PROJECTILES, THEORY OF, that branch of mechanics which treats of the motion of bodies thrown or driven some distance by an impelling force, and whose progress is affected by gravity and the resistance of the air. The most common cases are the balls projected from cannon or other firearms. If thrown horizontally, the body will move in a curved path, while it falls faster and faster toward the ground. A body pro jected obliquely has initially a certain horizontal velocity and a certain vertical velocity. It retains its horizontal veloc ity unchanged, but its vertical velocity is altered by the force of gravity, and in both of these cases we find that the path of the projectile is a parabola. With a given velocity the greatest range of a projectile is obtained by projecting at an angle of 45° with the vertical.
The velocity of projectiles fired from modern guns ranges from 1,500 to 3,000 feet per second. It is computed that the average velocity of the larger guns on the cruiser "New York" is 2,100 feet per second. Our small arm shoots a bullet
only one-third of an inch in diameter, which travels 2,000 feet in a second, or a mile in three seconds. It goes so fast that it becomes hot to the touch, due to the resistance of the air which it pushes aside. Strange to say, the heated bullet will cauterize the wound of its own mak ing and few of the wounded in the Spanish-American War in 1898 bled to death, except where struck in a vital spot.
Rotational Veloeity.—The projectile has, besides the forward velocity, a ro tational velocity, which is given to it by the rifling of the gun. Otherwise, since its length is much greater than its di ameter, it would soon begin to turn end on. The rifling prevents this by causing the bullet to bore a path through the air, and the higher the forward velocity the higher, too, must be the rotational.