The night aviator, however, has many things to think of besides a constant checking and readjustment of his course according to variations in direction and velocity of wind. On his own side of the lines he is constantly challenged by searchlights which must be answered immediately if the aviator wishes to avoid the risk of being shot down by his own anti-aircraft guns or of being attacked by his own night-patrol machines. The method of answering these challenges is extremely simple. All that is required of the aviator is to shoot at the searchlight with a large pistol loaded with an enormous cartridge. The aviator, intent on his calculations and annoyed by any interruption, often wishes that this pistol was a deadly weapon, but it is not. It merely fires a certain colored light which floats slowly down changing in its descent to certain other colors, which prove to the officer in charge of the challenging searchlight that an Allied aeroplane is above him. The colors which are shown on one night, however, will not do on another, for these "colors of the day," as they are inappropriately called, are changed every night and the utmost secrecy is maintained in regard to them. Even the aviators do not know the "color of the day" until ten minutes before the start of a raid, neither do the officers in charge of the anti-aircraft batteries. The reason for this secrecy became apparent to the Bedouins one night when a Hun flew over our aerodrome shooting down our "color of the day," blinking his navigation lights, and finally firing down a red light which was our prearranged forced-landing signal. The aerodrome officer, believing that one of the Bedouin machines was returning from that night's raid with engine trouble, lit up the "landing T" and brought upon himself a shower of bombs which carried him into the Unknown. After crossing the lines the aviators are intent on steering an accurate compass course, checking their position from time to time by various landmarks such as canals, rivers, cross-roads, and woods, and figuring changes in wind. The bursting shells of the enemy anti-aircraft batteries must be disregarded, for a slight detour around a particularly heavy barrage might mean an error of several degrees in their course which, unless corrected, would bring them twenty to thirty miles away from their objective after a flight of one hundred and seventy miles or more, and an accurate correction of a compass course after a wide detour is always difficult and sometimes impossible. Therefore, it is of the utmost importance for long-distance night bombers to hold their course regardless of the enemy's efforts at destruction.