Let us endeavour, first, to understand something of the movement of gun-shots in their simplest form. A gun-barrel, consisting of a bar of metal thicker at one end (where it has to withstand the first shock of the gunpowder) than at the other, is bored out throughout its length into a smooth hollow cylinder; this cylinder is closed at one end by the breech, which has a small opening in it, through which the charge is ignited. A charge of powder is placed in the closed end, and on the top of this the ball, say, a spherical one, such as our ancestors in their simplicity considered the best. The powder being ignited, rapidly, though not instantaneously, becomes converted into gas, and the permanent gases generated will, at the temperature estimated to be produced by the combustion (3,000° Fahr.), occupy a volume under the pressure of the atmosphere alone of over 2,000 times that of the bulk of the powder. This point, as well as the elasticity of the gases, both of the permanent ones and of the vapour of water or steam from the moisture in the powder, has never been accurately determined,[15] and various estimates have been formed; but if we take Dr. Hutton’s—a rather low one, viz.—that the first force of fired gunpowder was equal to 2,000 atmospheres (30,000 lbs. on the square inch), and that, as Mr. Robins computed, the velocity of expansion was about 7,000 feet per second, we shall have some idea of the enormous force which is exerted in the direction of the bullet to move it, of the breech of the gun to make it kick, and of the sides of the barrel to burst it. Notwithstanding Mr. Robins’ advice, we certainly never, till very lately, made the most of the power of committing homicide supplied by this powerful agent; but we used it in the most wasteful and vicious manner. All improvements—and many were suggested at different times to remedy defects, which he principally pointed out, like the inventions of printing and of gunpowder itself—lay fallow for long before they were taken up. They were premature. If our fathers had killed men clumsily, why should we not do the same? No one cared much, except the professionals, whether it required 100 or 1,000 bullets, on an average, to kill a man at 100 yards’ distance. Now we take more interest in such amusements; every one’s attention is turned to the best means of thinning his fellow-creatures; and we are not at all content with the glorious uncertainty which formerly prevailed when every bullet found its own billet: we like to kill our particular man, not his next neighbour, or one thirty yards off.
In order to see why we are so much more certain with our Whitworth, or Enfield, or Armstrong, of hitting the man we aim at, let us first examine how a bullet flies; and then by understanding how (badly) our fathers applied the force we have described to make it fly, we shall be able to appreciate how well we do it ourselves.
In consequence of the sudden generation of this enormous quantity of gas, then, in the confined space of the barrel, the bullet is projected into 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.
No. 1.
Looking down upon the spinning bullet.
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]
No. 2.
