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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 prin- cipally in or near it and over the communications imme- diately 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 be- tween 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 (6) 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 as- similated in a careful and comparatively slow manner. But directly a state of war arises, speed in the collection and trans- mission 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 lop 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 com- mander 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 aero- drome, 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, 10,000 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 i Starting up machines, " taxi-ing " and taking off . 5 Total (say) 17 Leaving the machines to get their heights in . . 10 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 de- fending 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 defen- sive 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, Graves- end, Wrotham, and Folkestone, two independent reports were received of an airship approaching London in broad daylight from