In all modern weapons we use conical projectiles, fitted near the base with a soft copper driving-band, the diameter of which is somewhat larger than that of the bore of the gun, and cut a number of spiral grooves in the barrel. The enormous pressure generated by the explosion of the charge forces the projectile down the bore of the gun and out of the muzzle. The body of the projectile, made of steel or iron, being smaller in diameter than the bore, easily passes through, but the driving-band being of greater diameter, and being composed of soft copper, can only pass down the bore with the projectile by flowing into the grooves, thus preventing any escape of gas, and being forced to follow their twist. It therefore rotates rapidly upon its own longitudinal axis while passing down the barrel, and on leaving the muzzle two kinds of velocity have been imparted to it;--first, a velocity of motion through the air; secondly, a velocity of rotation round its axis which causes it to fly steadily onward in the required direction, _i.e._ a prolongation of the axis of the gun. Thus extreme velocity and penetrating power, as well as correctness of aim, are acquired. The path of a projectile through the air is called its _trajectory_, and if uninterrupted its flight would continue on indefinitely in a perfectly straight line. But immediately a shot has been hurled from the gun by the explosion in its rear two other natural forces begin to act upon it:-- Gravitation, which tends to bring it to earth. Air-resistance, which gradually checks its speed. (Theoretically, a bullet dropped perpendicularly from the muzzle of a perfectly horizontal rifle would reach the ground at the same moment as another bullet fired from the muzzle horizontally, the action of gravity being the same in both cases.) Its direct, even course is therefore deflected till it forms a curve, and sooner or later it returns to earth, still retaining a part of its velocity. To counteract the attraction of gravity the shot is thrown upwards by elevating the muzzle, care being taken to direct the gun’s action to the same height above the object as the force of gravitation would draw the projectile down during the time of flight. The gunner is enabled to give the proper inclination to his piece by means of the _sights_; one of these, near the muzzle, being generally fixed, while that next the breech is adjustable by sliding up an upright bar which is so graduated that the proper _elevation_ for any required range is given.