that city, he was sent, in July, 1633, to the archiepiscopal palace of Sienna, the residence of the Archbishop Piccolimini, where he carried on and completed his valuable investigations respecting the resistance of solids. Here he continued five months, when, in consequence of the disappearance of the plague at Florence, he was allowed to retire to his villa at Bellosguardo, and afterward to that of Arcetri in the vicinity of Florence.

Though Galileo was now, to a certain degree, liberated from the power of man, yet the afflicting dispensations of Providence began to fall thickly around him. No sooner had he returned to Arcetri, than his favourite daughter, Maria, was seized with a dangerous illness, which soon terminated in her death. He was himself attacked with hernia, palpitation of the heart, loss of appetite, and the most oppressive melancholy; and though he solicited permission to repair to Florence for medical assistance, yet this deed of mercy was denied him. In 1638, however, the pope permitted him to pay a visit to Florence, and his friend, Father Castelli, was allowed to visit him in the company of an officer of the Inquisition. But this indulgence was soon withdrawn, and at the end of a few months he was remanded to Arcetri. The sight of his right eye had begun to fail in 1636, from an opacity of the cornea. In 1637 his left eye was attacked with the same complaint; so that in a few months he was affected with total and incurable blindness. Before this calamity had supervened, he had noticed the curious phenomenon of the moon's libration, in consequence of which, parts of her visible disk that are exposed to view at one time are withdrawn at another. succeeded in explaining two of the causes of this curious phenomenon, viz. the different distances of the observer from the line joining the centre of the earth and the moon, which produces the diurnal libration, and the unequal motion of the moon in her

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orbit, which produces the libration in longitude. It was left, however, to Hevelius to discover the libration in latitude, which arises from the inclination of her axis being a little less than a right angle to the ecliptic; and to Lagrange to discover the spheroidal libration, or that which arises from the action of the earth upon the lunar spheroid.

The sorrows with which Galileo was now beset, seemed to have disarmed the severity of the Inquisition. He was 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 visiter, and Gassendi, Deodati, 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 Astronomica 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, a Neapolitan philosopher, who published in 1666 a work on Jupiter's satellites.* In this work 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 primary planets in the same manner as the moon does round the earth, and that they all re volve 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.t

Our countryman Dr. Robert Hooke seems to have devoted much of his attention to the cause of the planetary motions. On the 21st March, 1666, he read to the Royal Society an account of a series of experiments for determining if bodies experience any variation in their weight 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 afterward, 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 dif

Theorica Medicearum planetarum ex causis physicis deducta, Flor 1666, 4to.

† M. Delambre maintains that these views of Borelli are only those of Kepler slightly modified. Newton and Huygens have attached to them a greater value. The last of these philosophers remarks," Refert Plutarchus, fuisse jam olim qui putaret ideo manere lunam in orbe suo, quod vis recedendi a terra, ob motum circularem, inhiberetur pari vi gravitatis, qua ad terram accedere conaretur. Idemque ævo nostro, non de Juna tantum sed et planetis ceteris statuit Alphonsus Borellus, ut nempe primariis eorum gravitas esset solem versus; lunis vero ad terram, Jovem ac Saturnum quos comitantur."-Huygen, Cosmotheor, lib. ii.; Opera, t. ii. p. 720.

ferent degrees of force; but though this experiment illustrated the production of a curvilineal motion, by 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 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 its motion, as in the same manner the corresponding attractive power of the earth hath 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 1

have 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, 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 not 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."

This passage, which has been considered as a remarkable one by the philosophers of every country, has, we think, been misapprehended by M. Delambre, when he asserts that every thing which it contains "is to be found expressly in Kepler."*

* Hist. de l'Astronomie aux Dix-huitieme Siècle, p.9.