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.
The greater the velocity the flatter is the trajectory, and the more dangerous to the enemy. Assuming the average height of a man to be six feet, all the distance intervening between the point where a bullet has dropped to within six feet of the earth, and the point where it actually strikes is dangerous to any one in that interval, which is called the "danger zone." A higher initial velocity is gained by using stronger firing charges, and a more extended flight by making the projectile longer in proportion to its diameter. The reason why a shell from a cannon travels further than a rifle bullet, both having the same muzzle velocity, is easily explained.
A rifle bullet is, let us assume, three times as long as it is thick; a cannon shell the same. If the shell have ten times the diameter of the bullet, its "nose" will have 10 × 10 = 100 times the area of the bullet's nose; but its _mass_ will be 10 × 10 × 10 = 1000 times that of the bullet.