However, the most useful trench mortar developed during the war was invented by Wilfred Stokes, a British inventor. In this a comparatively slow-acting powder was used to propel the missile, and so a thin-walled barrel could be used. The light Stokes mortar can easily be carried over the shoulder by one man. It has two legs and the barrel itself serves as a third leg, and the mortar stands like a tripod. The two legs are adjustable, so that the barrel can be inclined to any desired angle. It took but a moment to set up the mortar for action in a trench or shell-hole. [Illustration: FIG. 9. Sectional view of a 3-inch Stokes mortar showing a shell at the instant of striking the anvil] [Illustration: FIG. 10. A 6-inch trench mortar shell fitted with tail-vanes] Curiously enough, there is no breech-block, trigger or fire-hole in this mortar. It is fired merely by the dropping of the missile into the mouth of the barrel. The shell carries its own propelling charge, as shown in Fig. 9. This is in the form of rings, _A_, which are fitted on a stem, _B_. At the end of the stem are a detonating cap and a cartridge, to ignite the propellant, _A_. At the bottom of the mortar barrel, there is a steel point, _E_, known as the "anvil." When the shell is dropped into the mortar, the cap strikes the anvil, exploding the cartridge and touching off the propelling charge, _A_. The gases formed by the burning charge hurl the shell out of the barrel to a distance of several hundred yards. The first Stokes mortar was made to fire a 3-inch shell, but the mortar grew in size until it could hurl shell of 6-inch and even 8-1/2-inch size. Of course, the larger mortars had to have a very substantial base. They were not so readily portable as the smaller ones and they could not be carried by one man; but compared with ordinary artillery of the same bore they were immeasurably lighter and could be brought to advanced positions and set up in a very short time. The larger shell have tail-vanes, as shown in Fig. 10, to keep them from tumbling when in flight. CHAPTER III GUNS THAT FIRE THEMSELVES Many years ago a boy tried his hand at firing a United States Army service rifle. It was a heavy rifle of the Civil War period, and the lad did not know just how to hold it. He let the butt of the gun rest uncertainly against him, instead of pressing it firmly to his shoulder, and, in consequence, when the gun went off he received a powerful kick. That kick made a deep impression on the lad, not only on his flesh but on his mind as well. It gave him a good conception of the power of a rifle cartridge. Years afterward, when he had moved to England, the memory of that kick was still with him. It was a useless prank of the gun, he thought, a waste of good energy. Why could not the energy be put to use? And so he set himself the task of harnessing the kick of the gun.