That the choice of a profession was a wise one soon became evident. He was persuaded to study the great scientific writers of all ages—Archimedes, Huyghens, Slusius, Sir James Gregory and Sir Isaac Newton; and these, says his biographer, he readily understood without any assistance. His advance was extraordinarily rapid. When only fifteen years old he aimed so high as to confute the redoubtable John Bernouilli on the collision of bodies. His friends were already the leading mathematicians of the day, and there were many who took a strong interest in the brilliant and attractive lad. He certainly was gifted with qualities making for success; for, we are told, “besides his acquaintance with divers parts of learning, there was in him, to an ingenuous aspect, joined an activity of temper, together with a great facility in expressing his thoughts with clearness, brevity, strength, and elegance.”
Robins’ mind was of too practical a bent, however, to allow him to stay faithful to pure mathematics; his restless energy required another outlet. Hence he was led to consider those “mechanic arts” that depended on mathematical principles: bridge building, the construction of mills, the draining of fens and the making of harbours. After a while, taking up the controversial pen again, he wrote and published papers by which a great reputation gradually accrued. In 1735 he blew to pieces, with a Discourse on Sir Isaac Newton’s Method of Fluxions, a treatise written against the mathematicians by the Bishop of Cloyne. And shortly after followed further abstruse and controversial studies: on M. Euler’s Treatise on Motion, on Dr. Smith’s System of Optics, and on Dr. Jurin’s Distinct and Indistinct Vision.
His command of language now attracted the attention of certain influential gentlemen who, deploring the waste of such talent on mathematical subjects, persuaded their young acquaintance to try his hand at the writing of political pamphlets: party politics being at that time the absorbing occupation of the population of these islands. His success was great; his writings were much admired. And—significant of the country and the age—friendships and acquaintances were formed by the pamphleteer which were later to be of great value to the rising scientist.
This phase of his activities, fortunately, did not last long. Kindling the lamp of science once more, he now started on the quest which was to make him famous.
For thoughtful men of all ages, as we have already noted, the flight of bodies through air had had an absorbing interest. The subject was one of perennial disputation. The vagaries of projectiles, the laws governing the discharge of balls from cannon, could not fail to arouse the curiosity of an enthusiast like Robins, and he now set himself in earnest to discover them by an examination of existing data, by pure reason, and by actual experiment. Perusal of such books as had been written on the subject soon convinced him of the shallowness of existing theories. Of the English authors scarcely any two agreed with one another, and all of them carped at Tartaglia, the Italian scientist who in the classic book of the sixteenth century tried to uphold Galileo’s theory of parabolic motion as applied to military projectiles. But what struck Robins most forcibly about all their writings was the almost entire absence of trial and experiment by which to confirm their dogmatical assertions. This absence of any appeal to experiment was certainly not confined to treatises on gunnery; it was a conspicuous feature of most of the classical attempts to advance the knowledge of physical science. Yet the flight of projectiles was a problem which lent itself with ease to that inductive method of discovering its laws through a careful accumulation of facts. This work had not been done. Of all the native writers upon gunnery only four had ventured out of two dimensions; only four had troubled to measure definite ranges. All four asserted the general proposition that the motion of bodies was parabolic. Only one noticed that practice did not support this theory, and he, with misapplied ingenuity, called in aid the traditional hypothesis of a violent, a crooked, and a natural motion. Which wrong hypothesis enabled him, since he could choose for himself the point at which the straight motion ceased, to square all his results with his precious theory.
Leaving the books of the practitioners, Robins had more to learn from the great circle of mathematicians who in the first part of the eighteenth century lent a lustre to European science. The old hypotheses were fast being discarded by them. Newton, in his Principia, had investigated the laws of resistance of bodies to motion through the air under gravity, by dropping balls from the cupola of St. Paul’s Cathedral; and he believed that the trajectory of a cannon ball differed from the parabola by but a small extent. The problem was at this time under general discussion on the Continent; and led to a collision between the English and the German mathematicians, Newton and Leibnitz being the two protagonists.[81] But, whatever the merits or outcome of the controversy, one thing seems certain. None of the great men of the day understood the very great accession of resistance which a fast-travelling body encountered in cleaving the air, or realized the extent to which the trajectory was affected by this opposing force. It was in fact universally believed and stated, that “in the case of large shot of metal, whose weight many times surpasses that of air, and whose force is very great, the resistance of air is scarcely discernible, and as such may, in all computations concerning the ranges of great and weighty bombs, be very safely neglected.”[82]
In 1743 Robins’ New Principles of Gunnery was read before the Royal Society.
In a short but comprehensive paper which dealt with both internal and external ballistics, with the operation of the propellant in the gun and with the subsequent flight of the projectile, the author enunciated a series of propositions which, founded on known laws of physics and sustained by actual experiment, reduced to simple and calculable phenomena the mysteries and anomalies of the art of shooting with great guns. He showed the nature of the combustion of gunpowder, and how to measure the force of the elastic fluid derived from it. He showed, by a curve drawn with the gun axis as a base, the variation of pressure in the gun as the fluid expanded, and the work done on the ball thereby. Producing his ballistic pendulum he showed how, by firing a bullet of known weight into a pendulum of known weight, the velocity of impact could be directly ascertained. This was obviously a very important discovery. For an accurate measurement of the “muzzle velocity” of the bullet discharged from any given piece of ordnance was, and still is, the solution and key to many another problem in connection with it: for instance, the effect of such variable factors as the charge, the windage or the length of gun. In fact, as the author claimed, there followed from the theory thus set out a whole host of deductions of the greatest consequence to the world’s knowledge of gunnery. Then, following the projected bullet in its flight, he proceeded to tell of the continuous retardation to which it was subject owing to the air’s resistance. He found, he said, that this resistance was vastly greater than had been anticipated. It certainly was not a negligible quantity. The resistance of the air to a twenty-four pound cannon ball, fired with its battering charge of sixteen pounds of powder, was no less than twenty-four times the weight of the ball when it first issued from the piece: a force which sufficiently confuted the theory that the trajectory was a parabola, as it would have been if the shot were fired in vacuo. It was neither a parabola, nor nearly a parabola. In truth it was not a plane curve at all. For under the great force of the air’s resistance, added to that of gravity, a ball (he explained) has frequently a double curvature. Instead of travelling in one vertical plane it actually takes an incurvated line sometimes to right, sometimes to left, of the original plane of departure. And the cause of this departure he ascribed to a whirling motion acquired by the ball about an axis during its passage through the gun.
The reading of the paper provoked considerable discussion among the learned Fellows, who found themselves presented with a series of the most novel and unorthodox assertions, not in the form of speculations, but as exact solutions to problems which had been hitherto unsolved; and these were presented in the clearest language and were fortified by experiments so careful and so consistent in their results as to leave small room for doubt as to the certainty of the author’s theory. Of special interest both to savants and artillerists must have been his account of “a most extraordinary and astonishing increase in the resistance of the air which occurs when the velocity comes to be that of between eleven and twelve hundred feet in one second of time”: a velocity, as he observed, which is equal to that at which sounds are propagated in air. He suggested that perhaps the air, not making its vibrations with sufficient speed to return immediately to the space left in the rear of the ball, left a vacuum behind it which augmented the resistance to its flight. His statement on the deflection of balls, too, excited much comment. And, in order to convince his friends of the reality of this phenomenon, which, though Sir Isaac Newton had himself taken note of it in the case of tennis balls, had never been thoroughly investigated, Robins arranged an ocular demonstration.
One summer afternoon the experiments took place in a shady grove in the Charterhouse garden. Screens—“of finest tissue paper”—were set up at intervals of fifty feet, and a common musket bored for an ounce ball was firmly fixed in a vice so as to fire through the screens. By repeated discharges the various deflections from the original plane of departure were clearly shown; some of the balls whirled to the right, some to the left of the vertical plane in which the musket lay. But not only was the fact of this deflection established to the satisfaction of the visitors. A simple but dramatic proof was afforded them of the correctness of Robins’ surmise that the cause was the whirling of the ball in flight. A musket-barrel was bent so that its last three or four inches pointed to the left of the original plane of flight. The ball when fired would then be expected to be thrown to the left of the original plane. But, said Robins, since in passing through the bent part the ball would be forced to roll upon the right-hand side of the barrel; and as thereby the left side of the ball would turn up against the air, and would increase the resistance on that side; then, notwithstanding the bend of the piece to the left, the bullet itself might incurvate towards the right. “And this, upon trial, did most remarkably happen.”[83]