Its second purpose is to give the necessary spinning action to the shell. The old cannon ball suffered from the fact that it offered a considerable surface to the air in proportion to its weight. The idea arose, therefore, of making projectiles cylindrical and with a pointed nose, so that while the weight might be increased the resistance to the air might be even reduced. But it was clearly no use doing this unless the thing could be made to travel point foremost. Now for some rather mysterious reason, if you shoot a cylindrical object out of a gun, it will turn head over heels in the air, unless you give it a spinning motion. This motion, however, because of a gyroscopic effect, keeps the shell point first all the time. It has another effect, too, known as "air-boring." A spinning shell seems actually to bore its way through the air. Probably this is due to a centrifugal action, the spinning shell throwing the air outwards from itself and so to some extent sucking the air away out of its own path. Whether that be the true explanation or not, the fact remains that the spinning shell makes its way through the air better than a non-spinning one would do. The gun, therefore, has formed in its bore a very slow screw-thread called "rifling," from a French word meaning a screw. And it is the second function of the copper band to catch this rifling and by it be turned as the shell proceeds along the barrel. The soft copper conforms to the shape of the rifling and so itself becomes in a sense a screw engaging with the rifling. This band is situated near the base of the shell, lying in a groove turned in the shell for its reception. To prevent the band turning round without turning the shell there is a wavy groove turned in the bottom of the larger groove, and the band, being put on hot, is squeezed into the latter by a powerful press. The nose-bush is a little fitting of brass which screws into the smaller end of the shell and it has a hole in its centre into which another brass fitting, the nose itself, is screwed. The base of the shell is closed with a little disc of steel plate. People sometimes wonder why the original forging is not made solid at the bottom so as to save the necessity for this disc, but the reason is that if that were done defects might very possibly arise in the steel in the centre which, since it is the very spot whereon the propellant acts, might let some of the heat or force of the propellant through, causing a premature explosion of the charge inside the gun itself instead of among the ranks of the enemy.