freely permitted to enjoy the society of his friends, who now thronged around him to express their respect and their sympathy. The Grand Duke of Tuscany was his frequent visitor, and Gassendi, Diodati, and our countryman Milton, went to Italy for the purpose of visiting him. He entertained his friends with the warmest hospitality; and though simple and abstemious in his diet, yet he was fond of good wine, and seems even in his last days to have paid particular attention to the excellence of his cellar.

Although Galileo had nearly lost his hearing as well as his sight, yet his intellectual faculties were unimpaired; and while his mind was occupied in considering the force of percussion, he was seized with fever and palpitation of the heart, which, after two months' illness, terminated his life on the 8th of January, 1642.

Among the predecessors of Newton in astronomical research, we must not omit the names of Bouillaud, (Bullialdus,) Borelli, and Dr. Hooke. Ismael Bouillaud, a native of Laon in France, and the author of several valuable astronomical works, has derived more reputation from a single sentence in his Astronomiea Philolaica, published in 1645, than from all the rest of his labours. He was not a believer in the doctrine of attraction, which, as we have already seen, had been broached by Copernicus, and discovered by Kepler; but in speaking of that power as the cause of the planetary motions, he remarks, "that if attraction existed, it would decrease as the square of the distance." The influence of gravity was still more distinctly developed by Borelli,* an Italian philosopher, who published at Florence, in 1666, a work on Jupiter's satellites, in which he maintains, that all the planets perform their motions round the sun according to a general law; that the satellites of Jupiter and of Saturn move round their

* Born at Naples in 1608: died at Bome In 1679. Was Professor of Mathematics at Mossiua, and ailerward* at Pisa.—Editor.

primary planets in the same manner as the moon does round the earth ; and that they all revolve round the sun, which is the only source of any virtue, and that this virtue attaches them and unites them so that they cannot recede from their centre of action.*

Our countryman, Dr. Robert Hooke, seems to have devoted much of his attention to the cause of the planetary motions. On the 21st of March, 1666, he read to the Royal Society an account of a series of experiments for determining whether bodies experience any variation in thenweight at different distances from the centre of the earth. His experiments, as Hooke himself saw, were by no means satisfactory, and hence he was led to the ingenious idea of measuring the force of gravity by observing, at different altitudes, the rate of a pendulum clock. About two months afterwards, he exhibited to the Society an approximate representation of the forces which retain the planets in their orbits, in the paths described by a circular pendulum impelled with different degrees of force; but though this experiment illustrated the production of a curvilineal motion, hy combining a tangential force with a central power of attraction, yet it was only an illustration, and could not lead to the true cause of the planetary motions. At a later period, however, viz., in 1674, Hooke resumed the subject in a dissertation entitled, "An attempt to prove the Motion of the Earth from Observation," which

• M. Delambre maintains that thoso viows of Borelli are only those of Kepler, slightly modified. Newton and Huygons have attached to them a greater value. The last of these philosophers remarks, "Befert Plutarchus, fuisso jam olim qui pularet idoo manoro lunam in orbe suo, quod vis recedondi a terra, ob inotiim circularom, iuhiberotur pari vi gravitatts qua ad terram accodero conarotnr. Idemquo rovo nostra, non de lun& tantum sed et planetis ceteris statuit Alphonsus Tiorollus, ut nempe primariis eorum gravitas esset solem versus, lunis vero ad terram, Jovem 00 Saturnum quos comitautr.r."—Huygena, Cosmothoor., lib. iL, Opera, L ii, p. 720.

contains the following remarkable observations upon gravity.

"I shall hereafter explain a system of the world, differing in many particulars from any yet known, answering in all things to the common rules of mechanical motions. This depends upon three suppositions:—first, that all celestial bodies whatsoever have an attraction or gravitating power towards their own centres, whereby they attract, not only their own parts, and keep them from flying from them, as we may observe the earth to do, but that they also do attract all the other celestial bodies that are within the sphere of their activity, and consequently, that not only the sun and moon have an influence upon the body and motion of the earth, and the earth upon them, but that Mercury, Venus, Mars, Jupiter, and Saturn also, by their attractive powers, have a considerable influence upon every one of their motions also. The second supposition is this, that all bodies whatsoever, that are put into a direct and simple motion, will so continue to move forward in a straight line, till they are, by some other effectual powers, deflected, and sent into a motion describing a circle, ellipsis, or some other more compounded curve line. The third supposition is, that those attractive powers are so much the more powerful in operating by how much the nearer the body wrought upon is to their own centres. Now, what these several degrees are I luive not yet experimentally verified; but it is a notion which, if fully prosecuted, as it ought to be, will mightily assist the astronomers to reduce all the celestial motions to a certain rule, which I doubt will never be done without it. He that understands the nature of the circular pendulum and circular motion, will easily understand the whole of this principle, and will know where to find directions in nature for the true stating thereof. This I only hint at present to such as have ability and opportunity of prosecuting this inquiry,

SPECULATIONS OF DR. HOOKE.

131

and are not wanting of industry for observing and calculating, wishing heartily such may be found, having myself many other things in hand, which I would first complete, and therefore cannot so well attend it. But this I do promise the undertaker, that he will find all the great motions of the world to be influenced by this principle, and that the true understanding thereof will be the true perfection of astronomy."

In this remarkable passage, the doctrine of universal gravitation and the general law of the planetary motions are clearly laid down. Delambre, we think, scarcely does justice to Hooke, when he says that what it contains " is to be found expressly in Kepler."* Kepler did indeed mention as probable the law of the squares of the distances, but he afterwards, as Delambre admits, rejected it for that of the simple distances. Hooke, on tbe contrary, announced it as a truth; and in a letter which he addressed to Newton in 1679, relative to the curve described by a projectile influenced by the earth's daily motion, he asserted that if the force of gravity decreased as the square of the distance, the curve described by a projectile would be an ellipse, whose focus was the centre of the earth. But however great be the merit which we may assign to Hookc's experimental results and sagacious views, they cannot be regarded either as anticipating the discoveries of Newton, or diminishing his fame. Newton had made the same discoveries by independent researches; and there is no reason to believe that he derived any of his ideas from his contemporaries.

• llietoirt de VAslrtmomie du dix-kuitiime Stick, p. 8.

CHAPTER XL

The firtt Idea of Gravity occurs to Newton in 1666—Hit first Speculations upon it—Interrupted by his Optical Experiments—He resumes the Subject in consequence of a Discussion with Dr. Hooke—He discovers the true Law of Gravity and the Cause of the Planetary Motions—Dr. Halley urges him to publish his Principia—His Principles of Natural Philosophy—Proceedings of the Royal Society on this Subject—The Principia appears in 1687—General Account of it, and of the Discoveries it contains—Tfiey meet with great Opposition, owing to the Prevalence of the Cartesian System—Account of the Reception and Progress of the Newtonian Philosophy in Foreign Countries—Account of its Progress and Establishment in England.

Such is a brief sketch of the labours and lives of those illustrious men who prepared the science of astronomy for the application of Newton's genius. Copernicus had determined the arrangement and general movements of the planetary bodies: Kepler had proved* tbat they moved in elliptical orbits; that their radii vectores described arcs proportional to the times; and that the squares of their periodic times are as the cubes of their mean distances from the sun. Galileo had added to the universe a whole system of secondary planets; and several astronomers had

* Strictly speaking, Kepler had nbown (and great had Ixsen the thought aud labour employed in doing Bo) that those laws were true of the motions of Mars. and that the third resulted from a comparison of the orbits of the Earth and Mars. But by analogy " Kepler's laws " wore extended to the other planet*—Editor.