Another well-defined distinction is made between the true machine gun and the automatic rifle; the former being so heavy that it must be mounted on a substantial tripod or other base, while the latter is so light that it may be carried and operated by a single man. Of the former class, the Colt, (35 lbs.), the Vickers, (38 lbs.) and the Maxim, (63 lbs.) may be taken as representative. They are all mounted, for field work, on tripods weighing fifty pounds or more. In the latter class, the Lewis, Benet-Mercie, and Hotchkiss, running from 17 to 25 lbs., are fair examples. They are all equipped with light, skeleton "legs" or tripods, which, by the way, are never used in the field although they are still considered essential for training purposes. In the gas-operated arms, a small hole is drilled in the under side of the barrel, six to eight inches from the muzzle, so that, when the bullet has passed this point, and during the time it takes it to traverse the remaining few inches to the muzzle, a certain portion of the enclosed gas is forced through this hole, where it is "trapped," in a small "gas-chamber" and its force directed against a piston or lever which, being connected with the necessary working parts of the gun by cams, links or ratchets, performs the functions of removing and ejecting the empty cartridge case, withdrawing a new cartridge from the belt, clip or magazine, and "cocking" the gun: that is, forcing the "hammer" or striker back and compressing its spring. As the pressure generated in the barrel by our ammunition is not less than 50,000 lbs. to the square inch, very little gas is required to do all this. There must also be sufficient force to compress or coil a strong spring or springs called "main-springs" or retracting springs which, in their turn, force the mechanism forward to its original position, seating the new cartridge in the chamber and releasing the striker, thus firing another shot. This action continues as long as the "trigger" is kept pressed or until the belt or magazine is emptied. The Colt, Benet-Mercie, Hotchkiss and Lewis are in this class. They are all of the air-cooled type. In the recoil operated guns, the barrel itself is forced to the rear by the "kick," as we commonly call it, and the force applied directly to the working parts, thus performing the same operations above described. The Maxim, Vickers, Vickers-Maxim and Maxim-Nordenfeldt belong to this class. They are all water-cooled, having a water-jacket of sheet metal entirely surrounding the barrel. All the last-mentioned class, and also the Colt, have the ammunition loaded in belts containing two hundred and fifty rounds each. The Hotchkiss and Benet-Mercie use clips of from twenty to thirty rounds, while the Lewis is fed from a round, flat, pan-shaped magazine holding forty-seven rounds. (For aircraft guns these magazines are made larger; about double this capacity, I think.)