But the work of the catapult was not really satisfactory. The machine was clumsy; it occupied too much space, and it could not be aimed very accurately. It soon gave way to a more modern apparatus, fashioned after the old smooth-bore mortars. This was a miniature mortar, short and wide-mouthed. A rifled barrel was not required, because, since the missile was not to be hurled far, it was not necessary to set it spinning by means of rifling so as to hold it head-on to the wind. GIANT PEA-SHOOTERS Better aim was secured when a longer-barreled trench mortar came to be used. In the trench, weight was an important item. There was no room in which to handle heavy guns, and the mortar had to be portable so that it could be carried forward by the infantry in a charge. As the walls of a light barrel might be burst by the shock of exploding powder, compressed air was used instead. The shell was virtually blown out of the gun in the same way that a boy blows missiles out of a pea-shooter. That the shell might be kept from tumbling, it was fitted with vanes at the rear. These acted like the feathers of an arrow to hold the missile head-on to its course. [Illustration: Courtesy of "Scientific American" The Maxim Machine-gun Operated by the Energy of the Recoil] [Illustration: Courtesy of "Scientific American" Colt Machine-gun partly broken away to show the Operating Mechanism Gas from port _A_ pushes down piston _B_, rocking lever _C_, which compresses coil-spring _D_. The cartridge fed into the gun by wheel _E_, is extracted by _F_, raised by _G_ to breech _H_, and rammed in by bolt _I_. _J_, piston firing-hammer.] The French in particular used this type of mortar and the air-pump was used to compress the air that propelled the shell or aërial torpedo, or else the propelling charge was taken from a compressed-air tank. Carbon-dioxide, the gas used in soda-water, is commonly stored in tanks under high pressure and this gas was sometimes used in place of compressed air. When the gas in the tank was exhausted the latter could be recharged with air by using a hand-pump. Two or three hundred strokes of the pump would give a pressure of one hundred and twenty to one hundred and fifty pounds per inch, and would supply enough air to discharge a number of shell. The air was let into the barrel of the mortar in a single puff sufficient to launch the shell; then the tank was cut off at once, so that the air it contained would not escape and go to waste. THE STOKES MORTAR