The next marked step in the development of the machine gun was to make it fan itself and thus keep its barrel cool. Col. I. N. Lewis designed a gun operated by gas pressure somewhat on the principle of the Colt gun and around the barrel he fitted sixteen deep flanges or fins of aluminum that ran lengthwise of the gun. Around these fins he fitted a casing, thus forming 16 long narrow chambers about the barrel. The casing was open at the breech end, but at the outer end was contracted into a narrow mouthpiece that extended beyond the muzzle. The mouthpiece was so formed that as the bullets passed through it they sucked air through the chambers, thereby cooling the gun. The air travels through the casing at the rate of about seventy miles per hour. This design permitted Col. Lewis to build a very light gun. Its total weight was but 25½ pounds and it could be handled by a single man if the muzzle was supported on some sort of a rest. It represented a marked step toward a shoulder machine gun which would increase enormously the efficiency of infantry equipped with this weapon. The difference between rifle fire and machine-gun fire has been likened to the difference between trying to hit a tin can with a stone and with a stream of water from a hose. In the latter case the hose may be raised or lowered to correct the course of the stream and bring it to bear on the target. In the same way by watching the effect of the machine-gun bullets the leaden stream may be corrected to bring it directly upon the target. The advantages of a weapon such as this, which may be fired from the shoulder or from the hip, are perfectly obvious. As the war was nearing its end John M. Browning produced two machine guns, one a water-cooled heavy model fired from a stand. The total weight of this gun was 34½ pounds and with the water jacket empty it weighed but 22½ pounds. It was operated by the recoil of the gun, but the mechanism was greatly simplified and there were but few parts. These could be taken apart and replaced with new ones in a minimum of time in case of breakage. The other gun was a shoulder rifle weighing only 16 pounds, which carried a clip of twenty cartridges. These could be fired singly or in rapid succession in the space of two and a half seconds. Only a second was required to replace the empty clip with a filled one. No special cooling apparatus was provided because it was not likely that a shoulder rifle would be fired long enough at a time to become excessively heated. POCKET-SIZED MACHINE GUN