FOOLING THE WATCHERS IN THE SKY
In the early stages of the war, it seemed impossible to hide anything from the Germans. They had eyes everywhere and were able to anticipate everything the Allies did. But the spies that infested the sky were the worst handicap. Even when the Allies gained control of the air, the control was more or less nominal because every now and then an enemy observer would slip over or under the patrolling aëroplanes and make photographs of the Allies' lines. The photographs were carefully compared with others previously taken, that the slightest change in detail might be discovered. Airplane observers not only would be ready to drop bombs on any suspicious object or upon masses of troops moving along the roads, but would telephone back to their artillery to direct its fire upon these targets. Of course, the enemy knew where the roads were located and a careful watch was kept of them.
The French did not try to hide the roads, but they concealed the traffic on the roads by hanging rows of curtains over them. As these curtains hung vertically and were spaced apart, one would suppose that they would furnish little concealment, but they prevented an observer in an aëroplane from looking down the length of a road. All the road he could see was that which lay directly under his machine, because there he could look between the curtains; if he looked obliquely at the road, the curtains would appear to overlap one another and would conceal operations going on under them.
In one case, the Germans completely covered a sunken road with canvas painted to represent a road surface. Under this canvas canopy, troops were moved to an important strategic point without the slightest indication of such a movement.