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It is now possible to sum up the theoretical conditions which govern the concealment of gun positions, and other works, from the enemy in the air:

(a) The material of which the camouflage is composed must at all times appear on the photograph like the object or surface it represents, and likewise appear natural to the observer's eye. QuA material, it must be light, strong, impervious to weather, fire-proof and easily manufactured, (b) Disturbances of soil, tracks, shadows, blast-marks and regularity must never appear to be associated with an active gun position or occupied work.

Practical A pplication. We come now to the application of these principles. In the early part of the World War air photography was not the highly specialized art it subsequently became, and therefore the difficulties of combating it were not so great. At first, freshly cut branches and grass were used, being the mate- rials nearest to hand. These withered in the course of a few days and ceased to be efficacious. The next stage was the employment of sheets of canvas painted to represent the ground. The design was bold, and consisted of large masses of green, or brown and green as the case might be, with heavy black shadings,* to give the effect of texture. These covers were draped over the guns and came down to the ground on every side, being removed when the gun was in action and replaced immediately afterwards. This system also proved unsatisfactory. It is nearly impossible to reproduce on a smooth sheet of canvas the changing tones of the ground as recorded by the camera. Under certain con- ditions i.e. when the angle of light incidence is small, or after rain painted canvas, having no texture, reflects so much light that all trace of pattern or colour is lost.

Then came the introduction of fish netting. At first these nets were garnished sparsely with bunches of painted raffia (garden- ers' bast). The effect was excellent; the nets were light and portable; but the inflammability of the painted raffia was a grave disadvantage. Efforts made to dye the raffia and to render it fire-proof proved fruitless. The dyes, especially green, were too fugitive, and no method of rendering the raffia permanently fire- proof could be discovered. Strips of painted canvas, instead of raffia, proved more satisfactory from the manufacturing point of view, but these also suffered from the defect of inflammability, 1 though in a lesser degree. The final evolution of the gun cover was a net having an opaque centre of painted scrim, 2 the shape of which was boldly irregular, with a border of painted canvas strips decreasing in density towards the edges, erected horizon- tally, like a carpet, over the work and much larger in area than the work itself (see fig. 3). Thus, the excavation was concealed by the opaque centre, the shadow of which was blurred or masked by the bolder of strips which, in themselves, were not sufficiently dense to cast a shadow. If skilfully erected and maintained such covers were satisfactory. Installed before any work of excava- tion was started, subsequent construction and occupation re- mained concealed. Guns could be treated individually or col- lectively by increasing the area covered. Figs, i and 2 show the treatment of a battery position placed under the edge of a bank. The false edge of the " bank " should be noted.

The use of netting was practically confined to works whose nature demanded covers erected at a considerable height above ground level. Scrim was used, by itself, to conceal objects near, or on, the ground, such as short lengths of trench, ammunition, gas-projectors; it should always be reenforced by natural ma- terial to increase its texture effect. Further, this material must always be cut or assembled in large fantastic shapes, in order to appear natural, and to allow its edges to merge gradually into its surroundings.

Many gun positions, which had defied all attempts at location, were betrayed by snow, particularly in respect of blast marks, because the flash of discharge melts the snow over a large area immediately in front of the gun. Further, shadows were accentu- ated, and the normal method of combating shadows, by the adoption of thinned edges, proved fatal in snow, as such nets

when the Armistice put an end to its urgency.
 * A solution of this problem of fire-proofing canvas was in sight

2 A kind of loosely woven canvas whose meshes give the effect of texture by absorbing light.

did not hold the snow and consequently appeared as black holes in a sheet of white. White calico proved a palliative, especially in the case of blast marks, if boldly irregular in shape.

Evidence afforded by tracks is perhaps the most difficult of all to eliminate. Frequently positions, which are admirably con- cealed in every other way, are betrayed by the tracks leading up to them, so much so, that it is often possible to count the number of guns in a battery by the paths leading to each gun-pit and to distinguish between gun positions and other works. It is comparatively easy to plan the approach so that it may be con- cealed naturally or artificially; the difficulty is to ensure that this and no other route is used human nature being so strongly addicted to taking short cuts, barbed wire and discipline seem to be the only means of preventing it.

The following afford good illustrations of methods of concealing approaches that have been adopted with success: (a) Leading the track close past the gun position and on to join an existing track. The connexion to each pit being treated with camouflage material or cut grass, etc.. etc. (b) Similarly, but close in front of the gun- pits in order to use the track to hide blast marks. This method has the disadvantage of restricting traffic while the guns are in action. (c) Siting a battery in the midst of an existing network of tracks, taking precautions to reproduce on the camouflage any path inter- rupted by a gun-pit.

It is not practicable to conceal long trenches. If a covering sags or differs materially in tone from its surroundings the mere length and regularity will betray it. A covering, originally perfect, will require continual attention to keep it perfect, in- volving labour out of all proportion to its value. Short lengths of trench can be concealed, provided care is taken to support the camouflage adequately to prevent sag, and to conceal the spoil.

This applies equally to trench systems prepared far behind the lines for use in the event of a retirement. It is probable that the enemy, foreseeing the construction of such a defensive line, will be able to guess the approximate positions of such systems, and he is certain to have periodically photographed the suspected area. It is quite impossible to prevent some traces of work being evident in a long and deep system of defences. Camouflage must obviously be restricted to vital spots, and extreme care must be exercised

(2) Camouflage against Direct Observation. The concealment of observation posts was comparatively simple, being merely an adaptation of the craft of theatrical property-making. Natural features were selected, in places from which good observation could be obtained, and these were copied exactly. At night, the real was removed and replaced by the imitation. A large vari- ety of objects were so copied among which may be mentioned: trees, sand-bags, milestones, mounds of earth, chimney- stacks, walls. In all cases the copy was a thin outer shell con- taining a bullet-proof lining in order to give confidence to the occupier. The loopholes, when subject to scrutiny at short range, could be made quite invisible by the use of gauze, which, though painted to resemble the exterior of the O.P., remained trans- parent from the inside. This method was only adopted when absolutely necessary, because gauze interferes with vision especially through glasses; in other cases care was taken to give the loophole an irregular shape.

Certain conditions were found to govern the successful employ- ment of these observation posts, particularly in the case of the more elaborate examples such as trees.

a. Concealed access is essential.

b. The work connected with installation must, like other work, be concealed from the air.

c. They should not be erected in places that are normally sub- ject to heavy shelling, for the reason that careful observation will be prejudiced and accidental damage will probably reveal the obser- vation post to the enemy.

d. Provision must always be made to prevent daylight showing behind the loophole, so rendering it transparent to the enemy.

e. The comfort and security of the observer must always be studied, otherwise the full value of the observation will never be obtained.

Imitation trees (see fig. 6) were designed either to accommo- date an observer at a commanding height above the ground, or to conceal a long periscope, the user of which was protected in a strong dug-out. In the former case the observer had a better view, but was uncomfortably cramped. The periscope is limited