But what if the gun-barrel does become hot? The real trouble is not that the cartridge will explode prematurely, but that the barrel will expand as it grows hot, so that the bullet will fit too loosely in the bore. Inside the barrel the bore is rifled; that is, there are spiral grooves in it which give a twist to the bullet as it passes through, setting it spinning like a top. The spin of the bullet keeps its nose pointing forward. If it were not for the rifling, the bullet would tumble over and over, every which way, and it could not go very far through the air, to say nothing of penetrating steel armor. To gain the spinning-motion the bullet must fit into the barrel snugly enough to squeeze into the spiral grooves. Now there is another American machine-gun known as the Hotchkiss, which was used to a considerable extent by the French Army. It is a gas-operated gun, something like the Colt, and it is air-cooled. It was found in tests of the Hotchkiss gun that in from three to four minutes of firing the barrel was expanded so much that the shots began to be a little uncertain. In seven minutes of continuous firing the barrel had grown so large that the rifling failed to grip the bullet at all. The gun was no better than an old-fashioned smooth-bore. The bullets would not travel more than three hundred yards. It is because of this danger of overheating that the Colt and the Hotchkiss guns are always furnished with a spare barrel. As soon as a barrel gets hot it is uncoupled and the spare one is inserted in its place. Our men are trained to change the barrel of a colt in the dark in a quarter of a minute. But a gun that has to have a spare barrel and that has to have its barrel changed in the midst of a hot engagement is not an ideal weapon, by any means. And this brings us to still another invention--that, too, by an American. Colonel I. N. Lewis, of the United States Army, conceived of a machine-gun that would be cooled not by still air but by air in motion. This would do away with all the bother of water-jackets. It would keep the gun light so that it could be operated by one man, and yet it would not have to be supplied with a spare barrel.