the first, had a curiosity to see what was in the Sack, where he found a Wolfes skin, so properly accommodated for the purpose, that he had a strong fancy to disguise himself in it: whereupon he got it upon his back, and put his head within the Head-piece of the Skin, as ’twere to shew his Masters a Masquerade; but immediately a fury seized him, that in the Hall
where they supped, he made straight to the Company at Table, and falling on them with teeth and nails, made a dangerous rude havock, and hurt two or three of them, so as the Servants and others fled to their Swords, and so plyed the Wolf with wounds, that they laid him on the ground, and hurt in several places. But as they looked upon him, they were amazed when they saw under the Skin a poor Youth wallowing in blood. They were fain to lay him presently on a Bed, taking order for his wounds and hurts, whereof he was recovered; and was long before he could be cured: But this cured him of the like curiosity against another time. The Company by this means had but a bad seasoned Supper, and many of them were sick either of hurt or apprehension. For the old man Wolf, ’twas not known what became of him; but ’tis probable, that hearing of this tidy accident, he was cautious to appear.” Now if this relation be true, as there is nothing in it that seems either impossible or improbable, but that it might, then from it we may observe these two things. 1. To consider for what end the skin of the Wolf was so fitted and prepared, which might be to act some part of a Tragedy or Comedy in, or in sport to fright some persons withal; but then it is not likely, but that the old man would have appeared and sought for it again, which he might have done without fear or danger. But I rather conjecture it was for some more pernicious purpose, as in that disguise to fright Travellers and Passengers, that thereby they might (for without doubt the old man had other Companions) more securely rob them, and so escape, and not be discovered or apprehended, which might make him afraid to be seen, or to seek it again. 2. We may note the curiosity of the young man, and the strength of his fancy, being moved to see himself, so fitly to appearance, to be so like a Wolf, and not to the steams flowing from the Wolfes skin to work upon his imagination, which we leave to the inquisition of Naturalists, that live in Countries where Wolves are, to make tryal of.
So having sufficiently disproved their supposition or assumption, that Nebuchadnezzar was essentially transformed into a beast, we shall also shew the consequence that (if it had been true) they would draw from it, to wit, that if Nebuchadnezzar were really transformed into a beast, much more may the Devil transform himself into the shape of any Creature, and may change Witches into Cats, Dogs, Hares, and the like, which can by no true Rules of Argument be good, because it stands upon divers, or rather contrary efficients, namely
God and the Devil. The one having of himself an absolute and indeterminate power, and therefore of himself able to work what he will, where, when, and howsoever best pleaseth himself. And so by consequence he might (if it had so seemed good in his wisdom) have essentially transformed Nebuchadnezzar into an ox. The other (the Devil I mean) he hath only a finite and limited power, and therefore utterly unable of himself to accomplish any one work beyond the bounds of that power: and so by consequence he cannot possibly transform himself essentially into any Creature whatsoever, without a special power from God. Lastly we shall conclude all with this binding Argument: what transubstantiations soever are wrought, the thing transformed ceases to be what it was before, both in nature, and properties, as Lots Wife being transubstantiated into a Pillar of Salt, did cease to be flesh, blood, and bones, as she was before, and lost all the properties of humane Nature. So if Devils or Witches be transubstantiated into other Creatures, they cease to be what they were before both in Nature and Properties. And then by consequence the Devil should cease to be a Devil in Nature and Properties, and the Witches should cease to have humane Nature and Properties in them.
Having laid down these positive Arguments, we shall in the next place shew the horrid absurdities of these Tenents, to wit, of holding a visible Contract, that the Devil sucks upon the Witches bodies, that they have carnal Copulation together, or that they are essentially changed into Cats, Dogs, or Hares, or that they can flye in the air, or raise storms or tempests, and kill men or Cattel, and the like, and that in this order.
2 Pet. 2. 4.
Mat. 8. 31, 32.
Job 1. 11, 12. 2. 5. 6.
Absurd. 1.
1. These Tenents do derogate from the Wisdom and Power of God in his Government of the World by divine Providence, because by these it is supposed that the Devils and Witches do operate what, when, and howsoever it pleaseth them, and so the life and estate of all Creatures should be in their power to afflict, torment, or to destroy when they please, which is both false and blasphemous. For the Devils and wicked men are enemies and rebels against God, but
Jude 6.
Mar. 5. 9. to 14.
Joh. 19. 11. Rolloc. in loc.
Isa. 38. 29.
yet conquered, and imprisoned, and chained close up by his Almighty Power, that they are not able to act any thing at all (except the evil of their own wills) nor put that into execution, but as far as God doth license and order them, which we shall make plain in these two particulars. 1. The Devils are kept, in chains of darkness unto the judgment of the great day, that is, though their wills be corrupt, wicked, and evil, and that they have a continual desire, like a roaring lion, to seek whom they may devour; yet are they restrained from acting this evil, by the mighty Power of God, and can execute nothing at all, but only as far as God doth order and command them: so the Devils could not by their own power enter into the herd of Swine, until Christ gave to them leave: neither could Satan hurt Job either in his goods or body (though he strongly and earnestly desired it) until he had leave and commission given him from God. No more can Devils or Witches perform these things that are pretended; for it can never be proved that ever God did, or will give them order or leave to perform any such filthy or wicked thing, for which there can be no reason or end assigned why God should order such things to be done, so far different and opposite to the rules of his Justice, Wisdom, and Providence. 2. Nor can wicked persons act what they please, but God doth bridle and restrain them as he pleaseth; for though Pilate proudly thought and boasted that he had power to condemn Christ, or to let him loose, yet our Saviour tells him: Thou couldst have no power at all against me, except it were given thee from above. Upon which place learned Dr. Hammond saith thus: “So that thou hast neither right nor power to inflict any punishment on me, were it not that God, who is my Father, hath in his great Wisdom and divine Counsels, for most glorious ends, for the good of the World, determined to deliver me up into thy power to suffer death under thee.” Of which another saith thus: Verba hæc duobus modis accipi possunt: partim quia omnis potestas est à Deo, & divinâ ordinatione; partim quia qui cum potestate est, nihil planè potest, nisi ex Dei efficaci dispensatione ac providentia. So this is manifest in the excessive pride and boasting of Sennacherib of his own power, and taking no notice of Gods inevitable Decree in his Providence, that it was he, even the Lord of Hosts that had done it, and of ancient times had formed it, without which Sennacherib could have done nothing; but because he despised Gods Power and Providence, therefore saith the Lord: Therefore will I put my hook in thy nose,
and my bridle in thy lips, and I will turn thee back, by the way by which thou camest, which was performed by the slaughter of his Army, and the sending him back into his own Country: so little do mens purposes and counsels prevail, when the Lords will and purpose are against them.
Absurd. 2.
2. These Tenents do divert and obstruct the power and practice of Godliness: For while the Saints of God are taught, that they are to fight the good fight of Faith; and if they intend to be crowned, they must fight stoutly, and gain the victory, knowing, that they fight not against flesh and blood, but against spiritual wickedness in high places, and that therefore they are to take unto them the whole Armour of God: Therefore they knowing that this warfare is spiritual, and against spiritual enemies, and that the weapons both offensive and defensive are also spiritual; therefore they ought always spiritually to watch and stand upon their guard, lest their subtile and cruel enemy the Devil take them unawares, or by his Stratagems surprize them. For he is that old crafty Serpent, that hath innumerable wiles, and while he intendeth one thing, he pretendeth another; and like a cunning Enemy, gives a false Alarm at the one side of the Camp, while he assaulteth another, or making false fires or shews he seemeth to march away, when in the dark of the night he intendeth to fall on. So lest the Christian should be watchful and prevail, he laboureth by false Teachers, which are the Magicians and Sorcerers in the Mystery, to draw them from their vigilancy, by possessing their minds with these lying Tenents, that the Devil comes in the shape of a Cat or a Dog to a Witch, and bargains with her, and the rest, that whilst they are set at gaze to look for him in a bodily shape, they are made negligent in their spiritual watch, and so are diverted from the spiritual combate, and thereby the power and practice of Godliness is diverted and obstructed. Therefore we are to give heed unto the counsel of the Holy Ghost; to resist the Devil in his spiritual assaults with the spiritual weapons that God bestows upon us, and not to give heed to old Wives Fables, or the false Doctrine of Witchmongers, that make us watch for the Devil where he is not, and in the mean time not to resist him where he is, and that is within effectively in a spiritual manner, for he worketh in the children of disobedience, and therefore a Devil within us is more to be feared, than a Devil without us.
Absurd. 3. Considerat. about Witchcraft, pag. 95, 96.
3. These Tenents do uphold that horrid, lying, and blasphemous opinion, that our blessed Saviour did cast out Devils by Beelzebub the Prince of Devils: For when they could not deny, nor disprove the plain and open matters of fact, that our Saviour did really cast out Devils, then they devilishly invented and vented, that though he did so, yet it was but by the help of the Prince of Devils, with whom he had a compact, and so wrought by the greater power to over-power the less. Concerning which Mr. Glanvil is pleased to tell us this: “In his return to which he denies not the supposition or possibility of the thing in general, but clears himself by an appeal to the actions of their own children, whom they would not tax so severely.” But by Mr. Glanvils leave we must affirm, that though it be a bold assertion, yet it is not true; for our Saviour doth absolutely confute the supposition both in the general, and also in reference to himself, by shewing the absurdities of it, and that by these Arguments.
Vid. loc. citat.
Vid. Dr. Hammond in Math. 12. 25, &c.
1. They supposed that the Devils had a Prince or a Ruler that was able to cast out Devils that were his Subjects, and inferior unto him, to which his answer is: Every Kingdom divided against it self is brought to desolation, and every City or house divided against it self cannot stand. And if Satan cast out Satan, he is divided against himself, how shall then his Kingdom stand? Upon which a most learned Author doth thus paraphrase: “If any King mean to uphold his Kingdom, he will not quarrel and fall out with his own Subjects, and cast them out, which are doing him service; such divisions and civil dissentions as these will soon destroy his Kingdom, and therefore cannot probably be affirmed of any prudent Ruler or Prince. And Satans casting out Devils which are about his business (possessing those he would have possest) would be such a civil dissention as this, and a breach.” From whence he necessarily concludeth, that either Satan doth not cast out Satan, or else that his Kingdom is divided, and cannot stand, but come to desolation. But Satans Kingdom is not destroyed nor brought to desolation; therefore it is not divided against it self, and consequently Satan doth not cast out Satan. Of this passage Theophylact saith: Quomodo enim Dæmones seipsos ejiciunt, quum
magis inter se conveniant? Satan autem dicitur adversarius. And to the same purpose is that of S. Chrysostom: Si divisus est, imbecillior factus est, & perit: si autem perit, qualiter potest alium projicere?
Vid. Hieron. in loc.
2. Our Saviour saith further: And if I by Beelzebub cast out devils, by whom do your children cast them out? For as the fore-cited Author saith: “Why may not I cast out Devils by the Power and in the Name of God, as well as your Disciples and Country-men, the Jews among you (who being evil are therefore more obnoxious to suspicion of holding correspondence with Satans Kingdom) do, at least pretend to do. When they in the Name of God go about to cast them out, you affirm it to be the Power of God, and so do I. Why should you not believe that of me, which you affirm of your own?” Si expulsio (saith S. Hierom) Dæemonum in filiis vestris Deo, non Dæmonibus deputatur, quare in me idem opus non eandem habeat & causam?
3. Christ further urgeth: But if I cast out Devils by the Spirit of God, then the Kingdom of God is come upon you or unto you. “But if it be indeed by the Power of God, that I do all this, then it is clear, that although you were not aware of it, yet this is the time of the Messias, whose Mission God hath testified with these Miracles, and would not have done so, if it had been a false Christ.” So that he seemeth to conclude thus: “You Scribes and Pharisees seem to acknowledge, that there are real possessions by Devils, and that they may be thrown out, either by the Power of God or the power of Satan. But I have shewed the absurdity, that Satan doth not cast out the Devils his obedient Subjects that are doing his service; and therefore that what I do must be by the finger of God, and that must certainly denote unto you, that his Kingdom is come, and that I am the Messias.”
Chrysost. in loc.
4. He proceedeth: Or else how can one enter into a strong mans house, and spoil his goods? except he first bind the strong man, and then he will spoil his house. “My dispossessing Satan of his goods, and turning him out of those whom he possesses, is an argument that I have mastered him, and so that I do not use his power, but that mine is greater than his, and imployed most against his will, and to his damage.” Quòd enim (as saith a learned Father) non potest Satanas Satanam ejicere, manifestum ex dictis est: sed quoniam neq; alius potest eum
ejicere, nisi priùs eum superaverit, omnibus est manifestum: Constituitur ergo quod & anteà, cum manifestiori abundantia. Dicit enim: Tantum absisto ab hoc quòd utar Diabolo Coadjutore, quòd prælior cum eo, & ligo eum: Et hujus conjectura est, quòd vasa ejus diripio. Et sic contrarium ejus quod illi tentabant dicere, demonstrat. Illi enim volebant ostendere, quòd non propriâ virtute ejecit Dæmones. Ipse autem ostendit, quòd non solùm Dæmones, sed & eorum Principem ligavit: quod manifestum est ab his quæ facta sunt. Qualiter enim Principe non victo, hi qui subjacent Dæmones direpti sunt?
5. Lastly he concludeth: He that is not with me, is against me: And he that gathereth not with me, scattereth abroad. “And it’s proverbially known” (saith Dr. Hammond) “that he that is not on ones side, that brings Forces into the field, and is not for a mans assistance, he is certainly for his Enemy, engages against him, doth him hurt; and consequently my casting out Devils, shews that I am Satans declared Enemy.” By all which arguments he flatly overthrows the false supposition of the Pharisees.
Absurd. 4.
4. These Tenents do overthrow the chief Articles of the Christian Faith, to wit, the rational and infallible evidence of the Resurrection of Christ in the same individual and numerical body in which he suffered: and this we shall elucidate in these particular Considerations.
1 Cor. 15. 14, 15, 16, 17, 18.
1. The whole strength of the Christian Religion consists in the certainty of Christs Resurrection in his true and individual body. For as the Apostle argueth: And if Christ be not risen, then is our preaching vain, and your faith is also vain: yea and we are found false witnesses of God, because we have testified of God, that he raised up Christ: whom he raised not up, if so be that the dead rise not. For if the dead rise not, then is not Christ risen. And if Christ be not risen, your faith is vain, ye are yet in your sins. Then also they which are fallen asleep in Christ, are perished. If in this life only we have hope in Christ, we are of all men most miserable. So that all these sad consequences must needs follow, and the whole Christian Religion be found a lye, if Christ be not truly risen from the dead.
2. And though the Apostle do enumerate sufficient Witnesses of his Resurrection and appearance after death, and that he was seen of
Cephas, then of the Twelve, after that he was seen of above five hundred Brethren at once, then of James, then of all the Apostles, and lastly of himself: Yet all this Cloud of Witnesses will prove little, but dissolve into vapour, if there were or are either Angels or Spirits, that in their own or assumed bodies, may appear in his form, shape, and likeness, and to sight and tangibility be in all properties as his body was, to have flesh and bones, the print of the nails in the hands and feet, and to eat and drink.
Mat. 27. 51.
Mat. 28.6, 9.
Joh. 20. 1.
Joh. 20. 19, 26.
Mar. 6. 49.
3. That the Apostles held the opinion, that there was Apparitions and Spirits that did shew themselves in any form or likeness, is most plain and evident; for when they saw Christ walking upon the Sea, they supposed it had been a Spirit or Apparition, for the Greek is φάντασμα, and cryed out. That is, either being cruelly affrighted and amazed, their Phantasies did represent strange thoughts in their minds: or else (which doubtless was the truth) seeing Christ walking upon the Sea, which they thought was not possible for a man to do without sinking or drowning, they in great fear cryed out, and forgetting his former Miracles, did vainly suppose it some Spirit that had made an apparition in his likeness. But it is most strange, that the Disciples that had seen and been eye-witnesses of so many Miracles wrought by him during his life, and those that accompanied him at his death, as the renting of the veil of the Temple from the top to the bottom, and the Earth-quake, and the renting of the Rocks, and the Darkness that was over the Land from the sixth hour unto the ninth; and that after his Resurrection the Graves were opened, and many bodies of Saints that slept arose, and came out of the Graves, and went into the holy City, and appeared unto many, of which they could not be ignorant; It is (I say) most wondrous strange, that after all these they could doubt of the verity of his Resurrection, and imagine that it was a Spirit in his form and likeness. And most especially, considering that his Sepulchre was made sure, the stone sealed, and a Watch set to attend it, of which they could not be ignorant; and likewise the certain affirmation and evidence of the two Maries, from the mouth of the Angel, and their own sight who worshipped him, and held him
Mar. 15. 38.
Mar. 16. 1.
Luk. 24. 37.
Vid. Rolloc. in Joh. 20.
by the feet, and Peters finding the Sepulchre empty, and his appearing to the two Disciples that went to Emmaus, and yet for all this at his next appearance, not to be satisfied, but to be terrified and affrighted, and to suppose they had seen a Spirit, is beyond all wonder, but that doubtless the heavenly Father had so ordained it in his inscrutable Wisdom, that the infallible certainty of his Resurrection might be more evidently and punctually proved. For at his next appearing, when they were all together, Jesus himself stood in the midst of them, and said unto them, Peace be unto you. But they were terrified and affrighted, and supposed they had seen a Spirit, there the word is πνεῦμα. Now the cause of this supposing that they had seen a Spirit, doubtless was because as St. John tells us, That Jesus twice had stood in the midst of them, the doors being shut, because of the Jews, and therefore they could not possibly imagine, that he could have a body that could make penetration of dimensions, not considering that he had an omnipotent Power, and therefore nothing could be impossible unto him. Though it may well be conceived to be done without penetration of dimensions, because by his Almighty Power he might imperceptibly both open and shut the doors, and so enter, and suddenly stand in the midst of them, and no humane sense be able to discern it. But however it was, the Disciples did not then believe that it was Christ with his individual body in which he suffered, but either (as some of the Fathers believed) that it was his very Spirit that he yielded up upon the Cross, that appeared in his figure or shape, that was so pure, fine, and penetrable, that it could pass through any Medium, though never so dense or solid: or some other Spirit that assumed his form and shape, which is far more probable and sound. But howsoever it was, they did believe that it was some Spirit in his likeness, and not he himself, in that very numerical body in which he suffered, as may be apparently gathered from the words of Thomas called Didymus, who strongly affirmed, saying: Except I shall see in his hands the print of the nails, and put my fingers into the print of the nails, and thrust my hand into his side, I will not believe.
Rolloc. ubi supr. Vid. Caten. Aur. Tho. Aquin. in locum.
4. To the grounds of all these doubts our Saviour gives a demonstrative and infallible solution, which we shall explain in these particulars. 1. He doth not at all deny the existence
or beings of Spirits; neither that Spirits do not, or cannot make visible apparitions: but doth grant both. 2. But he restrains these apparitions to those inseparable properties that belong to Bodies and Spirits, that is, a body (that is to say an humane body) hath flesh and bones, but a Spirit hath neither, as Christs or humane bodies have; and therefore saith a learned Person upon the place: Docet se non esse Spiritum hoc modo: Spiritus, inquit, non habet carnem & ossa. Ego verò, ut conspicitis, habeo carnem & ossa: Ergo ego non sum Spiritus. Vide igitur ex sensu & sensibilibus: sensu nimirum visus; sensu tactus: ex visibilibus & tractabilibus se corpus esse non autem Spiritum edocet. Per sensum enim fides & gignitur & confirmatur. So that whether Spirits be taken to be corporeal (and so appear in their own bodies) or to be incorporeal (and so to appear in assumed bodies) yet are they both to sight, and especially to feeling, not as humane bodies are that have flesh and bones. So that however they do, may or can appear (for it must be considered in that latitude, else our Saviours argument would not be irrefragable and convincing) they to the resistibility of touching cannot be as flesh and bones are, for they to the sense of touching do resist, and are solid, but so the bodies of Spirits in what appearance soever have not, nor can have, otherwise our Saviours argument falls to the ground, and proves nothing. 3. He confirmeth this by the Disciples own proof of feeling and touching the prints or scars of the nails in his hands, and the print of the wound in his side, and thereby manifesteth that it was he himself, and the very same individual body in which he suffered, by which Thomas his great unbelief and doubting was unanswerably satisfied, by putting his fingers into or upon the very prints of the nails, and by putting his hand into or upon the wound or scar upon his side. And therefore though the same power that raised him from the dead, and rouled the sealed stone from the Sepulchre, could have perfected his body to be without prints or scars of the wounds; yet did the divine Wisdom reserve them, thereby to cure the infidelity of his Disciples, and undeniably to confirm the truth of his Resurrection; to which purpose one said well: Ibi ad dubitantium corda sananda, vulnerum sunt servata vestigia. And the further to establish and settle their Faith, he took a piece of a broiled fish, and of an honeycomb, and eat before them; all which concluded him to have a true body, and that he was not a Spirit: from whence we draw these conclusions.
1. That howsoever Spirits do or may appear, they have not, or can have such a body, that in respect of tangibility, is as flesh and bones. For flesh and bones are dense, solid, and make sensible resistance to the touch; but the bodies of Spirits in their apparitions are not, nor can be so. For as we deny not but there are and may be apparitions in any figure or shape, yet they can but be as the figures and shapes in the Clouds, which are often seen, and cause much wonder, though (we suppose) many of them may be rather attributed to the assimilation made in mens fancies, than to their real existence in those forms or shapes. So they may be as shadows, or the species of bodies that we see near or afar off, or as the images that we behold of our selves and other things in Mirrours or Looking-glasses: which though without doubt they be not non-entities, for nullius entis nulla est operatio, but these affect the senses, which is an operation or action; yet do they all easily yield to the touch, and have no firmness nor solidity, as flesh and bones have; and this is all that can be justly deduced from our Saviours argumentation.
2. Either we must believe that our Saviours argument is of no force and validity, which is blasphemous and horrid to affirm or imagine, he being the way, the truth, and the life, and in whose mouth there was found no guile, and thereby overthrow the whole foundation of the Christian Religion: or else we must for certain believe that Spirits whensoever they appear have no such solidity or resistibility as to touch, as flesh and bones have. And consequently that what strange things soever we may by sight and touch take to be the apparitions of Spirits, that to touch have the solidity of flesh and bones, we must conclude that they are not Spirits, but must be some other kind of Creatures, of whose nature and properties we are to inquire; for doubtless (as we shall manifest hereafter) there are many strange Creatures, that for their rarity or strange qualities, have been and are mistaken for the apparition of Spirits. For the Disciples doubts must still have remained unsatisfied, if Spirits could appear to have bodies to touch, of that solidity that flesh and bones are of, and then the truth of our Saviours Resurrection falls to the ground, and the Christian Faith is vain.
3. Therefore that Demons do appear in the shape of Dogs, Cats, and the like, and do carry the heavy bodies of Witches in the air, do suck upon their bodies, and have carnal copulation with them, must
suppose them to have bodies as solid and tangible as flesh and bones: and so overthrow the main proof of our Saviours Resurrection, and consequently the very foundation of the Christian Religion; For if Christ be not risen our faith is vain, we are yet in our sins, and are of all men most miserable, as having only hope in this life, and no further. And this is sufficient to shew the horrid and execrable absurdity of these opinions; which objection Mr. Glanvil calls spiteful and mischievous, but durst not undertake the solution, but with a plain shuffle leaves and over-runs it, as indeed being too hard a morsel for his tender teeth.
Gen. 18. 1, 4, 8.
And if any do object (as we have heard some do) that three Angels did appear unto Abraham in the Plains of Mamre, as he sate in the Tent-door, and did eat and drink, and washed their feet, and therefore that they had flesh and bones; to that we return this responsion.
1. It is a very froward and perverse way of arguing, to make one place of Scripture to clash with another, when they ought all to be expounded according to the Analogy of faith, and it is a perfect Harmony which we ought to labour to find out and rejoyce in.
Heb. 1. 1, 2.
2. It is no perfect way of arguing from the Dispensations in the time of the Patriarchs and Prophets, to those that God useth now in the time of the Gospel; for so they might argue that God should answer by Urim and Thummim, because he did so in the time of the Levitical Priesthood, but that is now ceased, and the Apostle tells us: God at sundry times, and in divers manners spake in times past unto the Fathers by the Prophets: But in these last days he hath spoken by his Son unto us. So though God did then vouchsafe to make himself manifest unto the Patriarchs by the visible appearance of Angels: yet it is no rational consequence that he doth so now in these days.
Ibid. v. 17.
3. It is manifest, that though they were in number three, yet it is true that it was Jehovah that appeared unto Abraham, and Jehovah said, Shall I hide from Abraham that thing which I do. Now we do not find that the word Jehovah is communicable to any Creature, but only to God himself; and therefore the best Expositors do understand (notwithstanding what Pererius doth say to the contrary) that one of them was Christ the second Person in the Trinity, who after was to take humane nature upon him, and therefore did so appear.
4. However these Angels had with them the assistance of a divine and omnipotent Power, which cannot rationally be affirmed of the common and ordinary apparitions of Demons to Witches, and therefore doth conclude nothing against what we have laid down before.
