There are several other types of machine guns, but the most ingenious, and perhaps the most effective, is the Maxim automatic gun. This has a single barrel, about two thirds of which, from the muzzle towards the breech, is surrounded by a water-jacket into which water is automatically injected at each discharge, thus rendering overheating impossible. The mechanism for operating the gun is at the breech, covering the remaining third of the barrel. All that is necessary is to draw back the trigger to fire the first shot; the recoil of the piece again cocks it, and the gun is then automatically fired, the process being kept up until the cartridges in the feed-belt are all expended. The cartridges are fed to the piece by means of belts holding 333 rounds, two or more of the belts being joined together if desired. The Maxim gun can easily fire ten shots a second, and if every man at the piece were killed the moment the first shot was fired the gun would keep on until it fired at least 332 more shots. The Gatling, Gardner, Maxim, and similar guns are known as machine guns. Of the same general family, so to speak, are rapid-fire guns, which are, however, distinguished from machine guns by having a larger calibre, loading by hand, having only one barrel, and being provided with artificial means of checking recoil and returning the piece to the firing position. They use metallic ammunition, and have a breech mechanism which cocks the firing pin and extracts the empty case by the same motion which opens the breech for reloading. Rapid-firing guns were first designed as a means of naval defense against torpedo boats. They deliver a rapid and easily aimed fire, and use projectiles of sufficient power to penetrate the plates of the boats. In the naval service the gun is mounted on a spring return carriage fixed to the vessel, so that the piece, when discharged, is brought back to the firing position without any derangement of aim. On land a rigid carriage is used. This carriage has a spade at the end of the trail, which is forced into the ground by the recoil and holds the gun and carriage in place. The principal rapid-fire guns are the Hotchkiss, Driggs-Schroeder, Nordenfelt, Krupp, Canet, and Armstrong, which fire from five to ten shots a minute, and use either shell or shrapnel. Experiments are now being made in different armies with a view to adopting rapid-fire guns for field artillery. The principle of rapid fire, or “quick fire,” has been successfully applied to guns having a caliber as great as six inches. The metallic cartridge used in rapid-fire guns is, in appearance, simply a “big brother” of the cartridge used in the infantry rifle.