A very busy program he worked out for that kick to perform. He planned to have the gun use up its exuberant energy in loading and firing itself. So he arranged the cartridges on a belt and fed the belt into the gun. When the gun was fired, the recoil would unlock the breech, take out the empty case of the cartridge just fired, select a fresh cartridge from the belt, and cock the main spring; then the mechanism would return, throwing the empty cartridge-case out of the gun, pushing the new cartridge into the barrel, closing the breech, and finally pulling the trigger. All this was to be done by the energy of a single kick, in about one tenth of a second, and the gun would keep on repeating the operation as long as the supply of cartridges was fed to it. The new gun proved so successful that the inventor was knighted, and became Sir Hiram Maxim. A DOCTOR'S TEN-BARRELED GUN But Maxim's was by no means the first machine-gun. During the Civil War a Chicago physician brought out a very ingenious ten-barreled gun, the barrels of which were fired one after the other by the turning of a hand-crank. Although Dr. Gatling was a graduate of a medical school, he was far more fond of tinkering with machinery than of doling out pills. He invented a number of clever mechanisms, but the one that made him really famous was that machine-gun. At first our government did not take the invention seriously. The gun was tried out in the war, but whenever it went into battle it was fired not by soldiers but by a representative of Dr. Gatling's company, who went into the army to demonstrate the worth of the invention. Not until long after was the Gatling gun officially adopted by our army. Then it was taken up by many of the European armies as well. Although many other machine-guns were invented, the Gatling was easily the best and most serviceable, until the Maxim invention made its appearance, and even then it held its own for many years; but eventually it had to succumb. The Maxim did not have to be cranked: it fired itself, which was a distinct advantage; and then, instead of being a bundle of guns all bound up into a single machine, Maxim's was a single-barreled gun and hence was much lighter and could be handled much more easily. A GUN AS A GAS-ENGINE Another big advance was made by a third American, Mr. John M. Browning, who is responsible for the Colt gun. It was not a kick that set Browning to thinking. He looked upon a gun as an engine of the same order as an automobile engine, and really the resemblance is very close. The barrel of the gun is the cylinder of the engine; the bullet is the piston; and for fuel gunpowder is used in place of gasolene. As in the automobile engine, the charge is fired by a spark; but in the case of the gun the spark is produced by a blow of the trigger upon a bit of fulminate of mercury in the end of the cartridge. [Illustration: Courtesy of "Scientific American" The Lewis Gun which produces its own cooling current]