der 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.)

During the early part of the war, before the advent of the Lewis and
other automatic rifles, the only machine guns in general use were of
the heavy, tripod-mounted type and it was necessary for them to
advance with or even ahead of attacking troops. As the guns and
tripods were very conspicuous objects they naturally became the
especial targe