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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 depôts 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.

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 Westerham 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 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 (May 6-7 1917) the attack was made by a solitary machine which dropped most of its bombs on Hackney Marshes.

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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 three to five in number, but at the end of the same month, the 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 seaplanes will find targets vulnerable by the marine torpedo. The which offer a direct line of approach from the sea of moderate length, launching of the torpedo involves a close approach by the torpedo carrying machine to the surface of the sea, and complete occupa tion 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, harass

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 attacking its aim, or deflecting the torpedo during the launching process. land objectives in the British Is. used to leave their sheds by day, 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.

L31 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.

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

duty it is to seek for enemy machines. Such patrols form targets Aeroplanes on hostile patrol constitute an armed guard whose 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

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seaplanes. Nevertheless the typical differences between the two The paragraphs which follow are applicable also in the main to preparations against attack by one or the other. The principal classes are not without importance from the point of view of the 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, com air defence, owing to the necessity they are under of landing on paratively limited. They come chiefly into the consideration of coastal 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 depôts, 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 100 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

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Total (say)

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Leaving the machines to get their heights in

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 assur 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

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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.

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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

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 responsi

bility 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) pres liminary 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:

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Signal by wire is only possible between stationary points, i.c. 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.

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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

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 10 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 10,000 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 70 to 100 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 10 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 10 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 able to do this throughout the vulnerable area, the adjacent gunfire area, the battle and patrol area, and for a sufficient distance outside the latter (say 4 to 5 m.) to enable the aeroplanes patrolling in defence to move into position to meet the attack. This gives the total area through which searchlights must be disposed, the projectors being at the angles of triangles whose sides measure approximately 2,500 to 3,000 yards. Owing to accidents on the ground, trees, houses, railway stations and the like, the actual distribution of searchlights throughout the area often appears to be indiscriminate; it is inadvisable as a rule to place a searchlight nearer than from 200 to 500 yd. from a gun. Again, by night, the difficulties of determining the height of the attack are so great, that it becomes necessary to dispose the aeroplanes in defence at different heights. Assuming this difference to be 1,000 ft., and that there are five machines one above the other, with the lowest at about 8,000 ft., the highest will be at 13,000 feet. The degree of endurance to be expected of a pilot flying on patrol at night may not exceed a tour of two hours in the air.

These data, combined with a knowledge of the average lengths of the summer and winter nights, will be sufficient to give some indication of the minimum numbers of machines and pilots required in the problem of night defence. The number by day is also affected by the probable frequency and size of the attacks.

It will now be easy to realize the enormous scale of defences required if any appreciable degree of efficiency is to be attained. A simple diagram will illustrate this general disposition of defences.

Few "vulnerable points" are as symmetrical as those indicated in these diagrams, but the principle illustrated can be

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By taking the maps of any state and applying these principles to the important towns, it will readily be seen that the matter is in reality considerably more complicated than it at first appears. For example, the defence area for Birmingham cannot be separated from that for Coventry. The defence of London is closely associated with that of Woolwich, and both of these are intimately linked with the defences of Gravesend and of Chatham; so that it eventually becomes necessary to look upon the whole district south of a line between the Wash and the Bristol Channel as a single area to be provided with defences under one command. Therefore this whole area will, for purposes of air defence, have an organization independent of all those ordinary commands and military formations whose activities are limited by conventional lines on a map.

The same line of reasoning applies to forces in the field with their "forward" areas, lines of communication, and bases; and necessitates the problem of air defence being considered with reference to the whole area of active operations, and not merely to that of all the independent vulnerable points within it.

The principle illustrated in the diagram will be found applicable to most cases, provided that consideration is given to the relative urgency of demands for gun and aeroplane defence combined, and of aeroplane defence alone. For the civil population, whilst applauding the courage and success of the airman, is ever apt to mingle with its praise a demand for a gun. A gun is tangible and comforting; it can be seen and heard; and so it produces on the population a moral effect which may be more than counterbalanced by the interference it may cause to the defending airmen. An instance, already alluded to, in which the principle requires modification, is that of coastal towns and harbours, few of which can be situated geographically so as to admit of the all-round disposition of defence illustrated. Here the sea intervenes to cut off observer posts, searchlights, and guns, in addition to restricting the area of manoeuvre for the defending aeroplanes by night. This inroad into the defences offers the enemy an avenue of approach, and necessitates considerable strengthening of the batteries within range of and covering the sea in the neighbourhood. A certain amount of defence may be afforded from vessels afloat, but reliance cannot be placed on them for anything more than a temporary assistance, as they may only be present for uncertain periods.

All that can be done is to increase the intensity of the gunfire belt to seaward, and to provide aircraft detector posts and instruments with a directional value in azimuth rather than vertically. The latter serve as a partial substitute for the observer cordon by giving somewhat distant warning of the approach of aircraft.

The defence of towns and ports separated from enemy territory by sea alone thus requires maintenance in a state of instant readiness for action, and so calls for a greater complement of personnel than would be the case in defences situated inland.

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The areas on either side of the dividing "line" between opposing forces in the field, up to a distance of some miles from the dividing forward line, were generally described during the World War as areas. The areas behind the forward areas were usually termed "back" areas; the latter term, however, was not generally taken to refer to places outside the " theatre of war," though from the point of view of aircraft action it was just as applicable.

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In "forward areas vulnerable points in the nature of men, guns, animals, and ammunition stores are numerous, but as a rule well distributed. In "back areas they all tend to greater concentra tion. Protection is therefore more easily afforded in the former than in the latter, and so the better targets for bombing machines will be found as a rule in "back areas.

The nearer the "line" the more intense will become the fire of hostile ground artillery; this precludes the free use of searchlights nearer than about 5,000 yd. from the "line," and necessitates the distribution of anti-aircraft artillery in smaller fire units than is possible at a greater range from the enemy.

Targets will be far more numerous in the forward area than in rear of it, throwing much more work on the anti-aircraft artillery situated near the line.

guns

Applying the principle, as illustrated in the figures, to the prob lem in the field, a distortion of the diagram results, as in the cases of coastal towns. The outer ring of guns (fig. 1) is formed by the in the line" and such as can be spared to protect the flanks and rear of the force. Within that ring, guns will be concentrated closely around vulnerable points such as munition dumps, hospi tals, etc., whilst the defending aeroplane will patrol in the space which may be available between. The maintenance of communication between the forward guns in the shell area becomes a matter of great difficulty and may require provision of special apparatus.

By night the guns in the line" must rest as far as possible, and employ themselves with observation duties. Searchlights in the aeroplane battle and pursuit area nearest the line must perforce be curtailed, and the aeroplane patrol lines withdrawn to points which will admit of sufficient searchlights operating between them and the attack.

VI. Some Possibilities of the Future.

Some limit to the speed of aircraft and the height at which they can fly must be assumed, and, as far as the possibilities can at present be imagined, heights up to 30,000 ft. and speeds of 200 m. per hour, together with powers of long endurance in the air, may come within the range of practicability during the next 20 years or so. A successful development of the helicopter would bring about a great change in the power of manoeuvre of aircraft, and enormously increase the difficulties of the defence. Detection of approaching aircraft will be rendered difficult by the silencing of the machinery; their destruction by fire will be hampered by the introduction of metal protection. Wireless aids to navigation will decrease the difficulties of the pilot in thick weather, improvements in the landing power and stability of machines will increase their immunity from storms; and all these conditions will call for a greater state of readiness in the defence. On the other hand, improvements in artillery will be necessary, and will follow as a natural consequence. Inventions for the detection of the locus of the source of sound will facilitate the accuracy of searchlight work. These factors, in their turn, will impose greater caution on the attack and give greater confidence to pilots patrolling in defence. Aeroplanes now used in defence will in the course of years become less localized in their work, and will develop a tendency to operate more and more like battlefleets at sea. Such aerial fleets operating from their bases will be likely to carry their own armaments and searchlights, and to be accompanied by what we may call their "destroyer flights,' which will assist them to seek out and find the enemy themselves.

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The improvements which will produce this tendency will only mature gradually, and danger will lie in the endeavour of the ground or air services to assume entire responsibility for defence against air raids before being in a position to do so. There must be a long transition period during which coöperation between air and ground units must be the strongest link in the chain of

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