The How and Why of
Cornhill Magazine, April 1860
On a windy, unpleasant day in 1746, a great mathematician and philosopher was exhibiting to a select company in the gardens of the Charter-house his skill in shooting round a corner with a bent gun-barrel. If he had requested the editor of the Cornhill Magazine of the day to publish his experiments, it is probable that he would have been refused. Now, when every morning paper informs us at breakfast, in its best type, of how far off we may be killed, and the evening papers analyze the same with the commencement of a hot debate on the French Treaty, to give us a pleasing subject for our dreams, we think that perhaps our unprofessional readers may like to know the how and the why of these far-reaching organs of peace on earth and good-will among faithful allies.
Supposing, then, reader - for it is to such that this article is addressed - that you are wholly ignorant of the science of gunnery, and of its principal establisher, Benjamin Robins, and have, therefore, been laughing at him, the poor silly philosopher, - if you will read the following extract from his work on Gunnery, you will see that if he did a foolish thing, he certainly sometimes wrote a wise one:- "I shall, therefore, close this paper with predicting that whatever State shall thoroughly comprehend the nature and advantages of rifled-barrel pieces, and, having facilitated and completed their construction, shall introduce into their armies their general use, with a dexterity in the management of them, they will by this means acquire a superiority which will almost equal anything that has been done at any time by the particular excellence of any one kind of arms; and will, perhaps, fall but little short of the wonderful effects which histories relate to have been formerly produced by the first inventors of fire-arms."
Now to our distinguished countryman, Mr. Benjamin Robins, is due the credit of having first pointed out the reasons why smooth bores - and smooth bore is now almost as great a term of reproach with us rifle volunteers as dog is with a Turk - were constantly, in fact, universally, in the habit "of shooting round corners, and the experiment mentioned was only a means of bringing the fact more strikingly before the obtuse faculties of the Royal Society, whom we may imagine to have been intense admirers of brown-bess - also now a term of reproach in constant use. Mr. Robins did more; he pointed out the advantage of elongated rifle bullets; showed us how to determine - and partially, as far as his limited means permitted, himself determined - the enormous resistance of the atmosphere to the motion of projectiles; in fact, smoothed the way for all our present discoveries; and, treason though it be to say so, left the science of gunnery much as we have it now. Though principally from increased mechanical powers of construction, better material and improved machinery, we have advanced considerably in the Art or practice of destruction.
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 deg. 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 (1), 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 first. 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.
1. It is not at all certain whether Marriott's law of the elasticity being as the density is true, when the gases are so highly condensed.