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510 they may not be apparent to the naked eye, but they always show clearly on a photograph. The first step, therefore, in examining a photograph is to discover from what direction the light is coming, that is, the position of the sun in relation to the photograph. This can always be discovered by examining the shadow cast by some known object such as a house or a tree. By a study of shadow not only can we discover whether an object has height or depth, but we can also get much valuable information about its shape and size. Thorough familiarity with the effects of shadows is in fact an absolute essential to the correct study and easy appreciation of aeroplane photographs.

Photographs must be examined systematically, detail by detail, and frequent comparison of photographs taken on differ- ent dates made. If a day's photographs are examined alone, many small details will be missed, and it is impossible to follow progress in the construction of enemy works, and to note in- crease and decrease in the size of enemy's dumps and aerodromes, etc., so important in studying enemy intentions.

Aeroplane photographs are of greatest value in position war- fare. In open warfare their use is not so great. Armies ad- vance over areas of so great an extent that it is often impossible to take photographs and get the information from them before the area photographed has ceased to be of interest. Moreover, the defences conducted in open warfare are usually of so simple a nature as not to be visible in photographs. Machine guns are concealed in hedges or in the windows of houses, guns are fired in the open from sunken roads and the edges of woods. Pro- tection is obtained by the utilization of natural cover. Such positions if they were occupied for any time would (unless elabor- ately camouflaged) betray themselves by the tracks made by men or vehicles approaching them; but if they are only occupied for a short period these tracks do not form. In open warfare therefore much more valuable results are obtained from air- men's reports than from their cameras. It is only when the enemy makes a stand for a few days on a definite line that photo- graphs become valuable. During the first part of the German retreat from the Somme to the Hindenburg position in March 1917 air photographs were of little value. When however the British troops were held up by an outpost line in front of the main Hindenburg position photographs again became of use. On one occasion photographs were taken of a temporary enemy position, th* photographs were brought back, developed and printed '. the results delivered by aeroplane to the divisional comr der on that front in about an hour and a half. During a p. if eat in open warfare photographs can be used in watching r'ae pushing forward of the enemy's, communications and aero- dromes, and during an advance in discovering what demolitions he has carried out.

We must now consider the tactical use of photographs. At the battle of Neuve Chapelle (1915) the maps used for the attack were simply ordinary topographical maps, with the enemy positions roughly marked. If defences are not of an elaborate nature, such a map may be sufficient. But when an attack is being made on a defensive system elaborately organized and several miles in depth, something more exact is necessary.

First of all thick wire entanglements must be faced. When these entanglements consist of several rows, each row from 20 to 50 yd. deep, wire cutters are useless, and gaps must be made either by artillery or tanks. In either case, for the ranging of the guns or for the drawing up of the plans for the tank attack, an accurate map of the wire entanglements is necessary. Numer- ous trench lines must be captured, dugouts both in the trench lines and in the terrain between the lines must be dealt with, machine-gun emplacements must be captured, and special ar- rangements must be made beforehand to subdue strong points of resistance, redoubts, fortified woods, farms, quarries, etc.

To make the careful preparations necessary for an attack on such a position, not only the commander, but also his subordi- nates, must possess an accurate map of the position, which may be, and in the latter part of the war was, so deep that ground observation of its real defensive heart, the " battle zone," is impossible. The information then must come from the air,

and moreover, though a competent airman may make valuable observations, the only means of plotting that network of trenches and other defensive organizations on to the map is by means of the aeroplane photograph. This may be defeated in some details by effective camouflage or by the successful use of natural cover, but in general an accurate map of the enemy's defences can be constructed from it or rather, them, for a particular photo is or should be only one of a series which shows the changing aspect of the ground as man's works are super- imposed on it. Besides trenches, wire, and close defence posi- tions of all sorts, the camera attacks the enemy's artillery posi- tions. In some cases innocent ground begins, in succession photos, to show works, tracks and the like, until it becomes so definite that the balloons and the sound-rangers and flash- spotters only confirm what is already certain. On the other hand, photo deductions may be doubtful or even impossible till the battery reveals itself to the other agencies by coming into action. In either case the work of the camera continues in aid of the artillery. Amongst the most important services rendered by the air photograph is that of recording the effects of bom- bardment upon battery positions, trenches, wire, strong points, dumps and communications.

Photographs are also of value in studying enemy organizations, roads and tracks used by the enemy, billets and positions of reserves, signal communications, i.e. buried cables, air telephone lines, light signal stations, railways (normal and light-gauge), unloading stations, ammunition and supply dumps, stations and railway sidings, hospitals and aerodromes.

(C. F. A.;F. C. H.)

(C) Secret Service and Counter-intelligence. -The section of military intelligence known in Great Britain as I (b) is charged with two functions somewhat opposed in character, but having this in common, that the methods employed in each are, generally, secret. This factor makes it difficult to submit those functions to public dissection except on general lines; what follows, there- fore, is confined within those limits.

Broadly speaking, the duties of Intelligence (B) are: (i) Offensive, in the acquisition of information as to the enemy's military resources, numbers, plans, movements and dispositions, by means other than those employed by I(a), which are iden- tifications by contact; examination of the enemy and other press; scrutiny of captured documents and prisoners; air recon- naissance and photography; sound-ranging and other means. (2) Defensive, in the prevention of the acquisition by the enemy of similar information about our own forces. These together make up what may be described as Secret Service, and both involve the use of secret agents and secret methods.

Apart from the close connexion between them, the knowledge and experience of enemy methods gained in either sub-section is so immediately beneficial to the other that the functions of the two sub-sections are complementary and indivisible. They should, therefore, be controlled by one directing brain, especially in the field, where rapidity of action and of the circulation of information is essential. In peace-time at home, where the urgencies and difficulties of active service conditions do not arise, separation is permissible, though not generally desirable.

The offensive sub-section, to which alone the name Secret Service is popularly applied, can only be referred to in general terms. Its duties are similar in peace and war, and are directed towards the collection of information in enemy territory. For this purpose secret agents, or spies, have to be employed. The duties of these agents again differ but little in war and peace; but war increases their importance, and with it their difficulties and dangers. Whether they work as agents <} paste fixe, like the agents of the notorious Sticber in France before and during the campaign of 1870; or whether they arc sent on definite missions, or on general roving commissions, their objective is the same: information about the enemy. This objective is unaltered whether they penetrate into enemy territory through the ports, in the guise of peaceful neutrals armed with all the necessary papers, or whether they get there by other means. In war these other means may include penetrating the enemy