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.
In other words, when two bodies are proportional in all their dimensions their air-resistance varies as the square of their diameters, but their mass and consequently their momentum varies as the cube of their diameters. The shell therefore starts with a great advantage over the bullet, and may be compared to a “crew” of cyclists on a multicycle all cutting the same path through the air; whereas the bullet resembles a single rider, who has to overcome as much air-resistance as the front man of the “crew” but has not the weight of other riders behind to help him.
As regards the effect of rifling, it is to keep the bullet from turning head over heels as it flies through the air, and to maintain it always point forwards. Every boy knows that a top “sleeps” best when it is spinning fast. Its horizontal rotation overcomes a tendency to vertical movement towards the ground. In like manner a rifle bullet, spinning vertically, overcomes an inclination of its atoms to move out of their horizontal path. Professor John Perry, F.R.S., has illustrated this gyroscopic effect, as it is called, of a whirling body with a heavy flywheel in a case, held by a man standing on a pivoted table. However much the man may try to turn the top from its original direction he will fail as long as its velocity of rotation is high. He may move the top relatively to his body, but the table will turn so as to keep the centre line of the top always pointing in the same direction.
Rifles.
Up to the middle of last century our soldiers were armed with the flint-lock musket known as “Brown Bess,” a smooth-bore barrel 3/4-inch in diameter, thirty-nine inches long, weighing with its bayonet over eleven pounds. The round leaden bullet weighed an ounce, and had to be wrapped in a “patch” or bit of oily rag to make it fit the barrel and prevent windage; it was then pushed home with a ramrod on to the powder-charge, which was ignited by a spark passing from the flint into a priming of powder. How little its accuracy of aim could be depended upon, however, is proved by the word of command when advancing upon an enemy, “Wait till you see the whites of their eyes, boys, before you fire!”