In the early stages of the war, when military and naval aviation was trying to adopt peace-time theories to war-time facts, Great Britain attempted to hunt the German submarines with aeroplanes, or hydroaeroplanes; but the method had its serious draw-backs. The aeroplane is of necessity a fast traveling machine; it must make at least forty miles an hour to be able to stay aloft. Whizzing through the air at such speed is not conducive to a careful scrutiny of the surface of the water below, necessary in order to detect the vague, dim outlines of a submerged submarine. At first the pilots of naval aeroplanes had considerable success in locating the submarines, and Germany lost quite a few of them, before the reason was discovered. Some one in Great Britain announced that it was easy to locate a submarine from an aeroplane by the peculiar reflection in the sunlight caused by the fine film of lubricating oil on the surface of the water. As soon as this "tip" was communicated to Germany, submarines discontinued the use of oil for lubrication, employing instead deflocculated graphite. The fuel oil used in the Diesel engines for propulsion on the surface is so thoroughly consumed and the exhaust now is so free of oil that an oil film as an indication of submarine proximity is no longer trustworthy. Besides, the submerged boat _might_ be a friendly one, a fact which was borne upon the British authorities on two separate occasions when scouting aeroplanes reported submarines near, and speedy motor boats rushed to the attack. In one case the British submarine is reported to have been rammed, and in the other--so the story goes--the commander of the submarine liberated a little buoy attached to the outside of the boat, which rose to the surface and informed the watchers above that "a friend is down below--not an enemy!"