The cause of these deflections we have seen is almost wholly rotation or _spin_. The object of the _rifle_ is to place this rotation under our control, and if the bullet must spin, to make it spin always in the same direction, and in the way which will suit our purpose best. With this object the interior of the cylindrical bore which we have been considering as smooth, is scored or indented with spiral grooves or furrows. As we are merely concerned with the principles, and not with the constructive details, we need only mention that the number of these grooves varies in different rifles from two to forty; that their shape and size, though dependent on certain conditions, is, we might almost say, a matter of fashion; and that Mr. Whitworth, in his almost perfect rifle, uses a hexagonal bore, and Mr. Lancaster makes a smooth oval-bored rifle; but that in all, the deviations from the circle of the interior cylinder do not pass straight from end to end of the barrel, but _spirally_, and constitute, in fact, a female screw. The bullet, fitting tight and entering the grooves, is constrained to rotate while being forced out of the barrel by the gunpowder, in the same manner that a screw is necessarily twisted while being drawn out of a hole or nut; and this rotation or spin being impressed upon it by the same force which projects it from the barrel, continues during the flight. This spin is different in direction from those we have been considering previously; it is like the spin of a top thrown point foremost, the axis of rotation coincident with the line of flight. While it remains in this position (coinciding with the line of flight) none of the deflecting effects of the air we have mentioned can come into operation, as the resistance is equal on all sides; and not only that, but if there are any irregularities on the surface of the ball, as they are brought rapidly first on one side and then on the other of the point or pole of rotation, they can have no effect in deflecting it to one side more than to the other. Hence the accuracy, or straight shooting, of our modern gun, the rifle.