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1911 Encyclopedia Britannica

Air Defence

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"AIR DEFENCE. - Even before aerial navigation and aviation had been developed to a practical point, the employment of aircraft in war for the attack of vulnerable places was discussed from time to time in a speculative way, and in the seven or eight years preceding the World War the types and characteristics of aircraft became so far definite that technical study could be brought to bear on the problem of defence, especially that of artillery defence. But this period of seven or eight years was short; military history could give no lead; practical experiments were almost impossible. Moreover, in the existing state of international law, liability to air attack was understood to depend not on whether a place was vulnerable, in the sense that its destruction would impair the capacity of the nation for carrying on the war, but on whether it was " defended," i.e. fortified in the conventional sense of the word, or at any rate held by a ground garrison against ground attack. Attention was therefore directed chiefly to the question of air attack on what according to the prevailing ideas were " military " objectives, and in view of the small numbers of aircraft then available such attacks were regarded as unlikely to affect the course of operations seriously.

In these conditions, and especially in the absence of all data based on practical experience, it is not surprising that defence against air attack was everywhere in a rudimentary state at the outset of the World War. In the war itself, on the contrary, experience, data and methods crowded upon one another from first to last, and through the clearer definition of the problems to be faced on the one hand and the constant process of trial and error on the other, it has become possible to formulate the main principles of air defence with some approach to precision.

I. General Considerations. Air defence, as discussed in the present article, deals with the arrangements which deny to enemy aircraft access to vulnerable points. By " access " is implied the gaining of a position either directly over the objective or sufficiently close to it for the success of the attack. Amongst " vulnerable points " are included bodies of troops and their materiel in the field, centres of population, large magazines, arsenals, harbours, ports, dockyards, ships and convoys at sea, big industrial centres, and the like. As with a fleet, the primary duty of the air forces is to seek out the enemy air forces and destroy them; but the problem in the air is far more difficult than on the sea, as a third dimension has to be taken into account, i.e. that of height. When we remember what difficulties have been introduced into naval war by the introduction of the submersible warship, slow as this is and small as is its up-and-down range, and when we realize further that, in the air, opposing craft can pass at great heights both above and below each other, move at speeds that are enormous relatively to any rate of movement on or under the sea and have to cope with extraordinary difficulties in detecting one another's proximity, it will be seen that the task in front of the air forces of any nation is the most difficult of all.

There is no " command of the air " while the enemy disputes it. Therefore, against attacks by air, it is a logical necessity to provide ground defences, and to limit the radius of the air units allotted to cooperate with them. But though defence against attack by air, as on the ground, may be active or passive, yet to be effective it must be both. Further, the conditions under which aircraft move, by day and by night respectively, are so widely different, that the conditions of the defence must be correspondingly varied to meet them.

It is not intended here to deal with operations launched over long distances against enemy aerodromes and depots, such being the role of the air forces alone, but solely with the local defence of areas occupied by " vulnerable points " of the kind which have been enumerated above.

The instruments of air defence are: - (a) the machine-gun on the ground; (b ) the machine-gun in the aeroplane; (c ) the heavier guns on the ground; (d ) the searchlight, the sound locator, the observer post; (e ) the aerial obstacle; (f) local protection on the ground, i.e. against bomb splinters and machine-gun fire, and camouflage.

Each of these weapons supplements a deficiency in one or more of the others; none is complete without one or more of the others acting in conjunction with it. From this it follows that, in any anti-aircraft organization, cooperation in effort can only be effected by organizing units of the air force, artillery and engineers under a single command, as in a formation of all arms in ground warfare.

The following are some of the forms which attacks by air may take: - bombing by airships, aeroplanes or seaplanes, singly or in formation; the harassing of troops on the ground or sea with machine-gun fire by aeroplanes or seaplanes, singly or in pairs; torpedoing ships at anchor, by seaplanes, probably in pairs or escorted by " scout " (i.e. air fighting) machines. To these may be added, though they only indirectly affect the problems here discussed: - photographic or visual reconnaissance, by aeroplanes or seaplanes, singly; and aerial engagements, by aeroplanes in formation on hostile patrol, i.e. ready to engage air-fighting groups of the enemy, or by aeroplanes, singly or in groups, attacking machines which are engaged in observation duties (especially artillery observation) in connexion with ground operations.

Anti-aircraft units are concerned primarily with hostile attacks the objectives of which are on the ground or sea; the defence of objectives in the air is a secondary matter, but nevertheless important when air-force units are not at hand to undertake the duty themselves.

It may not be amiss at this stage to mention a few of the peculiarities of sound and light. Although these properties are generally known, their full importance in relation to air defence is seldom recognized by those who have little experience in antiaircraft work. Sound travels at a certain known rate, namely about ',too ft. per second. Aircraft are generally first detected by ear. By the time the sound of a machine reaches the ear, the machine making it will have moved some distance away from the spot where it made the sound. A path of sound is deflected by the different velocities of the various air currents through which it passes on its way to the ground. A machine in the air is only visible to an observer on the ground by reason of light rays reflected from the surfaces of the machine, reaching his eye in sufficient intensity to enable him to detect it. Thus a machine flying " straight into the sun " is invisible to an observer also facing the sun. A spherical or cylindrical surface reflects light chiefly in the direction from which the illumination comes; hence, in the case of a balloon or airship, the observer sees the target best when he and a searchlight are on the same side of it.

A flat surface reflects light towards an observer further from the source of illumination than its own position; thus an aeroplane flying steadily in a searchlight beam is generally seen best when it is between the observer and a searchlight. does not follow that because aircraft are invisible to observers on the ground the ground is invisible to an observer in the machine, and vice versa. The greatest difficulty is frequently experienced in gaining the correct focus for the eye, and, having gained it, to maintain that focus. This is a difficulty common to the observer both on the ground and in the air, but whilst the former has only to look upwards, the latter has in addition to look all round and below his machine. Neither has the assistance of intermediate objects by means of which the focus of the eye can be altered and held at definite stages. By day the observer in the air is deaf, by night he is deaf and blind. A searchlight shining into the sky is only visible by reason of the particles of " dust " or moisture in the path of the beam. In a perfectly clear or clean atmosphere the beam is invisible.' 1 Unusually clear atmospheric conditions with a few high clouds were responsible for popular rumours prevalent during the war in 1914-8, to the effect that a new invisible searchlight had been discovered which simply threw a disc of light far away up in the sky, and was otherwise quite invisible.

II. Conditions affecting the various Instruments of Air Defence. (a) The machine-gun on the ground, on account of its comparatively short range, can only deal with targets flying at low heights, e.g. up to 3,000 or 4,000 feet. On the other hand, the ease with which it can be handled enables it to cope with the rapid change in angular velocity of low-flying targets in a way which the heavier guns cannot do. Low-flying machines move over their ground targets with a very rapid angular velocity, and owing to their small height they are often invisible from the ground objective until at a close range. Every unit of an army, therefore, requires anti-aircraft machine-gun equipment for its own local protection.

(b) The machine-gun mounted in the aeroplane can attack its target at close range, and, if its own aeroplane is superior to the target in speed and in climbing and manoeuvre power, can maintain that attack until the combat ceases. It is therefore of great importance in air defence.

Mention has already been made of the difficulties of seeing objects other than on the ground, and of hearing; and to overcome them it usually becomes necessary to direct the pilot and observer by signal (visual or wireless) from the ground, to assist them in finding the target which has been selected for attack.' Other serious handicaps to the observer in the air are the unstable platform for his gun and the difficulty of estimating the range to his target.

(c) The heavier gun on the ground acts in cooperation with the machine-gun in the air, and in substitution for it when weather conditions or other reasons prohibit the use of the aeroplane.

The gunnery problem is an extremely intricate one owing to the difficulties involved in range finding, the rates of change in angular velocity, the ease with which a target can change its height and course, and the fact that the target can only be engaged for a very limited space of time. The difficulties of the artilleryman originate from the time of flight of his projectile. On the other hand, he is not hampered as is the airman by the unstable platform and by the noise made by the engine.

(d) The searchlight has three roles to fulfil in air defence. (r) It points out the selected target to the defending aeroplanes. At night the pilot is deaf and practically blind, and, unless he carries a searchlight in his machine, he must depend on those on the ground to show him where the target is moving. (2) It illuminates it for the artillery. The artilleryman on the ground is blind, and cannot use his sights unless his target is well illuminated so that he can see it. (3) It exerts a moral effect deterrent to the attack. It is necessary to read the personal narratives of night-flying pilots, and to listen to their conversations, to appreciate the great moral effect which the systematic and unhesitating use of searchlight beams has upon them when they are approaching their objectives on the ground. They know that once the searchlight succeeds in laying on them they become the target for every gun and aeroplane within reach - an experience to be avoided as far as possible.

There are peculiarities in a searchlight beam which handicap the detachment to a large degree; the principal one is the frequent inability of a man standing close to a projector to see a target in the beam from it. This is sometimes due to a general prevalence of a local mist which diffuses light in all directions in the neighbourhood of the projector, but it may also be due to the blinding effect of a secondary cone of light close to the base of the main searchlight beam, which prevents the man close to the projector from detecting the light reflected from the target. At distances, however, varying from 6 to 20 ft. to one side of the projector, the effect of this secondary cone of light is generally so slight as to cause no interference. Projectors have accordingly been provided with long control arms fitted with wheels or handles at the end, so that the man whose duty it is to manipulate the beam can do so with the minimum of outside assistance.

An aeroplane can, by " side slipping " or otherwise executing some unexpected manoeuvre, generally escape from a single illuminating 1 By day the visual signal may be given by gunfire. By night it is more often made by the searchlight. For this purpose the Germans in Belgium erected large illuminated " arrows " composed of incandescent lamps in troughs of wood, designed to revolve on the roofs of concrete shelters.

searchlight, owing to the differences in the reflecting surfaces of an aeroplane when viewed from different angles. If, however, three beams manned by good detachments succeed in training on it, they can generally hold it whatever the gymnastics attempted by the target. On the other hand, if a target is illuminated by a comparatively large number of beams, say eight or ten, some of which must be at a considerable range from the target, there is a marked tendency for the latter beams to drop below the target altogether; although the detachments at those projectors are unaware of the fact that their beams are now useless, and may even interfere with the vision - and consequently the work - of detachments nearer the target.

An incident which took place during the German airship raid on London on the night of Sept. 2-3 1916, has been attributed to a reason of this nature. The Schiitte-Lanz airship SLi i, which was eventually burned that night and fell at Cuffley, was entering the area over London from a northerly direction. London itself was lying in what looked like a lake of mist, and the searchlights, which could hear the attack approaching over them, were seeking for it through the mist. Presently the airship was " picked up," and immediately from all quarters of the defences searchlights could be seen moving across to get on to it, until there may have been any number up to 20 beams shining in its direction.

The airship seemed to hesitate, and then swung round until she was steering north again. She was seen to empty one or more of her water ballast tanks and suddenly disappear. The searchlights lost her entirely for some minutes. Though, as is well known, she was eventually detected again and then held until she fell, there is little doubt that the searchlights were, in the first portion of the engagement, hampering each other in their work owing to the exposure of too many beams.

In the same engagement a searchlight near Kenton was quite useless owing to a dense mist surrounding it and the gun station near by. The local reflexion of the light by the mist was so great that it prevented either gun or light detachment from seeing the target.

During the first years of the war discussions were often heard as to the advisability of throwing out searchlight beams, on the assumption that the target might not know where it was, and might therefore pass away without dropping any bombs; in other words that the exposure of searchlights invited the arrival of bombs. The answers to these suggestions are simple, viz.: (i.) The searchlights are there to be used, because guns and aeroplanes are blind without them. Guns and aeroplanes cannot defend efficiently without seeing their target.

(ii.) There is no justification for the assumption that the enemy has lost his way and does not know where he is.

During the spring of 1917 a general display of searchlights round London was arranged to test their efficiency as a moral deterrent on airship commanders. Every searchlight in the London area was given a prearranged arc through which the beam was to be moved slowly and regularly at a given signal, the movement to be continued indefinitely until the signal was cancelled or enemy aircraft became audible or visible to any of the detachments. The intention was to expose all the beams (some 120) simultaneously as soon as an attack was heading towards the London area, but whilst it was still sufficiently far away to give the airship commander plenty of time to think matters over, and remind him of the aeroplanes and guns which were waiting for him, and of the fate of some of his

predecessors. On the two occasions when this scheme was put into force the attacks stopped short of the defended area and never came near it, though the German official communique announced on each occasion that they had dropped bombs on London.

A searchlight and its detachment are very vulnerable when within range of shell and machine-gun fire. On some occasions aircraft have occupied themselves in deliberately bombing searchlights, though the instances have been rare in England, and in no case was harm done either to the searchlights or to their personnel. On the other hand, in the areas where ground fighting was in progress, searchlights formed a vulnerable target for machine-gun fire from low-flying aeroplanes, though actual casualties were comparatively few.

The sound locator is an instrument which is intended to indicate the angle of elevation, and the bearing in azimuth, of aircraft audible but invisible from the ground.

Many types of sound locator were invented and tried by the various nations involved in the war, but none was eminently satisfactory. The fact of the matter was that but little was known of the vagaries of the paths of sound waves in the atmosphere. During and since the war, however, students have begun to appreciate and learn a few of its peculiarities, though at the present time knowledge of the subject is still little more than in its infancy. The pattern of sound locator most commonly used in the war was one with four trumpets, two for obtaining the direction in elevation and two for obtaining it in azimuth. In order to convey the information to the searchlight itself in as instantaneous a manner as possible, the locator was provided with a " ring sight," on the edge of which the searchlight beam was kept in contact by a " layer," who gave suitable directions to the men at the projector.

A defect of this type was its inability to eliminate certain sounds which had nothing to do with aircraft, such as those from petrol tractors, motor bicycles, etc., on the ground in the neighbourhood. The Germans in Belgium had to give up using their instrument at a searchlight near Bruges, because of the noise made by the frogs in the dykes all round it.

Another pattern of sound locator has been constructed in cliffs on the shore by cutting a concave surface in the face of the cliff, and providing an appliance for collecting the sound waves at their point of maximum concentration, in such a way as to indicate approximately the direction of the source of the sound. This pattern was used on the British coasts and warned observers of the approach of machines from Belgium when the latter were as far away as 15 to 20 miles. These locators also, however, were liable to error; and on one occasion a fleet of motor-boats caused an alarm which was only prevented from becoming public by the perspicacity of the local anti-aircraft commander.

The functions of the observer post, which may or may not be equipped with detector apparatus, are of great importance. The duty of the observers is to detect the passage of aircraft and report their movements to the authority controlling the air defences. On these reports depend the warnings to the civil and military authorities within the defended areas. Such duties demand considerable physical strength to bear the severe strain incurred by watching and waiting; and a high degree of refinement in hearing and eyesight, owing to the necessity of detecting and identifying aircraft at great distances. The speed of aircraft in the attack is the factor which determines the minimum distances of the posts from the objective, and those distances may involve the disadvantage of great isolation for many posts. The necessity of good and speedy means of communication between such posts and the controlling air defence authority to which they belong is obvious. However excellent an observer's training may be, a report based solely on what he has heard must be open to doubt, should it attempt identification of the class of the aircraft in question.

(e ) The aerial obstacle consists of some form of wire impediment hung from balloons, and intended to be such a menace to a flying machine that it will either pass beside the obstacle or, more probably, fly at a higher level than the balloons supporting it.

The Italian authorities claimed extraordinary success with the contrivances used in their defences. The French authorities were not so optimistic over the type adopted by themselves, and there is no proven case of success with the pattern used in Great Britain.

The Germans at Bruges and Zeebrugge flew kites and balloons by wires of a very high tensile strength, and one Handley Page bombing machine with its crew was destroyed at Bruges by these means. The balloons were about 15 ft. in diameter, and were used when wind power was insufficient to raise a kite. The kites were of at least two patterns, but both were of the double box-kite type. The lower ends of the wires were wound on vehicles provided with gauges, oil baths, and lightning " earths." They were managed by a few small boys, pressed into the German service at the rate of a few francs a night. It was calculated that the wire provided an obstacle up to about 3,000 feet.

In Great Britain balloons moored by wire cables were arranged in lines, and at some distance below the balloons was a bridle connecting all the cables. From this bridle at equal intervals were suspended long thin wires of considerable tensile strength.' Any arrangement of obstacles suspended from balloons must be particularly vulnerable, both from the shell fire of the defence and from any machine-gun fire brought to bear by the attack.

Many other forms of obstacles have been suggested from time to time, and perhaps one of the most ingenious was that of an aerial minefield. The inventor proposed to attach a small charge of explosive, sufficient to destroy an aeroplane wing, to a revolving vane by a length of fine cord. The charge was fitted with suitable percussion firing arrangements. The vane was attached for the purpose of delaying the fall of the explosive through the air. Charge, cord, and vane were neatly packed together so that considerable numbers could be carried in a box provided with a simple release. The pro 1 A curious incident occurred during an air-raid alarm in London during the war. To the astonishment of the detachments one complete series of balloons came down with unexpected suddenness, all being deflated by the rupture of their ripping panels. On examination, it was found that moisture had condensed on the ripping ropes and frozen there, until each cord was about as thick as a man's forearm. The weight had gradually increased on all with remarkable regularity until the ripping point was reached, when each balloon in the series was deflated almost simultaneously. There was a heavy mist that night, and the temperature at the ground level was above freezing-point.

cedure proposed was to send up a group of machines loaded with these " mines," on patrol well outside the defences, on any occasion when conditions were so favourable that a raid was probable. The " mines " were to be released across a broad belt through which the attack would probably pass, as soon as a signal was made from the ground that it had reached a suitable point on its course. The idea, however, was never carried into effect - probably owing to the danger involved to friendly machines, - but nevertheless it had possibilities which gave considerable promise, especially for use over the sea.

1 (f) Bombproof and Splinterproof Protection

2 Camouflage

3 Aeroplane and Seaplane Attack

4 (b) Communications.

(f) Bombproof and Splinterproof Protection

Local protection for personnel, animals and stores involves the provision of shelters proof against the bombs themselves and their splinters.

A bomb with a stout and heavy point, and provided with a means to keep it revolving so that the point travels first, will, if launched from a great height, penetrate most practicable forms of shelter. A stout double roof of concrete with the sides sloping fairly steeply, and provided with a " sandwich " of some resilient material between the roofs, will probably give protection to what is beneath it, provided that the foundation supporting the roofs is a good one.

Many bombproof shelters made of concrete or big stones cemented together were constructed in France and Belgium by all belligerents, but they were generally of small capacity, and provided for particular detachments whose duties necessitated their remaining in the vicinity at all times. It is not, however, possible to provide such protection universally; in most cases all that can be done is to minimize the danger as far as is practicable, and to accept the fact that a direct hit on, or an explosion very close to, the person or animal will finish the matter as far as they are concerned. A little-known fact is that the open spaces in a big city like London may amount in total area to nearly ten times that on which houses are actually built, the chance that a bomb will fall on a house being therefore far less than is generally recognized.

The heaviest bombs used by both sides in the war made craters about 35 ft. in depth when dropped on ordinary soil and open ground. These bombs were fitted with fuses with a slight delay action. A light bomb with a very sensitive fuse was used by the Germans with deadly effect against men and animals. The crater made by it was practically negligible, all the fragments flying outwards and upwards. Protection against this type of bomb was afforded by low parapets of sandbags or sods, close to which troops could live; but horses were extremely difficult to protect against these so called " daisy cutters." In the open, protection during a bombing attack will generally be best found by lying down - in a depression if one be available.

In houses it is difficult to say which position is the safest; a bomb with a delay-action fuse will probably blow the whole house up from roof to cellar, while one with an instantaneous fuse will probably blow the roof in. On the whole, it would appear that the safest position of all is near the chimney breast in a room on the first floor, and below the level of the window sill. Such a spot may give protection from debris falling from the roof, and from splinters from a bomb bursting in the road outside, but is of course not likely to be of any use if the whole house is blown up.

Torpedo nets arranged in tiers about 10 ft. above each other may provide a certain amount of protection against small bombs fitted with instantaneous percussion fuses, but they are costly and difficult to erect.

Camouflage

Concealment of the ground target may take more than one form. The landscape may be studied from the air, and the vulnerable points treated in such a way with painting and netting and so on, as to assimilate their appearance as far as possible with the surrounding country. Again, attempts may be made to hide an important point with smoke clouds during a raid, but unless the work is very carefully done the smoke may invite attention to the possible objective rather than conceal it. In any case it involves much careful organization, and may in the end prove very expensive. Lights and dummy buildings may be placed in exposed positions so as to form attractive targets for hostile bombing machines, at a safe distance but not too far from the point actually sought by the enemy. Thus a carefully arranged target of green, red, and white lights may successfully simulate and so protect an important railway junction.

Concealment of the principal leading-in marks has frequently been suggested; but success would only be likely with objects which were of small size, and therefore probably of comparatively small importance. For a big objective such as London, where there are such prominent guides in the nature of rivers, railroads and valleys the expense of concealment would be enormous and the probability of success negligible. Moreover, the developments of wireless teleg raphy for directional and position-finding purposes would almost entirely neutralize any such work if it were attempted, on account of the size of the target. (See also CAMOUFLAGE.) III. Forms of Attack. The effects of bombing are moral and material. There is no doubt that the moral effect is far greater than the material - particularly in thickly populated districts where self-control, as a general rule, will be found lacking in the population to a greater degree than amongst armed forces in the field. No result decisive to a campaign has been brought about by a raid of any kind of itself alone. This fact will probably be true of aircraft bombing operations, provided that a country has taken suitable precautions in peace against the chance of an overwhelming attack at the very outbreak of war.

Written evidence was found during the war of the nervous apprehension reigning in a certain German town after the British special raiding force known as the " Independent Force, R.A.F.," had been operating for a comparatively short time. One of the inhabitants described a night of terror in which Allied aeroplanes had come in the early night and dropped their bombs and gone away. No sooner had the inhabitants come out of their shelters to go to bed than they were again summoned under cover, and the bomb dropping was repeated. Again they went to bed, and again they had to take cover - the performance continuing in this manner for some three or four hours. As a matter of fact one solitary Allied aeroplane paid a single visit to the town that night; the rest of the raid was purely imaginary, and the result of demoralization! Over another large town six long air raids took place during eight nights. One effect was that the clothing output from that district was temporarily reduced by 80% - a serious matter for the army, as a large proportion of the force was depending on the district for its clothing.

Bombing operations over disciplined forces in the field constitute on the whole a form of annoyance rather than a potential danger, provided that store and ammunition depots are so designed as to be separated from each other, and subdivided within themselves, in such a way that a fire arising in one section may be properly isolated and prevented from spreading to its neighbours. Interference with movements of troops and stores by rail can be, and has been, caused by low-flying bombing machines.

Airship Attack. - Airships form targets of great size, and, if filled with inflammable gas - as were those of the Central Empires during the war - are objects of considerable danger to their crews. If and when a suitable non-inflammable gas is discovered which can be produced cheaply for commercial purposes, the airship will become a serious factor in air-defence considerations. It possesses greater endurance, radius of action, carrying capacity, accommodation, and facilities for observation than " heavierthan-air " machines. Meteorological conditions, however, will always militate more against the free use of airships than of aeroplanes, which possess higher powers of manoeuvre and performance.

During the war bombing operations by airships were not intentionally undertaken by the Germans over land targets by day, but ships at sea were frequently made the objects of such attention between dusk and dawn. Airships intending to attack land objectives in the British Is. used to leave their sheds by clay, and make their landfall while still over the North Sea. There they would wait until it was dark enough to cross the coastline without prospect of serious interference, and make for their various objectives - as a rule more or less independently, but sometimes in pairs. The return journeys were made independently.

It has been held that at night it is hardly necessary to attack with more than one airship at a time, but there is no doubt whatever that simultaneous attacks by two or more airships on the same course add enormously to the difficulties of the defence.

The German raid on London during the night of Sept. 23-4 1916 affords a notable instance of airships setting out to attack in pairs, but failing to carry out their intention. L31 and L32 sailed on the task in company and reached Dungeness together. Thence L31, commanded by a bold and skilful pilot, set her course straight across London at high speed, and eventually won through. Her consort hesitated, and was lost.

L3 i passed over Purley and Croydon, and dropped a very brilliant flare as she turned on a northerly course. This undoubtedly had the effect of distracting the ground defences from herself; for she was scarcely seen as she passed over the metropolis, and bombed it heavily without damage to herself. She reached home in safety.

L32 waited near Dungeness for about 40 minutes, and then flew north over Tunbridge Wells, instead of following L31. She avoided London, and dropped her bombs between W'esterham and Ockenham. Near Billericay she was destroyed by fire.

Although there would appear to be much to commend such a course, " fleet " movements of airships in formation with the intention of bombing were not carried out by the Germans. However, it does not necessarily follow that a big attack of airships, either by themselves or convoyed by aeroplanes, will not form part of an extensive bombing operation in the future. The arrival of such an aerial flotilla over a capital city at the very outset of a war would do much to spread despondency and alarm; and if such a fleet succeeded in getting away unscathed, the attack might suffice to overturn all government in the state attacked.

Aeroplane and Seaplane Attack

Bombing aeroplanes 1 by reason of their speed, difficulty of destruction from the ground, and comparative ease of handling in unfavourable weather, form the most serious factor in air attack.

The first aeroplane raid on London by day took place about noon on Nov. 28 1916. This was carried out by a two-seater machine carrying about half a dozen light bombs and flying at a high altitude. It was a courageous effort. Engine trouble brought the pilot to the ground on French territory, where he was captured with his observer. London was covered with clouds of dust which prevented all but a very few from ever seeing the machine. The success of the effort made it all the more surprising that it was never repeated; subsequent attacks in daylight were all made by machines flying together in considerable numbers and not singly. The most notable was that which took place on July 7 1917.

Before Sept. 1917, only a single attack on London was made by aeroplane by night. In that particular case (l-Iay 6-7 1917) the attack was made by a solitary machine which dropped most of its bombs on Hackney li'larshes.

With these two exceptions, aeroplane and seaplane raids on England by day and night were limited practically to coastwise towns and shipping at anchor till the beginning of Sept. 1917, when aeroplane attacks on London by night were commenced seriously. These seem to have been made at first by machines in groups of three to five in number, but at the end of the same month, tree groups appear generally to have split up on reaching the English coast, each machine taking its own line independently from that time onwards.

Machine-gun fire from low-flying aeroplanes and seaplanes will be encountered wherever targets present themselves: troops in action, in camp, or on the march, transport in movement, troops crowded on shipboard. But here again the principal effect will be moral rather than material.

Where ships lie at anchor in open roadsteads, or in harbours which offer a direct line of approach from the sea of moderate length, seaplanes will find targets vulnerable by the marine torpedo. The launching of the torpedo involves a close approach by the torpedocarrying machine to the surface of the sea, and complete occupation for the crew of the machine. These facts render it necessary that such machines be escorted by one or more fighting machines, whose duty it is to protect them from attacks by air and if possible from fire from the shore and ships. Various methods of active protection suggest themselves - the destruction of the machine, harassing its aim, or deflecting the torpedo during the launching process.

Photography of the ground for intelligence purposes forms a highly important feature in aircraft work. With good lenses, photographic machines can do their work at immense heights, thus rendering their detection by the defence a matter of considerable difficulty.

Aeroplanes on hostile patrol constitute an armed guard whose duty it is to seek for enemy machines. Such patrols form targets for air defence formations when they are within range and the air forces proper are not at hand to take up their challenge.

Friendly machines acting as auxiliaries to ground operations - especially artillery machines observing the results of gun fire - are 1 The paragraphs which follow are applicable also in the main to seaplanes. Nevertheless the typical differences between the two classes are not without importance from the point of view of the preparations against attack by one or the other. The principal difference is that seaplanes require no landing ground or special arrangements for landing on ships. They can also take in their fuel from ships. On the other hand they find difficulty in " taking off " in rough water. Their powers of manoeuvre are, however, comparatively limited. They come chiefly into the consideration of coastal air defence, owing to the necessity they are under of landing on water. But amphibious machines are certain developments of the near future, and wide canals such as that between Bruges and Zeebrugge have served as landing places and enabled seaplanes to operate from a point inland and safe from interference from the sea.

at a serious disadvantage if attacked by enemy aircraft, as their duties tie them to a comparatively small area at a fairly low height. To defend each of such machines by an aerial escort would absorb too great a number of fighting aircraft, and so the duty falls most frequently on the anti aircraft artillery and such machines as are allotted for air defence work - provided that the latter can be directed to the spot in sufficient time to provide the protection required.

IV. The Defence in General. It will now be realized that air defence is required both in the actual theatre of active operations in the face of the enemy, and in areas far to the rear of the fighting line, so long as the enemy has machines capable of reaching those distant points and returning again from them. Bombing attacks may be met anywhere, i.e. both in the forward area of ground operations - the " Front "- and also in store depots, bases, ports, and large cities far removed from them. Low-flying machines with bombs or machine-guns may be encountered far in rear of the " fighting line," but principally in or near it and over the communications immediately behind it; so that, as a broad general rule, the nearer the " line " the greater will be the proportion of low-flying targets, and vice versa. Torpedo-carrying machines will be met with over the sea; and photography machines anywhere between the " line " and points far in rear of it on the lines of communications.

In order to place defending aeroplanes in positions favourable for engaging their targets, it is necessary to obtain information of the attack in sufficient time. This leads to two great essentials in any scheme of air defence, namely: - (a) intelligence, and (b) communications.

(a) Intelligence can be treated under three headings: (1) during peace, and before the beginning of an attack in war; (2) during an attack; (3) immediately after an attack. Intelligence before the beginning of an attack includes information obtained during peace of all the resources of a possible enemy; his preparations and probable intentions; with the numbers, details and performances of his machines both civil and military. On such information will the whole scheme of air defence of a country and its forces in the field depend. In peace such information can be collected, compiled, and assimilated in a careful and comparatively slow manner. But directly a state of war arises, speed in the collection and transmission of that intelligence to those whom it most concerns, i.e. the executive in the air defence services, becomes the prominent factor. The authority responsible for the collection of that information has to add comparatively suddenly to his ordinary peace-time duties that of rapidly tracing the movements of both hostile and friendly aircraft, as by no other method can an officer check information sent to him by his observers. Only on the efficiency of the preparations made for the use of telephone, telegraph, and other signals can he hope to issue the warnings which will be required by the population to enable them to take cover during a raid. The state of war may even be heralded by the air attack itself, and there may only be a matter of a few hours for the transition from " intelligence duties during peace and before an attack " to " intelligence during an attack." It will be best to consider a concrete example, which will show perhaps more than anything else the necessity for speed. Take an imaginary city with an average radius of 12 m., with its centre situated 30 m. west of the sea. One night a ship 60 m. east of that city reports a number of aeroplanes as having been heard passing high overhead, going west at an estimated ground speed of too m. per hour. The message, which is probably sent " in clear," is picked up by some coastguard station, which sends it to the local senior naval officer and so to the military garrison commander near at hand. These officers, after digesting the report, and confirming it if possible, send it on through their respective headquarters to the central organ of the system. Thence it goes to the railways, to the police, and to air defence headquarters, who give the alarm to the railroad men, to the civil population, and to the squadrons, guns, and lights, etc., of the defences, respectively. The defending squadrons will probably be situated from 15 to 20 m. from the centre of the city, i.e. about 40 to 45 m. from the source of the report. At the squadron aerodromes the pilots, who are waiting ready to start up the machines, " taxi " over the aerodrome, and then " take off " and begin to climb to predetermined heights, as the real height of the attack cannot be known at the moment.

A little time-table will show the time probably left to them to get up to, say, io,000 feet.

minutes

Time taken by attack to travel 40 to 45 m., say

. 27

Ship to shore. .. .

5

Coastguard to local H.Q.

2

Local H.Q. to main H.Q .

2

Main H.Q. to Air Defence H.Q. .

2

Air Defence H.Q. to units

1

Starting up machines, " taxi-ing " and taking off

5

Total (say) 17 Leaving the machines to get their heights in.. io A single report of this nature would suffice to send out an alarm far and wide, and turn the defence posts over a vast area into seething points of activity; whilst there might be nothing whatever to show that those machines were hostile, or that if hostile they were going to attack the city in question. The initial probability was that they were hostile; and as they happened to be going west at a point 70 odd miles east of the city, the time required to get the defending aeroplanes into position would leave no option but to assume that the attack was coming to that city. Yet the attack in this instance might easily turn aside as soon as the coastline was made, in order to proceed to some other objective; there was no certain indication beforehand of the real one, and there may never be.

The foregoing example shows that the observer system of a defensive organization for a big " vulnerable point " must extend to a radius of from 70 to 100 m. from the probable main objective of hostile attack by air, if the executive is to have sufficient time to get its defences into a state of readiness for action, and the civilian population and railways properly warned of the approaching danger.

As soon as the attack enters the area in which anti-aircraft posts exist, each of such posts within sight or earshot of the attack becomes a potential source of information. It remains then for the commander of the air defences to organize a system of speedy intelligence within his own command, which can be supplemented by reports collected from police and railways, which may or may not assist in checking the reports received from the defence posts themselves. This system continues its work until such time as the attack withdraws to a point outside its boundaries, when intelligence is again required from outside sources until it is certain that the engagement is over.

Directly after the attack it becomes of importance immediately to check the commander's ideas of the battle, to supplement them with local details of what actually happened, and to compile as complete an account as possible, showing: - Nature and numbers of aircraft employed on each side; routes followed by attack and defence; casualties to personnel and material; number and nature of bombs dropped; expenditure of ammunition; size, speed, and manoeuvres of enemy machines; new features of machines, if any; efficacy of communications; weather conditions, etc. This report is of high importance and may enable a commander, if it is compiled and issued rapidly, to dispose his forces afresh in sufficient time should features in the attack show this to be necessary.

In this connexion, it is important to note certain peculiarities of air-defence information. A report on the position of aircraft in movement is incorrect the instant after the observation is made, unless the time of the observation is given. The value of the report decreases with every moment that elapses after the observation. To be of value at all it must specify whether the aircraft was seen or only heard; if the former, whether friendly or hostile; and the time of the observation. To be of real value, it should contain data as to the direction of flight, the number and type of the machines and their height. One of the outstanding curiosities of the air raids over England was the remarkable inaccuracy of the reports rendered by eyewitnesses which were received at the various headquarters. To men who have been in the services the hypothesis that the man " on the spot " knows what is going on and therefore knows best what should be done, will be familiar. The history of anti-aircraft operations during the war abounds with instances showing the fallacy of that hypothesis.

During the aeroplane raid of June 1917, over Sheerness, Gravesend, Wrotham, and Folkestone, two independent reports were received of an airship approaching London in broad daylight from a point between Gravesend and Wrotham. They were the only intimations of any airship being present. One of the reports came from an officer, and one from a searchlight detachment: all had been used to seeing airships at night and knew what they were like. They were closely questioned, and there is no doubt that they were mistaken, but none of them was ever shaken in his conviction that he had seen an airship.

The gun detachment at Hyde Park were threatened by an angry crowd one afternoon in June 1917, because they would not open fire on a British machine flying high overhead. An air raid was actually in progress over East Kent at the time.

Bombs were reported one night as dropping in places up and down the eastern portions of Lincolnshire and Yorkshire, but a duty officer sitting over a map in London could only trace the noises to echoes of a serious explosion which had taken place a short time before in Lancashire; so he assumed the responsibility of declining to give an alarm; he was right.

An airship was reported as visible and audible over the scene of the great Silvertown explosion in east London within a few minutes after the last explosion there had taken place; it was identified with a curious wisp of smoke which many spectators had noted in the glare of the flames. The sound of the engines was purely imaginary. Thin long clouds were frequently reported as airships on moonlight nights.

These few examples will show the unreliability of reports concerning aircraft, and bring into prominence the enormous responsibility resting on the shoulders of the " duty officer," who, sitting miles away from the scene in a closed room, has to decide whether an observer really has seen or heard what he has said he did.

In making observations on the movements of large cylindrical airships, a common cause of error is due to the lack of an appreciation of the effect of perspective. An airship travelling horizontally and straight away from an observer may give the impression of falling vertically, nose downwards. An obliquely approaching airship may appear to be gaining height, and vice versa, although travelling at a constant height. Further, the observer on the ground is unable to assist himself by comparison of the size of the machine with other objects, the sizes of which may be familiar, placed at gradually increasing distances from him, and between himself and the airship.

The vagaries of the path of sound emanating from aircraft have proved extraordinarily deceptive. An officer accustomed to living in a shelter on a roof in the heart of London was able, while inside the hut, to detect sounds of aircraft which were quite inaudible to him when he was outside it. Local slopes and wooded country lead to confusion in the intensity and direction of the source of sound.

During the raid of May 23-4 1917 on the London area, airships were reported independently as "almost overhead" by three experienced anti-aircraft detachments in the neighbourhood of Hoddesdon and Hatfield, though no airships came nearer than within 25 m. of them; the mistake was probably due to peculiar and dense cloud formations which lay over the London area at the time. During the same engagement, bombs dropped between Braintree and The Wash were reported as clearly audible from Putney Heath and southwest of it.

In a civilized country, warning of an approaching attack by air is required by both civil and military populations. Here again the organization must be based on " areas." It is not possible to decide beforehand the objective of attack by air, but it is possible to fix the degree of probability of attack on the different vulnerable points in any country. In each of such vulnerable points certain precautions are necessary, such as the evacuation of the workers from an explosive factory, the dowsing of bright lights, or the control of railway traffic. These precautionary measures take time to bring into force but it is nevertheless desirable to bring them into force only at the very last moment, in order not to delay output or cause unnecessary alarm and congestion. It becomes necessary therefore to keep a quick and careful record of the enemy aircraft movements, to divide up the country into `. ` warning districts," and to provide a good system of distributing the warnings.

The movements of the attack are recorded by the " intelligence " system. The sizes of the warning districts depend on the speed with which the attack may move, as well as on the time required to bring the precautionary measures into force. The system of distributing the warnings will rest with those civil authorities who act as guardians of the public safety, who will probably use the civil telephone system.

Warnings and orders will normally be divided into: (a) preliminary warnings as to the approach to the area of an attack; (b ) definite military orders as soon as the attack has entered the area; (c ) messages cancelling (a) and (b).

As regards railways, special arrangements are necessary. Complete stoppage of railway traffic creates such disorganization that weeks may be taken to recover from it. Failure of train service causes the assembly of huge crowds of would-be passengers at railway stations, and so the formation of " vulnerable points " in which a single bomb would cause immense destruction of life. The dislocation of the traffic suspends the punctual delivery of goods, and upsets transport arrangements throughout the whole country traversed by the railway system, as well as in the ports to which it is connected. The control of the traffic therefore remains in the hands of the railway authorities, who are advised by the military authorities of the assistance the system may afford to hostile aircraft under certain circumstances. Both the railway and military authorities render each other mutual assistance in the interchange of information regarding the progress of an attack by air.

(b) Communications.

The rapidity with which aircraft move, and the uncertainty of their objectives, render necessary a very complete system of communications. Without such provision the intelligence gained cannot be collected or information and orders distributed in sufficient time to meet an attack before it arrives over its objective, or to enable precautionary measures for the public safety to be taken.

Signals may be sent by wire, wireless, and visual means. Means of communication are required between: Points on land, l (points on land.

Ships on the sea, and (ships on the sea.

Machines in the air, I l machines in the air.

Signal by wire is only possible between stationary points, i.e. those on land or the shore and anchored vessels afloat. Visual signalling between machines in the air and points on the ground is limited chiefly by atmospheric conditions, but also by the necessity of concealing the position of machines in the air. As between points on the surface of the earth, intervening ground features as well as atmospheric conditions may interfere.

In order to minimize the inevitable congestion which arises where the same wire circuit is used for the dual purpose of collecting and distributing information, independent methods must be provided for the two processes wherever this can be arranged. As far as possible information should be collected by wire circuits, but after verification it may be distributed by any method available. Wireless is of value between machines in the air to enable formation commanders to communicate with each other and with the machines under them. Wireless signal facilities are also required to enable machines to check their navigation reckonings, and to assist them in locating landing grounds, particularly when fog or cloud prevail. For the communication of intelligence before the latter has been thoroughly investigated its use is a source of danger, owing to the ease with which wireless messages can be " picked up " and to the large proportion of inaccuracies to be found in messages concerning aircraft.

These broad principles apply to all " back " areas; but in " forward " areas, where shell fire renders the maintenance of wire circuits almost impossible, resort to wireless alone may be necessary, if the passage of aircraft intelligence is essential in the area affected. That the highest standard of accuracy and rapidity is required to make the service of communication efficient for crises in which minutes are precious goes without saying.

V. The Application of the Various Instruments of Defence. In order always to be as economical as possible, air defences must not be disposed too far from the area they are intended to defend. As the attack can come from any direction, they must be disposed all round that area. To dispose ground defences along the boundary of a state with aeroplanes on patrol on either side of them, in order to keep the invader out of the state at the outset, is to be " strong everywhere," and consequently " strong nowhere." Such a policy involves dispersion of available strength over unimportant localities, reduction of control, loss of cohesion in effort, extravagance, and the achievement of a minimum of efficiency. The close defence of the localities which are important to the state is the only sound policy.

For the defence to be effective, the attack must be met and defeated at the right height and outside the line from which it can achieve its object. Such at least must be the aim of the defence, however difficult it may be of achievement. That is to say, the defence must be outside the objective of the attack. This necessity plunges the matter at once into difficulties with what is known in the army as the " chain of command." An army works by definite boundaries shown by real or imaginary lines on the ground. The air knows no boundaries. It follows, therefore, that those units of air defence formations which are tied to the ground must be sited and organized for purposes of command with no regard to those imaginary territorial boundaries necessary to the ordinary army of the ground, and solely with regard to the whole area in which the vulnerable point or points are situated.

In one respect the sea has an important bearing on the nature of aeroplane attacks. The risk of being shot down on the return journey while still over water, with little or no hope of rescue, tends to make a circumspect pilot fly high over his objective, even if this be some distance inland, as he must evade detection till he has gained such a start over the pursuers as will enable him to pass the sea in safety even with a damaged engine. This was apparently the policy of the Germans during the raids on London in Sept. 1917. Whether, in any given case, the pilot will thus sacrifice some of the effectiveness of his attack in order to give himself better chances of a safe return, will depend on his personal character, the traditions of his corps and the free hand or limiting instructions that he receives from his superiors. From the point of view of the defence this has its drawbacks. It is difficult to decide a priori, or even during the progress of the attack itself, as to the probable height of the enemy when the basis of the decision is practically conjecture. Another effect of the seacoast on anti-aircraft defences may be to limit them in area. The defences must extend over an area outside the vulnerable point; but, in cases of ports on the open sea, that area is limited to the ranges of gun and searchlight on the edge which borders on the sea.

Some typical instances of the use of the various instruments of defence may now be considered. The defence has to provide against attacks both by day and by night. By day the instruments of defence and their adjuncts are: the machine-gun in the air, the heavy gun on the ground, the sound locator, and the observer post. By night the machine-gun in the air must be manned by a crew specially trained in night fighting, and in addition there is the searchlight. By day and by night the object of the defences is to break up the enemy attack and destroy it in detail. By day the massed attack must be broken up by gunfire before the aeroplanes on the defensive are launched against it; this entails guns outside the defensive aeroplane patrols, which again are outside the vulnerable point. Then in support of the aeroplanes (i.e. in rear of them) more guns again are required to repel such of the attackers as succeed in penetrating the aeroplane patrol area. And lastly, throughout the area of the vulnerable point itself, provision must be made for attacking by gunfire any hostile machine which may succeed in penetrating so far.

The attack will probably be audible and visible throughout the greater part of its course. In certain conditions of thick cloud or haze it may be invisible from the ground, but this fact, though increasing the difficulties, does not alter the disposition of the defences.

By night the attack is broken up in an entirely different manner. Both attacking and defending machines being in darkness, the attack is, as it were, reconnoitred by the searchlight, and the targets selected by the latter are isolated for engagement by the apparently simple process of keeping them illuminated. Unless the searchlights succeed in their object, the attack is invisible.

It is not possible as a rule to illuminate several targets in a searchlight beam simultaneously, although during the war as many as five have been held in the beam simultaneously for a few minutes; nor is it likely that any method of illuminating a formation of, say, 22 machines simultaneously, for any length of time, would be practicable. The outer ring of guns, therefore, would normally remain inactive by night unless the absence of a defending aeroplane gives an opportunity for a gun to engage an enemy target.

By day and by night the aeroplane in defence can only move a certain maximum distance on patrol without running the risk of allowing an attack to slip past in rear of it; the aeroplane also requires a certain minimum distance on one side or other of its patrol line in which to manoeuvre and bring its enemy to battle. Suppose for the purposes of illustration these measurements be taken at 15 and io m. respectively. The aeroplane patrol area, and the battle and pursuit area, must be kept as clear as possible of gunfire areas and areas containing vulnerable points of any size.

The width of the gunfire area will depend on the probable height at which the attack is delivered. Assuming that the latter is io,000 ft. and that the gun can command a horizontal range of three miles at that height, the belts of gunfire may be taken at six miles in width. Observer posts must be between 7 0 to roo m. away, as has been shown, in order to gain time for the defences to get into position, if they are to meet the attack as it comes in and not bring it to account merely as it is returning home.

In the case of a vulnerable area represented by a circle of a radius of 5 m., the area immediately outside that will be a belt for gunfire from 3 to 4 m. in width; the next a belt of io m. for the aeroplane battle and pursuit area; then one of 6 m. for the outer gunfire area; and a final belt from 45 to 75 m. wide covered with a network of observer posts, each of which can be from io to 15 m. from each other. This arrangement provides for the problem of defence by day.

By night it is necessary to consider the disposition of the searchlights, and it will have been seen that one of their functions is to indicate the approximate position of attacking aircraft. To be of any value they must be


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These files are public domain.

Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Air Defence'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/bri/a/air-defence.html. 1910.

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