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 the air, and if it were not acted on by any other force, would proceed for ever in the line in which it started; gravity, however, at once asserts its sway, and keeps pulling it down towards the earth. These two forces together would make it describe a curve, known as the parabola. There is, however, another retarding influence, the air; and though Galileo, and Newton in particular, pointed out the great effect it would have, several philosophers, in fact the majority, still believed that a parabola was the curve described by the path of a shot. It remained for Mr. Robins to establish this point and to prove the great resistance the air offered: to this we shall have to recur again presently. Let us first see how a shot is projected. If the bullet fitted the bore of the gun perfectly, the whole force in that direction would be exerted on it; but in order that the gun might be more easily loaded—and this was more especially the case with cannon—the bullet was made somewhat smaller than the bore or interior cylinder; a space was therefore left between the two, termed windage, and through this windage a great deal of gas rushed out, and was wasted; but the bad effect did not stop there: rushing over the top of the bullet, as it rested on the bottom of the bore, it pressed it down hard—hard enough in guns of soft metal, as brass, after a few rounds to make a very perceptible dint—and forcing it along at the same time made it rebound first against one side and then the other of the bore, and hence the direction in which it left the bore was not the axis or central line of the cylinder, but varied according to the side it struck last. This was one cause of inaccuracy, and could, of course, be obviated to a great extent, though at the cost of difficulty in loading, by making the bullet fit tight; but another and more important cause of deflection was the various rotatory or spinning motions the bullet received from friction against the sides of the bore, and also from its often not being a homogeneous sphere; that is, the density of the metal not being the same throughout, the centre of gravity did not coincide with the centre of the sphere as it should have done.

Let us try to understand the effect of this rotation. A bullet in moving rapidly through the air, separates it; and if its velocity is at all greater than the velocity with which the air can refill the space from which it has been cleared behind it, it must create a more or less complete vacuum. Now when the barometer stands at thirty inches, air will rush into a vacuum at the rate of 1,344 feet per second; and if the bullet is moving at a greater velocity than this, there will be a total vacuum behind it. But it can be easily understood that even when moving with a less velocity, there will be a greater density of air before than behind. If the bullet be rotating on a vertical axis—that is, spinning like a top, point downwards, as in the diagram No. 1, from left to right, in the direction indicated by the crooked arrow, at the same time that

No. 1.

Looking down upon the spinning bullet.