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.
Shot were constructed in 1851 to try the effect of rotation in the above-mentioned and in the opposite directions. They were made excentric, that is, lop-sided, by taking out a portion of the metal on one side, and replacing it either with a heavier or lighter body. The manner in which they would rotate was, therefore, known; for, not to use too scientific language, the light side moved first, and according to the relative positions of the heavy and light side when placed against the charge so the rotation took place. Thus, when the light side was resting against the bore of the gun, the rotation was exactly contrary to the direction shown in diagram No. 2; and a range of 5,566 yards was obtained from a 10-inch gun, being 916 yards farther than with a concentric shot from the same gun. The deflections to the right and left were proportionately large, according as the light side was placed to the left or right.
We need not specify further; this will be sufficient to show the reason why the smooth bore with a spherical bullet never made a straight long shot, for it was not only that the bullet did not go in the direction in which it was aimed, but it did not even follow the direction in which it started. This was well shown by Mr. Robins in the experiment we commenced with. He bent the end of a gun barrel to the left, and aimed by the straight part. As would be naturally expected, the shot passed through the first tissue-paper screen 1½ inches to the left of the track of a bullet, which had been previously fired from a straight barrel in the same line with which the crooked barrel had been aimed, and 3 inches to the left on the second screen; but as he had predicted, and as the company could hardly have expected, on the wall which was behind, the bullet struck 14 inches to the right of the track, showing that though it had gone at first as directed by the bent portion of the barrel, yet as the bullet in being turned had rolled against the right-hand side of this portion of the barrel, it had a rotatory motion impressed upon it, by which the anterior portion moved from left to right, and the bullet, after moving away from, turned back and crossed the track of the other bullet again, or was incurvated to the right.
We now see why spherical bullets from a smooth bore, though they may fly almost perfectly accurately a short distance, cannot be depended on in the least for a long distance, as the bullet which might strike within 1 inch at 100 yards would not strike within 2 inches at 200 yards, and still less within 3 inches at 300 yards of the mark at which it was fired.
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.
We have before mentioned that Robins pointed out the enormous effect of the resistance of the atmosphere to the passage of a shot; and “because,” as he says, “I am fully satisfied that the resistance of the air is almost the only source of the numerous difficulties which have hitherto embarrassed that science,” viz. gunnery, he considered it above all things necessary to determine its amount; for which purpose he invented the Ballistic Pendulum and Whirling Machine. His experiments were made principally with small bullets; but a more extended series of experiments was made by Dr. Hutton with the same machines, and on the Continent and in America by Major Mordecai, with a ballistic pendulum of improved construction. It appears from these that when a ball of two inches diameter is moving with a great velocity, it meets with a resistance of which the following examples will give an idea: at a velocity of 1,800 feet per second the resistance is 85½ lbs., and at a velocity of 2,000 feet, 102 lbs. If we wish to increase the range, then, we must overcome this resistance in some way. As the resistance is nearly proportionate to the surface, that is, twice as great on a surface of two square inches as on a surface of one square inch, we must do so by increasing the weight of the shot. For it is evident that if two shot of different weights start with the same velocity, and meet with the same resistance, the heavier one, having the greater momentum, will maintain its velocity the longest. Throw a cork and a stone of the same size with the same force—the cork will only go a few yards, while the stone will go perhaps ten times as far. In the smooth-bored cannon this could only be effected partially by increasing the size of the shot, when the surface exposed to the resistance of the air increased only as the square of the diameter, while the weight increased in a greater ratio, as the cube of the diameter. Hence the longer range and greater penetration of heavy guns. As, however, with a rotating body the tendency is always for the axis of rotation to remain parallel to its original direction—thus a top while spinning may move about the floor, but remains upright on its point, and does not fall till the spin is exhausted—we have with rifles a means by which we can keep a bullet always in the same direction. In order to comply with the condition, then, of exposing a small surface to the resistance of the air while the bullet’s weight is increased, we reject the spherical form, and make it a long cylinder; and to make it the more easily cut through the air, we terminate it with a conical point.
Thus compare Mr. Whitworth’s 3-pounder with the ordinary or old 3-pounder; the shot weigh the same, but the diameter of Mr. Whitworth’s 3-pounder shot is 1·5 or 1½ inches, while the diameter of the old 3-pounder shot is 2·91 inches, or nearly three inches; and the surfaces they expose to the resistance of the air are 2·25, or 2¼ square inches, and 8·47 nearly, or nearly 8½ square inches; that is, Mr. Whitworth’s bullet, with the same weight to overcome it, meets with a resistance of a little more than a quarter that which the old bullet met with, and has the advantage of a sharp point to boot. Hence the enormous range attained,—9,688 yards.
The very same causes which make the fire of a rifle accurate, tend also to make it inaccurate, paradoxical as it may seem; but this inaccuracy being to a certain extent regular and known beforehand, is not of so much consequence, though it is a decided disadvantage. It may—not to be too mathematical—be explained thus:—The axis of rotation having, as we said, a tendency always to remain parallel to its original direction, when a rifle bullet or picket (the long projectile we have described) is fired at a high angle of elevation—that is, slanting upwards into the air, in order that before it fells it may reach a distant object,—it is evident from the diagram, that if the direction of the axis of rotation remains, as shown by the lines p p p, which represent the shot at different portions of the range parallel to the original direction in the gun, the bullet or picket will not always remain with its point only presented in the direction in which it is moving, but one side of the bullet will be partially opposed to the resistance of the air. The air on that side (in front) will be denser than behind, and the disturbing or deflecting influences before described will come into operation, the two opposite tendencies described in the text and the note to a certain extent counteracting one another. While at the same time the resistance of the air has a tendency to turn the bullet from the sideways position in which it is moving with respect to the line of flight (and the effect of this is the greater the less spin the bullet has to constrain it to keep its original direction), the result of which force, conspiring with the force described in the note, is to give it a slight angular rotation round another axis, and deflect the bullet by constantly changing its general direction (this second axis of rotation) to the side to which the rifling turns. This was exemplified in the late practice with Mr. Whitworth’s gun. When firing at the very long range of 9,000 yards the 3-pounder threw constantly to the right from 32 to 89 yards.
No. 3.