Not the least of the calculations in this activity was to screen every detail from aerial observation. New hangars had risen at the edge of level fields, whence the swift fighting machines of an aircraft concentration in keeping with the concentration of guns and all other material rose to reconnaissance, or to lie in wait as a falcon to pounce upon an invading German plane. Thus the sky was policed by flight against prying aerial eyes. If one German plane could descend to an altitude of a thousand feet, its photographs would reveal the location of a hundred batteries to German gunners and show the plan of concentration clearly enough to leave no doubt of the line of attack; but the anti-aircraft guns, plentiful now as other British material, would have caught it going, if not coming, provided it escaped being jockeyed to death by half a dozen British planes with their machine guns rattling. To "camouflet" became a new English verb British planes tested out a battery's visibility from the air. Landscape painters were called in to assist in the deceit. One was set to "camouflet" the automobile van for the pigeons which, carried in baskets on the men's backs in charges, were released as another means of sending word of the progress of an attack obscured in the shell-smoke. This conscientious artist "camoufleted" the pigeon-van so successfully that the pigeons could not find their way home. Night was the hour of movement. At night the planes, if they went forth, saw only a vague and shadowy earth. The sausage balloons, German and Allied, those monitors of the sky, a line of opaque, weird question marks against the blue, stared across at each other out of range of the enemy's guns, "spotting" the fall of shells for their own side from their suspended basket observation posts from early morning until they were drawn in by their gasoline engines with the coming of dusk. Clumsy and helpless they seemed; but in common with the rest of the army they had learned to reach their dugouts swiftly at the first sign of shell fire, and descended then with a ridiculous alacrity which suggested the possession of the animal intelligence of self-preservation. Occasionally one broke loose and, buffeted like an umbrella down the street by the wind, started for the Rhine. And the day before the great attack the British aviation corps sprang a surprise on the German sausages, six of which disappeared in balls of flame. A one-armed man of middle age from India, who offered to do his "bit," refused a post at home in keeping with his physical limitations. His eyes were all right, he said, when he nominated himself as a balloon observer, and he never suffered from sea-sickness which sausage balloons most wickedly induce. Many a man who has ascended in one not only could see nothing, but wanted to see nothing, and turning spinach lopping over the basket rail prayed only that the engine would begin drawing in immediately.