Of hazard of one Christian. form some idea of the novelty this it is said that (on a wager) and profundity of the discoveries he gave proof upon a large plain which it contained, on learning in Scotland, to the destruction that, when it was first published, of a great many head of cattle not more than two or three and flocks of sheep, whereof among Newton's contemporaries some were distant from others were capable of understanding half a mile. To continue the it. Huygens himself, a man thread of my story as I have it, whose mind was peculiarly suited I must not forget that, when he to appreciate its merit, only in was most earnestly desired by part adopted the idea of gravian old acquaintance and pro- tation, and that merely as refessed friend of his, even about garded the heavenly bodies, the time of his contracting the while he rejected its influence malady whereof he died, that between the separate particles he would be pleased, for the of matter, being preoccupied honour of his family, and his by the hypothetical ideas he own everlasting memory to pos- had formed respecting the cause terity, to reveal unto him the of gravity. Leibnitz, perhaps manner of the contrivance of so through rivalry, or perhaps by a ingenious a mystery, subjoin- prepossession in favour of his ing thereto, for the better per- own metaphysical system, comsuading him, that it were a pletely mistook the beauty and thousand pities that so excellent the certainty of the method eman invention should be buried ployed by Newton in this work, with him in the grave, and that and even went so far as to pubafter his decease nothing should❘lish a dissertation, in which he be known thereof, his answer endeavoured to demonstrate was, "That for the ruin and overthrow of man there were too many devices already framed, which if he could make to be fewer, he would with all his might endeavour to do; and that therefore, seeing the malice and rancour rooted in the heart of mankind will not suffer them to diminish the number of them, by any new conceit of his they should never be increased."' SIR ISAAC NEWTON. Newton's Principia appeared complete in 1687. We may the same truths on different principles. Even many years after the publication of the Principia, several most profound mathematicians (John Bernoulli, for instance) opposed it; and Fontenelle, though in advance of his age on most subjects of philosophy, expressed somewhat more than doubts concerning the law of attraction, and persisted during his whole life in upholding the vortices of Descartes. In fact, more than fifty years elapsed before the great physical truth contained and demonstrated in the Prin cipia was, we do not say followed up and developed, but even understood by the generality of learned men. Whatever difficulty, however, the just appreciation of such a work may present, we can give here a brief account of it with entire confidence, by translating the words of that illustrious man whose genius has so much contributed to Newton's glory, in having by his own discoveries subjected all the movements of the celestial bodies to the law of universal gravitation. After having exhibited him as setting out from the laws of Kepler, to discover the nature and the law of the force that governs the motions of the planets and the satellites in their orbits, and afterwards generalizing this idea according to the phenomena that presented themselves until he had ascended to the certain and mathematical knowledge of universal gravitation, 'Newton,' says Laplace, 'having arrived at this point, saw all the great phenomena of the universe flow from the principle he had discovered. By considering gravity at the surface of the heavenly bodies as the result of the attractions of all their particles, he discovered this remarkable and characteristical property of a law of attraction reciprocal to the square of the distance, namely, that two spheres formed of concentric layers, and with densities varying according to any law whatever, attract each other mutually, as it their masses were united at their centres. Thus the bodies of the solar system act upon each other, and upon the bodies placed at their surfaces, very nearly as if they were so many centres of attraction,-a result which contributes to the regularity of their movements, and which made this illustrious mathematician recognise the gravity of the earth in the force that retains the moon in her orbit. He proved that the earth's movement in rotation must have flattened it at the poles; and he determined the laws of gravitation in the degrees of the meridian, and in the force of gravity at the earth's surface. He saw that the attractions of the sun and moon excite and maintain in the ocean those oscillations which are there observed under the name of tides. He recognised several inequalities in the moon's motion, and the retrograde motion of her nodes to be owing to the action of the sun. Afterwards, considering the excess of matter in the terrestrial spheroid at the equator as a system of satellites adhering to its surface, he found that the combined actions of the sun and the moon tend to cause a retrogradation in the nodes of the circles they describe round the axis of the earth, and that the sum of these tendencies being communicated to the whole mass of the planet, ought to produce in the intersection of its equator with the ecliptic that slow retrogradation known by the name of the successors of this great mathematician. The imperfection of the infinitesimal calculus when first discovered did not allow him completely to resolve the difficult problems which the theory of the universe offers; and he was oftentimes forced to give mere hints, which were always uncertain till confirmed by rigorous analysis. Notwithstanding these unavoidable defects, the importance and the generality of his discoveries respecting the system of the ing points of natural philosophy, the great number of profound and original views which have been the origin of the most brilliant discoveries of the mathematicians of the last century, which are all presented with much elegance, will ensure to the Principia a lasting preeminence over all other productions of the human mind.' of the precession of the equinoxes. The true cause of this great phenomenon could not have even been suspected before the time of Newton, since he was the first who made known the two leading facts on which it depends. Kepler himself, urged by an active imagination to explain everything by hypothesis, was constrained to avow in this instance the failure of his efforts. But with the exception of the theory of the elliptical motions of the planets and comets, the attrac-universe, and the most interesttion of spheres, the ratio of the masses of the planets accompanied by the satellites to that of the sun, all the other discoveries respecting the motions and figures of the heavenly bodies were left by him in an incomplete state. His theory of the figures of the planets is limited by supposing them to be homogeneous. His solution of the problem of the precession of the equinoxes, though very ingenious, and notwithstanding the apparent agreement of its result with observations, is defective in many particulars. Among the numerous perturbations in the motions of the heavenly bodies, he has only considered those of the moon, the greatest of which, viz. evection, has wholly escaped his researches. Newton has well established the existence of the principle he had the merit of discovering; but the development of its consequences, and advantages, has been the work The simple falling of a stone to the ground, has been found to involve principles which are the basis of all we know in mechanical philosophy. Without accurate experiments on the descent of bodies at the surface of the earth, the objections against the earth's motion could not have been answered; the inertia of body would have remained unknown; and the nature of the force which retains the planets in their orbits could never have been investigated. In 1665, when the tremendous visitation of the plague raged in London, and threatened Cam |