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88 II. Conditions affecting the various Instruments of Air Defence. (a) The machine-gun on the ground, on account of its com- paratively 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 main- tain 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 ob- jects other than on the ground, and of hearing; and to overcome them it usually becomes necessary to direct the pilot and ob- server by signal (visual or wireless) from the ground, to assist them in finding the target which has been selected for attack. 1 Other serious handicaps to the observer in the air are the un- stable 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, (i) It points out the selected target to the defending aeroplanes. At night the pilot is deaf and practically blind, and, unless he carries a searchh'ght 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 conversa- tions, 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 inabil- ity 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 detect- ing the light reflected from the target. At distances, however, vary- ing 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 ioter- ference. Projectors have accordingly been provided with long con- trol 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 mini- mum of outside assistance. An aeroplane can, by " side slipping " or otherwise executing some unexpected manoeuvre, generally escape from a single illuminating 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 in- candescent 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 com- paratively 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 SLll, which was eventually burned that night and fell at CufHey, 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 engage- ment, 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 assump- tion that the target might not know where it was, and might there- fore pass away without dropping any bombs; in other words that the exposure of searchlights invited the arrival of bombs. The an- swers to these suggestions are simple, viz. : (i.) The searchlights are there to be used, because guns and aero- planes 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 con- tinued 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 with- in range of shell and machine-gun fire. On some occasions air- craft have occupied themselves in deliberately bombing search- lights, 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 prog- ress, searchlights formed a vulnerable target for machine-gun fire from low-flying aeroplanes, though actual casualties were com- paratively 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 satis- factory. 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.