The cam _B_ on the propeller shaft lifts the rod _C_, rocking the angle lever _D_ which moves the rod _E_ and operates the firing-piece _F_. Firing may be stopped by means of lever _H_ and Bowden wire _G_. _I_ is the ejection-tube for empty cartridges.] [Illustration: It would take a Hundred Horses to Supply the Power for a Small Airplane] A cam placed on the propeller-shaft worked the trigger of the machine-gun. This did not slow up the fire of the machine-gun. Quite the contrary. We are apt to think of the fire of the machine-guns as very rapid, but they usually fire only about five hundred rounds per minute, while an airplane propeller will make something like twelve hundred revolutions per minute. And so the mechanism was arranged to pull the trigger only once for every two revolutions of the propeller. FIGHTING AMONG THE CLOUDS There was no service of the war that began to compare with that of the sky fighter. He had to climb to enormous heights. Air battles took place at elevations of twenty thousand feet. The higher the battle-plane could climb, the better, because the man above had a tremendous advantage. Clouds were both a haven and a menace to him. At any moment an enemy plane might burst out of the clouds upon him. He had to be ready to go through all the thrilling tricks of a circus performer so as to dodge the other fellow and get a commanding position. If he were getting the worst of it, he might feign death and let his machine go tumbling and fluttering down for a thousand feet or so, only to recover his equilibrium suddenly and dart away when the enemy was thrown off his guard. He might escape into some friendly cloud, but he dared not hide in it very long, lest he get lost. It is a peculiar sensation that comes over an aviator when he is flying through a thick mass of clouds. He is cut off from the rest of the world. He can hear nothing but the terrific roar of his own motor and the hurricane rush of the wind against his ears. He can see nothing but the bluish fog of the clouds. He begins to lose all sense of direction. His compass appears to swing violently to and fro, when really it is his machine that is zig-zagging under his unsteady guidance. The more he tries to steady it, the worse becomes the swing of the compass. As he turns he banks his machine automatically, just as a bicyclist does when rounding a corner. He does this unconsciously, and he may get to spinning round and round, with his machine standing on its side. In some cases aviators actually emerged from the clouds with their machines upside down. To be sure, this was not an alarming position for an experienced aviator; at the same time, it was not altogether a safe one. A machine was sometimes broken by its operator's effort to right it suddenly. And so while the clouds made handy shelters, they were not always safe harbors.