It was really a very difficult matter to hit a rapidly moving airplane; and even if it were hit, there were few spots in which it could be mortally wounded. Hundreds of shots could go through the wings of an airplane without impairing its flying in the least. The engine, too, could be pretty well peppered with ordinary bullets without being disabled. As for the men in the machine, they furnished small targets, and even they could be hit in many places without being put entirely out of business. And so the dangers of air scouting were not so great as might at first be supposed. One of the most vulnerable spots in the airplane was the gasolene-tank. If that were punctured so that the fuel would run out, the airplane would have to come to the ground. Worse still, the gasolene might take fire and there was nothing the aviator dreaded more than fire. There were occasions in which he had to choose between leaping to earth and burning to death, and the former was usually preferred as a quicker and less painful death. In some of the later machines the gasolene-tank could be pitched overboard if it took fire, by the throwing of a lever, and then the aviator could glide to earth in safety. THE SELF-HEALING GASOLENE-TANK One of the contributions which we made to military aëronautics was a gasolene-tank that was puncture-proof. It was made of soft rubber with a thin lining of copper. There are some very soft erasers on the market through which you can pass a lead pencil and never find the hole after it has passed through, because the rubber has closed in and healed the wound. Such was the rubber used in the gasolene-tank. It could be peppered with bullets and yet would not leak a drop of gasolene, unless the bullet chanced to plow along the edge of the tank and open a long gash. The Germans used four different kinds of cartridges in their aircraft guns. The first carried the ordinary bullet, a second type had for its bullet a shell of German silver filled with a phosphor compound. This was automatically ignited through a small opening in the base of the shell when it was fired from the gun and it left a trail of smoke by which the gunner could trace its course through the air and correct his aim. At night the bright spot of light made by the burning compound would serve the same purpose. Such a bullet, if it hit an ordinary gasolene-tank, would set fire to its contents. The bullet would plow through the tank and out at the opposite side and there, at its point of exit, is where the gasolene would be set on fire. Such incendiary bullets were repeatedly fired into or through the rubber tanks and the hole would close behind the bullet, preventing the contents from taking fire. The two other types of bullets referred to were an explosive bullet or tiny shell which would explode on striking the target and a perforating steel bullet which was intended to pierce armor or penetrate into vital parts of an airplane engine.