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Rh in the early stages of the war. This was the dart, a heavy bullet p] steel sharpened to a point at one end. These darts, released in showers, were intended to be effective against personnel. (C. F. A.) AIR DEFENCE. Even before aerial navigation and aviation had been developed to a practical point, the employment of aircraft in war for the attack of vulnerable places was discussed from time to time in a speculative way, and in the seven or eight years preceding the World War the types and characteristics of aircraft became so far definite that technical study could be brought to bear on the problem of defence, especially that of artillery defence. But this period of seven or eight years was short; military history could give no lead; practical experiments were almost impossible. Moreover, in the existing state of international law, liability to air attack was understood to depend not on whether a place was vulnerable, in the sense that its destruction would impair the capacity of the nation for carrying on the war, but on whether it was " defended," i.e. fortified in the conventional sense of the word, or at any rate held by a ground garrison against ground attack. Attention was therefore directed chiefly to the question of air attack on what according to the prevailing ideas were " military " objectives, and in view of the small numbers of aircraft then available such attacks were regarded as unlikely to affect the course of operations seriously. In these conditions, and especially in the absence of all data based on practical experience, it is not surprising that defence against air attack was everywhere in a rudimentary state at the outset of the World War. In the war itself, on the contrary, experience, data and methods crowded upon one another from first to last, and through the clearer definition of the problems to be faced on the one hand and the constant process of trial and error on the other, it has become possible to formulate the main principles of air defence with some approach to precision. /. General Considerations. Air defence, as discussed in the present article, deals with the arrangements which deny to enemy aircraft access to vulner- able points. By " access " is implied the gaining of a position either directly over the objective or sufficiently close to it for the success of the attack. Amongst " vulnerable points " are in- cluded bodies of troops and their materiel in the field, centres of population, large magazines, arsenals, harbours, ports, dock- yards, ships and convoys at sea, big industrial centres, and the like. As with a fleet, the primary duty of the air forces is to seek out the enemy air forces and destroy them; but the problem in the air is far more difficult than on the sea, as a third dimension has to be taken into account, i.e. that of height. When we remember what difficulties have been introduced into naval war by the introduction of the submersible warship, slow as this is and small as is its up-and-down range, and when we realize further that, in the air, opposing craft can pass at great heights both above and below each other, move at speeds that are enor- mous relatively to any rate of movement on or under the sea and have to cope with extraordinary difficulties in detecting one another's proximity, it will be seen that the task in front of the air forces of any nation is the most difficult of all. There is no " command of the air " while the enemy disputes it. Therefore, against attacks by air, it is a logical necessity to provide ground defences, and to limit the radius of the air units allotted to cooperate with them. But though defence against attack by air, as on the ground, may be active or passive, yet to be effective it must be both. Further, the conditions under which aircraft move, by day and by night respectively, are so widely different, that the conditions of the defence must be correspondingly varied to meet them. It is not intended here to deal with operations launched over long distances against enemy aerodromes and depots, such being the role of the air forces alone, but solely with the local defence of areas occupied by " vulnerable points " of the kind which have been enumerated above. The instruments of air defence are: (a) the machine-gun on the ground; (6) the machine-gun in the aeroplane; (c) the heavier guns on the ground; (d) the searchlight, the sound locator, the observer post; (e) the aerial obstacle; (/) local protection on the ground, i.e. against bomb splinters and machine-gun fire, and camouflage. Each of these weapons supplements a deficiency in one or more of the others; none is complete without one or more of the others acting in conjunction with it. From this it follows that, in any anti-aircraft organization, cooperation in effort can only be effected by organizing units of the air force, artillery and engineers under a single command, as in a formation of all arms in ground warfare. The following are some of the forms which attacks by air may take: bombing by airships, aeroplanes or seaplanes, singly or in formation; the harassing of troops on the ground or sea with machine-gun fire by aeroplanes or seaplanes, singly or in pairs; torpedoing ships at anchor, by seaplanes, probably in pairs or escorted by " scout " (i.e. air fighting) machines. To these may be added, though they only indirectly affect the problems here discussed: photographic or visual reconnaissance, by aeroplanes or seaplanes, singly; and aerial engagements, by aeroplanes in formation on hostile patrol, i.e. ready to engage air-fighting groups of the enemy, or by aeroplanes, singly or in groups, attacking machines which are engaged in observation duties (especially artillery observation) in connexion with ground operations. Anti-aircraft units are concerned primarily with hostile attacks the objectives of which are on the ground or sea; the defence of objectives in the air is a secondary matter, but never- theless important when air-force units are not at hand to under- take the duty themselves. It may not be amiss at this stage to mention a few of the peculiarities of sound and light. Although these properties are generally known, their full importance in relation to air defence is seldom recognized by those who have little experience in anti j aircraft work. Sound travels at a certain known rate, namely about 1,100 ft. per second. Aircraft are generally first detected by ear. By the time the sound of a machine reaches the ear, the machine making it will have moved some distance away from the spot where it made the sound. A path of sound is deflected by the different velocities of the various air currents through which it passes on its way to the ground. A machine in the air is only visible to an observer on the ground by reason of light rays reflected from the surfaces of the machine, reaching his eye in sufficient intensity to enable him to detect it. Thus a machine flying " straight into the sun " is invisible to an observer also facing the sun. A spherical or cylindrical surface reflects light chiefly in the direction from which the illumination comes; hence, in the case of a balloon or airship, the observer sees the target best when he and a searchlight are on the same side of it. A flat surface reflects light towards an observer further from the source of illumination than its own position; thus an aeroplane flying steadily in a searchlight beam is generally seen best when it is between the observer and a searchlight. It does not follow that because aircraft are invisible to observers on the ground the ground is invisible to an observer in the machine, and nice versa. The greatest difficulty is frequently experienced in gaining the correct focus for the eye, and, having gained it, to maintain that focus. This is a difficulty common to the observer both on the ground and in the air, but whilst the former has only to look upwards, the latter has in addition to look all round and below his machine. Neither has the assistance of intermediate objects by means of which the focus of the eye can be altered and held at definite stages. By day the observer in the air is deaf, by night he is deaf and blind. A searchlight shining into the sky is only visible by reason of the particles of " dust " or moisture in the path of the beam. In a perfectly clear or clean atmosphere the beam is invisible. 1 Unusually clear atmospheric conditions with a few high clouds were responsible for popular rumours prevalent during the war in 1914-8, to the effect that a new invisible searchlight had been dis- covered which simply threw a disc of light far away up in the sky, and was otherwise quite invisible.