Like the Colt and the Hotchkiss, the Lewis gun takes its power from the gas that comes through a small port in the barrel, near the muzzle. In the plate facing page 44 the port may be seen leading into a cylinder that lies under the barrel. It takes about one ten-thousandth part of a second for a bullet to pass out of the barrel after clearing the port, but in that brief interval there is a puff of gas in the cylinder which drives back a piston. This piston has teeth on it which engage a small gear connected with a main-spring. When the piston moves back, it winds the spring, and it is this spring that operates the mechanism of the gun. The cartridges, instead of being taken from a belt or a clip, are taken from a magazine that is round and flat. There are forty-seven cartridges in the magazine and they are arranged like the spokes of a wheel, but in two layers. As soon as forty-seven rounds have been fired, the shooting must stop while a new magazine is inserted. But to insert it takes only a couple of seconds. USING THE BULLET TO FAN THE GUN The most ingenious part of the Lewis gun is the cooling-system. On the barrel of the gun are sixteen flanges or fins. These, instead of running around the gun, run lengthwise of the barrel. They are very light fins, being made of aluminum, and are surrounded by a casing of the same metal. The casing is open at each end so that the air can flow through it, but it extends beyond the muzzle of the barrel, and there it is narrowed down. At the end of the barrel there is a mouthpiece so shaped that the bullet, as it flies through, sucks a lot of air in its wake, making a strong current flow through the sixteen channels formed between the fins inside the casing. This air flows at the rate of about seventy miles per hour, which is enough to carry off all the heat that is generated by the firing of the cartridges. The gun may be regulated to fire between 350 and 750 rounds per minute, and its total weight is only 25-1/2 pounds. [Illustration: Lewis Machine-guns in action at the front] America can justly claim the honor of inventing and developing the machine-gun, although Hiram Maxim did give up his American citizenship and become a British subject. By the way, he is not to be confused with his younger brother, Hudson Maxim, the inventor of high explosives, who has always been an American to the core. Of course we must not get the impression that only Americans have invented machine-guns. There have been inventors of such weapons in various countries of Europe, and even in Japan. Our own army for a while used a gun known as the Benèt-Mercié, which is something like the Hotchkiss. This was invented by L. V. Benèt, an American, and H. A. Mercié, a Frenchman, both living in St. Denis, France. THE BROWNING MACHINE-GUN