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a point between Grayesend 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 actu- ally 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 imagin- ary. Thin long clouds were frequently reported as airships on moon- light nights. These few examples will show the unreliability of reports con- cerning 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 ap- preciation 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 com- parison 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 expe- rienced anti-aircraft detachments in the neighbourhood of Hoddes- don 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 south- west of it. In a civilized country, warning of an approaching attack by air is required by both civil and military populations. Mere 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 precaution- ary 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 con- gestion. It becomes necessary therefore to keep a quick and care- ful 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 " in- telligence " 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) pre- 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 passen- gers 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 provi- sion 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, "j f points on land. Ships on the sea, > and < ships on the sea. Machines in the air, i ( 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 neces- sity 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 com- munication 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