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concentration may lie in an area where movement in several directions is equally easy, as in the German concentration on the Mezieres transversal in March 1918. But the less an intelligence service knows about the location of the enemy reserves, the more danger is there of a surprise.

For the watching of the enemy reserves there are four chief sources of information: the statements of prisoners and deserters,
 * aptured documents and correspondence, agents' reports, and the

interception of enemy wireless messages. Much of this information is of an uncertain character and powers of deduction and imagina- tion are necessary to piece together and coordinate the mass of material.

(a) When frequent contact is maintained it is easy to discover what new formations have arrived in line and what formations have gone into reserve. Prisoners can sometimes give information about the movements of the unit or formation they have relieved. They may also be able to say what formations they have seen in the journey to the line and what formations were grouped in the area from which they have moved.

(6) Captured documents and correspondence are of high value. Even a small attack results in the capture of many documents. The facilities with which maps and documents can now be produced has resulted in the issue of numerous orders, instructions and summaries; as regards maps, the contrast with previous wars is even more marked. In 1870 the Government of National Defence at Bordeaux was only able with difficulty to assemble one set of 1/80,000 maps of France for reproduction and issue to staffs, whereas nowadays a single divi- sion going into line may receive as much as a ton of maps. One of the most fruitful sources of information is letters written from men of formations in reserve to their comrades in line, which often reveal the location of an unidentified reserve.

(c) The work of agents is dealt with in another part of this article. The usefulness of agents in matters of tactical intelligence varies according to the kind of warfare which is being fought. In a war where the opposing armies are manreuvring over a large tract of country and where the front is not fixed, the passing of agents across the lines and their return with the information gained is compara- tively easy. On the other hand information becomes out of date far more quickly in manoeuvre warfare than in position warfare. On the contrary, when the front is fixed, as in the late war, the pass- age of agents is more difficult but their information holds good for a longer time.

(d) The picking up of enemy wireless messages is also a fruitful source of information. These messages are in cipher, 1 and can sometimes be deciphered quickly enough to yield useful informa- tion. But in any case the positions of enemy wireless masts can be discovered by means of direction-finding wireless and valuable deductions can be drawn from their groupings and activity, even if not one of the intercepted messages can be decoded. At one period in the campaign of 1918, a silent battle of which few were aware was fought between wireless intelligence and wireless " Camou- flage " so called, in which one side sought now successfully, now in vain, to mislead the other by varying the positions of masts and the volume of traffic.

Enemy Works and Installations. Information about the de- fensive system and the organizations of the enemy is obtained from reconnaissance, and to some extent from (b) sources, but the most fertile and certain source of information is the aero- plane photograph. Aeroplane photographs are of two types, the oblique and the vertical. Those of the first type are taken from heights of 200 to 1,000 ft. with a tilted camera. Taken at a 1,000 ft. they show the ground as it would appear to an observer from the top of a mountain. Not much detail is visible, but an excellent idea of the general lie of the land is given. Taken at a lower altitude, such details as trench construction, loopholes and machine-gun emplacements, entrances to dugouts, roads, trees and hedges are apparent. The more important type of photograph, however, is the vertical, that is, a photo- graph taken from directly above, with the camera pointing straight downwards. The appearance of objects on the vertical photograph is stranger, and puzzling to the uninitiated student. All objects are seen from above, so that only their tops and shadows are visible. Everything is seen in plan as on a map and to be able to appreciate a vertical photograph one must, so far as circumstances permit, accustom oneself to see the ground from above, and in any case cultivate a sympathetic under- standing of maps as maps.

Vertical photographs may be taken at almost any height. If taken too low the result may be blurred owing to movement,

1 In the earlier campaigns of the war, strange to say, messages in clear were sent on several important occasions.

but clear photographs may be obtained from 2,000 to 20,000 ft. The scale of the photograph varies according to the height at which the photograph is taken and the focal length (that is the distance between the lens of the camera and the photographic plate) of the camera used.

Different types of cameras are used according to the scale of photograph required. If a forward trench system is required to be photographed a short focal-length camera (say 8 in. or 10 in.) will be used on a machine flying at a low altitude (say 6,000), so as to get a photograph on a comparatively large scale. On an extensive photographic reconnaissance of an area some miles behind the line, where the object is to get photographs of a large area, not for study in detail, but to discover what con- structive work is engaging the enemy, a " wide-angle " type of camera (i.e. a camera of short focal length, but, since the photo- graphs are taken at a great height, [15,000 ft. to 18,000 ft.,] on a full plate, show a large area on a small scale) is employed. With this type of camera a larger area can be covered in a short time. If these wide-angle photographs show details of which a more thorough examination is desirable, large-scale photographs can be taken from a height of 15,000 ft. and more with a long focal-length camera (20 in. or even 48 in.) which will show clearly small dumps of material or even the actual barrels of guns.

In working on aeroplane photos there are two stages, the reading of the photo (often called interpretation, though the word is avoided here as having been used in another significance in this article) and its " annotation," that is, the redrawing of its indications in map form for the use of the army generally.

Aeroplane photographs record colours and accidents of the ground (such as bare earth, vegetation, woods, etc.) in terms of light and shade. The ground appears as a simple or complex pattern, in black, grey and white. Though the aeroplane photographic plate is affected by colour, that effect is not so marked as the effect of texture and shadow. For instance, a stretch of dry earth which to the eye appears dark will appear almost white on the photograph. The reason for this is that being smooth it has no texture or contained shadow, and con- sequently reflects light. Vegetation, on the other hand, which to the eye appears light will photographically be at the dark end of the scale because of its texture and contained shadow. It absorbs rather than reflects light. So when the nature of objects in an aeroplane photograph is to be determined colours must be judged principally in relation to texture. Ti, > ground must be visualized vertically not obliquely. .

The reading of aeroplane photographs, which necessitate ? a keen, trained eye, consists in the " spotting " of the r.umeroL ; details which the photograph contains; its annotation, which is in effect the labelling of the various objects shown, presupposes ability to appreciate these details and their relative importance in the enemy's system of defence and organization. When a detail has been discovered, the examiner of the photograph must decide its probable nature and its role and importance in the enemy's system of defence, offence or supply. Details are often very similar in photographs and their nature can only be discovered by considering them in relation to their position and the surrounding details as well as in relation to the current tactical practices of the enemy. The reading and translation of aeroplane photographs indeed is not a solitary science. The interdependence of all branches of intelligence work has already been emphasized and certainly this is no exception. Many details can be seen on photographs and their nature determined from photographs alone; but there is much that will be doubtful and must be reexamined in the light of prisoners' statements, ground and air observations, captured documents and captured maps. Conversely, the aeroplane photograph may supply miss- ing links in a chain partially established otherwise.

Unlike photographs taken obliquely, which convey some- thing to the merest novice, the vertical photograph must b^ carefully studied before it reveals its secrets. Only the tops of objects and their shadows are visible and it is only through the latter that the nature of many objects seen on a photograph can be determined. All objects have shadows. On a dull day