Why the difference? someone may ask. The answer is that the longer ones are an improved type. The extra length gives longer range and harder hits, as is quite apparent after a little thought. The explosive "goes off" and forthwith commences to drive the shell towards the muzzle. So long as it is in the gun the shell is being pushed faster and faster, but so soon as it leaves the muzzle the pushing ceases and the shell is left to pursue its course with its own momentum. Therefore, generally speaking, one may say that the longer the gun the faster will be the speed of the shell as it leaves the muzzle, the farther will it go and the harder will be the blow at a given range. Incidentally this explanation reveals the need for different kinds of explosive. The propellant whose function it is to drive the shell out of the gun is different from that with which the shell is itself filled. The former needs to act comparatively slowly, so that it may continue its pushing action during the whole time that the shell is travelling along the gun. It might be ever so powerful, but were its action too sudden it would simply tend to burst the gun, without imparting very much speed to the shell. On arrival at its destination, however, the shell needs to burst suddenly and violently. Another interesting question arises at this point. Seeing how fast is even the slowest speed at which a projectile travels, how can it be possible to measure the rate at which a shell issues from one of these monster guns. Needless to say, it is electricity which makes a thing apparently so difficult really quite easy. Near the gun is set up a frame with a wire zigzagging to and fro across it, in such a manner that when the gun is fired the shell is bound to cut the wire. Electric current is made to pass through this wire on its way to a suitable house in which are recording instruments, where it energises a magnet and so holds something up. Now it is easy to see that as soon as the shell cuts the wire the current will stop, the magnet will "let go" and the "something" will drop. At a certain distance farther on there is a second frame with wires upon it, through which passes a second current, which is also led to the instrument house, where it again operates a second magnet. When the first magnet releases its hold it drops something, to wit, a long lead weight. When the second magnet lets go it permits a second weight to fall against the first and make a dent or scratch upon it. The longer the interval between the action of the two magnets the higher up upon the lead weight will the scratch be. The apparatus, in short, will register the distance fallen through by the lead weight between the breaking of the wire in the first frame and the breaking of the wire in the second frame.