A Memoir of the Chevalier Louville, among those of the Academy of Sciences for 1720, is the first in that collection, and, I believe, the first, published in France, where the elliptic motion of the planets is supposed to be produced by the combination of two forces, one projectile and the other centripetal. Maupertuis soon after went mach farther; in his elegant and philosophic treatise, Figure des Astres, published about 1730, he not only admitted the existence of attraction as a fact, but even de fended it, when considered as- an universal property of body, against the reproach of triag a metaphysical absurdity. These were considerable advances, but they were made slowly ; and it was true, as Voltaire afterwards remarked, that though the author of the Principia survived the publication of that great work nearly forty years, he had not, at the time of his death, twenty followers out of England.
We should do wrong, however, to attribute this slow conversion of the philosophic world entirely to prejudice, inertness, or apathy. The evidence of the Newtonian philosophy was of a nature to require time in order to make an impression. It im plied an application of mathematical reasoning which was often difficult ; the doctrine of prime and ultimate ratios was new to most readers, and could be familiar only to those who had studied the infinitesimal analysis.
The principle of gravitation itself was considered as difficult to be admitted. When presented indeed as a mere fact, like the weight of bodies at the earth's surface, or their tendency to fall to the ground, it was free from objection ; and it was in this light only that Newton wished it to be considered' But though this appears to be the sound and philosophical view of the subject, there has always appeared a strong desire in those who speculated concerning gravitation to go farther, and to inquire into the cause of what, as a mere fact, they were sufficiently disposed to admit. If you said that you had no explanation to give, and was only desirous of having the fact ad mitted ; they alleged, that this was an unsatisfactory proceeding,—that it was admitting the doctrine of occult it amounted to the assertion, that bodies acted in places where they were not,—a proposition that, metaphysically considered, was un doubtedly absurd. The desire to explain gravitation is indeed so natural, that Newton himself felt its force, and has thrown out, at the end of his Optics, some curious conjec tures concerning this general affection of body, and the nature of that elastic ether to which he thought that it was perhaps to be ascribed. " Is not this medium (the ether) much rarer within the dense bodies of the sun, stars, and planets, than in the empty celestial spaces between them ? And, in passing from them to great distances, does it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies to one another, every body endeavouring to go from the denser' parts of the me dium to the rarer ?" 1 Notwithstanding the highest respect for the author of these conjectures, I cannot find any thing like a satisfactory explanation of gravity in the existence of this elastic ether. It is very true that an elastic fluid, of which the density followed the inverse ratio of the distance from a given point, would urge the bodies immersed in it, and impervious to it, toward that point with forces inversely as the squares of the dis tances from it ; but what could maintain an elastic fluid in this condition, or with its density varying according to this law, is a thing as inexplicable as the gravity which it was meant to explain. The nature of an elastic fluid must be, in the absence of all inequality of pressure, to become every where of the same density. If the causes that produce so marked and so general a deviation from this rule be not assigned, we can only be said to have substituted one difficulty for another.
A different view of the matter was taken by some of the disciples and friends of Newton, but which certainly did not lead to any thing more satisfactory. This phi losopher himself had always expressed his decided opinion' that gravity could not be considered as a property of matter; but Mr Cotes, in the preface to the second edition of the Principia, maintains, that gravity is a property which we have the same right to ascribe to matter, that we have to ascribe to it extension, impenetrability, or any other property. This is said to have been inserted without the knowledge of
Newton,—a freedom which it is difficult to conceive that any man could use with the author of the Principia. However that be, it is certain that these difficulties have been always felt, and had their share in retarding the progress of the philosophy to which they seemed to be inseparably attached.
There were other arguments of a less abstruse nature, and moie immediately con nected with experiment, which, for a time, resisted the progress of the Newtonian phi losophy, though they contributed, in the end, very materially to its advancement. Nothing, indeed, is so hostile to the interests of truth, as facts inaccurately observed ; of which we have a remarkable example in the measurement of an arch of the,meri dian across France, from Amiens to Perpignan, though so large as to comprehend about seven degrees, and though executed by Cassini, one of the first astronomers in Europe. According to that measurement, the degrees seemed to diminish on going from south to north, each being less by about an 800th part than that which immedi ately preceded it toward the south. From this result, which is entirely erroneous, the conclusion first deduced was correct, the error in the reasoning, by a very singu lar coincidence, having corrected the error in the data from which it was deduced. Fontenelle argued that, as the degrees diminished in length, on going toward the poles, the meridian must be less than the circumference of the equator, and the earth, of course, swelled out in the plane of that circle, agreeably to the facts that had been ob served concerning the retardation of the pendulum when carried to the south. This, however, was the direct contrary of the conclusion which ought to have been drawn, as was soon perceived by Cassini and by Fontenelle himself. The degrees growing less as they approached the pole, was an indication of the curvature growing greater,' or of the longer axis of the meridian being the line that passed through the poles, and that coincided with the axis of the earth. The figure of the earth must, therefore, be that of an oblong spheroid, or one farmed by the revolution of an ellipsis about its longer axis. This conclusion seemed to be strengthened by the prolongation of the meridian from Amiens northward to Dunkirk in 1713, as the same diminution was observed ; the medium length of the degree between Paris and Dunkirk being 56970 toises, no less than 137 less than the mean of the degrees toward the south.' All this seemed quite inconsistent with the observations on the pendulum, as well as with the conclusions which Newton had deduced from the theory of gravity. The Academy of sciences was thus greatly perplexed, and uncertain to what side to incline. In these circumstances, J. Cassini, whose errors were the cause of all the difficulty, had the merit of suggesting the only means by which the question concerning the figure of the earth was 'likely to receive a satisfactory solution,—the measurement of two degrees, the one under the equator, and the other as near to the pole as the nature of the thing would admit. But it was not till considerably beyond the limits of the pe riod of which I am now treating, that these measures were executed; and that the in crease of the degrees toward the poles, or the oblateness of the earth's figure, was completely ascertained. Cassini, on resuming his own operations, discovered, and candidly acknowledged, the errors in his first measurement ; and thus the objections which had arisen in this quarter against the theory of gravity became irresistible arguments in its favour. This subject will occupy much of our attention in the history of the second period, till which, the establishment of the Newtonian philosophy on the Continent cannot be said to have been accomplished.