RIFLED ARMS were invented for the purpose of remedying certain defects essentially connected with cylindrical smooth-bore guns. These defects, which are chiefly owing to atmospheric resistance, showed themselves in the erratic motion of the ball, especially when fired at a long range, and arose from the following causes: First, The ball never fitted tightly, and, in consequence of this, its center was below the center of the bore. A portion of the explosive force of the powder escaped over the top of the bullet, and was not only wasted, but exercised a downward pressure on the ball, tending to squeeze it into the tinder side of the barrel, and so great was this pressure that in guns of soft metal, as brass, a perceptible dint was produced after a few rounds. Another and more important consequence of the looseness of the ball was that the action of the powder on it was necessarily irregular, and its resulting motion along the barrel was a series of oblique impacts, now against one side, now against the other, and the direction of its motion after expulsion was necessarily not in line with the axis of the barrel, and cepended upon the side of the barrel with which it was last in contact. Secondly, Balls can never be perfectly homogeneous; and the violent and sudden pressure of the exploded powder produces a slight change of shape; consequently the center of gravity can never accurately coincide with the center of the sphere, the air resists its forward motion unequally, and true flight is precluded. Thirdly, As a consequence of the friction of the ball against the sides of the barrel, it acquires a rotatory motion, the direction of its rotation after expulsion being determined by the particular point of the muzzle with which it was last in contact. Thus, if it finally touchei. the top or bottom of the muzzle, the plane of rotation of the anterior surface of the ball would be in line with its progressive motion, and the rotation would be in an upward or downward direc tion; if it last rebounded from the right side, the plane of rotation would be in line with its path, and the rotation of the anterior surface from left to right, and so on. The ball,
in its rapid flight, compresses the air in front, and produces a vacuum behind; the denser because more compressed, air in front, attempts to rush round the sides of the ball to fill up the vacuum. Now let us suppose that the ball,while in rapid advance, is also revolving in a horizontal plane, and from left to right, the left side, whose rotation conspires with the motion of translation, resists, by its friction, the attempt of the air to reach the vacuum by that side; while the right side, whose rotation is against the motion of trans lation, conspires to aid the air in reaching the vacuum. It follows from this that the air is denser in front of the left side than in front of the right; its resistance on the left side is greater than that on the right, and the ball, in consequence, is deflected toward the side on which the resistance is least (toward the right in this instance). If the ball struck the top of the muzzle, its revolution would be in a vertical plane in line with the barrel, and in an upward direction, under which circumstances the ball would tend, first, downward from the first reason, and then upward from the third; while, if it struck the bottom of the muzzle, the contrary would be the case. These aberrations of the ball from its true theoretical path, as was evident to artillerists, could never be wholly annihilated while smooth-bores were used, and they set themselves to discover how they might be counteracted. It occurred to them that this could best be managed by securing that the plane of rotation of the ball should be at right angles to its motion of translation, as the irregularities in its structure, which produce aberrations of the first and second kind, would thus act equally in all directions. producing an exact coun terbalance, while the aberration from the ball's rotation would wholly disappear; and the constancy of the vertical transverse position of the plane of the ball's rotation was obtained by making one or more spiral grooves along the interior of the barrel.