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Outer Gunfire Area [Aircraft Battle and (Pursuit Area. Inner Gunfire Area Vulnerable Point Aircraft Battle and Pursuit Area. Inner Gunfire Area Vulnerable Point FlG. 2 Night. 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 associ- ated with that of Woolwich, and both of these are intimately linked with the defences of Gravesend and of Chatham ; so that it eventu- ally 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 forma- tions 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 necessi- tates 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 applaud- ing 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 dispo- sition 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 instru- ments 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 readi- ness for action, and so calls for a greater complement of personnel than would be the case in defences situated inland. 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 line, were generally described during the World War as " forward " 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. 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. 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. l) is formed by the guns " 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 ammunition dumps, hospi- tals, etc., whilst the defending aeroplane will patrol in the space which may be available between. The maintenance of communica- tion 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, im- provements 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 battle- fleets at sea. Such aerial fleets operating from their bases wilh 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. 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 cooperation between air and ground units must be the strongest link in the chain of