In the water-cooled guns the barrel is en cased in a jacket filled with water, and the heat generated by the explosions is absorbed in the heating of the water to the boiling point (which takes but two and one-half minutes) and keeping it boiling. The steam is led by a flexible tube into a reservoir of cool water standing on the ground beneath the gun, and is there condensed. The water from this reser voir is used to replenish the supply in the jacket as it boils away. The Maxim, Gardner, Schwarzlose, Vickers and Browning machine guns are water-cooled. When used in aero planes however, water jackets are not needed, and are removed, the stripped guns operating satisfactorily as air-cooled guns. The water cooled guns show greater endurance than the air-cooled type. The Schwarzlose (Austrian) gun, which is essentially a modified Maxim, is notable as having fired continuously 35,000 rounds before the mechanism required repair. The Browning gun at its official trial fired 39,500 rounds continuously after having fired 20,000 rounds and been cooled down, before the mechanism gave out. Most machine guns have their ammunition arranged in pockets of belts made of fabric, with 250 cartridges on each belt. A few types have the ammunition attached to metal bands, from 25 to 30 cart ridges to each band. The Lewis gun carries its cartridges in a drum-shaped magazine. Formerly the machine gun was mounted on a gun carriage, but a stout tripod is now preferred and the gun is normally placed so as to be about two feet from the ground. The common method of transporting it is by pack-horse, though in the Great War these guns were usu ally carried on automobile trucks to the nearest point possible and carried by hand or dragged on a sled-mounting to the position where they were to be used. The German machine gun has a stand with four legs which may be ex tended like the poles of a stretcher, and the gun thus carried by two strong men, in case of emergency and easily by four men. Some types have adjustable skeleton wheels.
The principle of the machine gun has been applied to weapons of larger than rifle calibre and with a measure of success. The Maxim "pompom* or 37-millimetre gun throws one pound shells at the rate of 300 per minute. The Hotchkiss gun is also made for one-pound shells. The Maxim combination automatic nine-pounder gun fires 60 shots per minute, but cannot be aimed at this speed. It is arranged to fire also as slowly as desired.
The machine gun is regarded from a tactical point of view as an infantry gun — as dis tinguished from artillery. That is, its effect is that of concentrated infantry fire. It is con sidered as equivalent to 100 riflemen, is far more easily concealed and is much more effect ive, due to the elimination of the human factor. It can deliver a sweeping fire over a wide front, or a concentrated fire upon a small area. While it is wasteful of ammunition, the effect of machine gun fire is peculiarly demoralizing to troops in the field. It is primarily a defensive weapon most useful in stopping advancing forces; hut, given the attainment of a com manding position, it is supremely effective in putting hostile artillery out of commission. It is of comparatively little use against an enemy in line directly in front, but is invaluable in enfilade. For this reason machine-gun emplace
ments often occupy salients, and have loopholes that open obliquely. In the field the machine guns are moved forward with the attacking troops to hold the ground won in an advance and are often useful in firing over the'heads of a force advancing to an attack. It is effective up to 2,800 yards. As used in aeroplanes it sprays solid formations and supply trains on the march with a most deadly and demoralizing fire, besides being the chief weapon in air battles.
In 1917 the Maxim, gun was in use by the armies of Great Britain, Russia, Italy, Spain, Portugal, Norway, Sweden, Holland, Turkey, Greece and most of the countries of South America; the Hotchkiss gun in France, Japan, Spain and Portugal; the Schwarzlose gun in Germany, Austria and Bulgaria. Norway, Sweden and Denmark also used the Madsen automatic rifle. The Browning guns of both light and heavy types were introduced in 1918 in the United States army and Lewis auto matic rifles in the navy and the aeroplane serv ice. The Lewis rifle is used also by the armies of Great Britain and France.
The first machine gun tp be pro duced was the Gatling gun, invented by Dr. R. J. Gatling of the United States. It had 10 rifle barrels arranged in the form of a, hollow cylinder and was fired by turning a crank., The several barrels were discharged in suc cession, and delivered 500 shots per minute, scattered over the field at any angle. As each barrel fired only one shot out of 10, there was no difficulty from overheating. This gun was adopted by the British army in 1875. Gatling gun was followed by the Gardner gun which had fewer barrels and in the latest type had two barrels encased in a water jacket. The Nordenfelt gun was the next (1878), and worked on the same principle as tlie Gatling. In Hiram Maxim first exhibited his automatic ma chine gun, and this was followed by the Hotch kiss gun. The chief differences were that the Maxim gun was water-cooled and the Hotchkiss air-cooled, and that the former operated by the recoil, and the latter by gas pressure. The Colt machine gun followed. in 1906 the Schwarzlose gun created a sensation.. It was modeled upon the Maxim gun, but remarkably simplified. The Lewis gun, or, more correctly., automatic rifle was adopted by the British army in 1909. It weighs 25% pounds, and can be fired from the shoulder. Its magazine holds 49 cartridges. The Madsen automatic rifle is similar in type, but weighs only 15 pounds, and its magazines hold from 20 to 30 cartridges. In 1916 the Browning guns were invented es pecially for the United States army. The 13rowning automatic rifle resembles closely the Madsen rifle in appearance, weighs 15 pounds,' and its detachable magazines carry generally 20 rounds although special magazines holding 40 rounds are also provided. The Browning machine gun resembles the Colt gun in some respects, and the Schwarzlose gun in others. Its ammunition is carried in belts holding 25CI cartridges, which feed into the gun from a box mounted above the tripod. The Browning gun can he stripped of its water-cooling-jacket for aeroplane work, and in that form weighs only 2234 pounds