During the War of the Revolution, cannon were fired at short range, and it was the custom to load them with grape-shot, or small iron balls, when firing against a charging enemy, because the grape would scatter like the shot of a shot-gun and tear a bigger gap in the ranks of the enemy than would a single solid cannon-ball. In modern warfare, guns are fired from a greater distance, so that there will be little danger of their capture. It is impossible for them to fire grape, because the ranges are far too great; besides, it would be impossible to aim a charge of grape-shot over any considerable distance, because the shot would start spreading as soon as they left the muzzle of the gun and would scatter too far and wide to be of much service. But this difficulty has been overcome by the making of a shell which is really a gun in itself. Within this shell is the grape-shot, which consists of two hundred and fifty half-inch balls of lead. The shell is fired over the lines of the enemy, and just at the right moment it explodes and scatters a hail of leaden balls over a fairly wide area. It is not a simple matter to time a shrapnel shell so that it will explode at just the right moment. Spring-driven clockwork has been tried, which would explode a cap after the lapse of a certain amount of time; but this way of timing shells has not proved satisfactory. Nowadays a train of gunpowder is used. When the shell is fired, the shock makes a cap (see drawing facing page 77) strike a pin, _E_, which ignites the train of powder, _A_. The head of the shell is made of two parts, in each of which there is a powder-fuse. There is a vent, or short cut, leading from one fuse to the other, and, by the turning of one part of the fuse-head with respect to the other, this short cut is made to carry the train of fire from the upper to the lower fuse sooner or later, according to the adjustment. The fire burns along one powder-train _A_, and then jumps through the short cut _B_ to the other, or movable train, as it is called, until it finally reaches, through hole _C_, the main charge _F_, in the shell. The movable part of the fuse-head is graduated so that the fuse may be set to explode the shell at any desired distance. In the fuse-head there is also a detonating-pin _K_, which will strike the primer _L_ and explode the shell when the latter strikes the ground, if the time-fuse has failed to act. When attacking airplanes, it is important to be able to follow the flight of the shell, so some shrapnel shell are provided with a smoke-producing mixture, which is set on fire when the shell is discharged, so as to produce a trail of smoke. [Illustration: (C) Committee on Public Information Putting on the Gas Masks to Meet a Gas Cloud Attack]