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 it is moving forward (sideways it would be in the top) as indicated by the straight arrow,—it is evident that the left half rotates _with_ the general motion of translation of the bullet, and the right half backwards _against_ this motion, and therefore that on the left side it is moving quicker relatively to the air through which it is passing than on the right side. And its rough surface preventing the air escaping round it on that side, while it, as it were, assists it on the other side, the air becomes denser where shown by the dark lines, and tends to deflect the bullet in the other direction, that is, in the direction in which the anterior or front surface is moving.[16] [Illustration: No. 2. Looking at the bullet sideways.] If the bullet rotate on a horizontal axis at right angles to the direction of its motion of translation (that is, like a top thrown spinning with its point sideways, when it would strike the object thrown at with its side), shown in the diagram No. 2; if the anterior portion be moving, as shown by the arrow, from above downwards, it is evident, for the same reasons, that the air will become denser, as shown, and assist the action of gravity in bringing the ball to the ground—that is, decrease the range. A spherical bullet resting on the bottom of the bore of a gun would always have a greater tendency to rotate in this manner than in a contrary direction; for the friction against the bore would be augmented by the weight of the ball in striking against the bottom, and diminished by it when striking against the top.