Memoirs of the Life, Writings, and Discoveries of

2 Vols., London, 1855, volume 1: 282-285

Among the astronomers who preceded Newton in astronomical inquiries, and contributed some ideas to establishment of the true system of the planets, we must place the names of Bouillaud,1 Borelli, Hooke, Huygens, Wren. and Halley. After refuting the magnetic notions of Kepler, Bouillaud maintained that the force of attraction must vary reciprocally as the square, and not as Kepler asserted, in the simple ratio of the distance; but Delambre does not allow him any credit in this respect, and remarks that he has done nothing more for astronomy than to introduce the word evection into its language

The influence of gravity as a central force in the planetary motions has been very distinctly described by Borelli, Professor of Mathematics at Pisa, in his work on the theory of Jupiter's Satellites.2 He considers the motions of the planets round the sun, and of the satellites round their primaries, as produced by some virtue residing in the central body. In speaking of the motion of bodies in circular orbits, he compares the tendency of the body to recede from the centre of motion to that of a stone whirled in a sling. When this force of recession is equal to the tendency of the body to the centre, a balance is effected between these tendencies, and the body will continually revolve round the centre, and at a determinate distance from it. Delambre attaches no value to these speculations of Borelli. He has in his opinion pointed out no physical cause,3 and has merely made a series of reflexions which every astronomer would necessarily make who was studying the theory of the satellites. He gives him the credit, however, of being one of the first who conjectured that the comets described round the sun elliptical or parabolic orbits.4

The speculations of our distinguished countryman, Dr. Hooke, respecting the cause of the planetary motions, exceeded greatly in originality and value the crude views of Borelli, and form a decided step in physical astronomy. On the 21st of March 1666, he communicated to the Royal Society an account of a series of experiments to determine if bodies experienced any change in their weight at different distances from the surface of the earth "either upwards or downwards." Kepler had maintained that this force, namely, that of gravity, was a property inherent in all celestial bodies, and Hooke proposed "to consider whether this gravitating or attractive power be inherent in the parts of the earth; and if so, whether it be magnetical, electrical, or of some other nature distinct from either." The experiments which he made with the instrument described in this communication, were far from being satisfactory, and he was therefore led to the ingenious idea of measuring the force of gravity "by the motion of a swing clock," which would go slower at the top of a hill than at the bottom.5

About two months afterwards, namely, on the 23d May 1666, Hooke communicated to the Society a paper "On the inflexion of a direct motion into a curve by a supervening attractive principle."5 After maintaining that the celestial bodies moving in circular and elliptical orbits "must have some other cause beside the first impressed impulse to bend their motion into these curves," he considers the only two causes which appear to him capable of producing such an effect. The first of these causes, which he considers an improbable one, is that the tendency to a centre is produced by a greater density of the ether in approaching to the sun. "But the second cause," he adds, "of inflecting a direct motion into a curve may be from an attractive property of the body placed in the centre, whereby it continually endeavours to attract or draw it to itself. For if such a principle be supposed, all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions; and possibly the prosecuting this speculation, may give us a true hypothesis of their motion, and from some few observations their motions may be so far brought to a certainty that we may be able to calculate them to the greatest exactness and certainty that can be desired." After describing the circular pendulum6 for illustrating these views, he adds that "by this hypothesis the phenomena of the comets, as well as of the planets, may be solved; and the motions of the secondary as well as of the primary planets. The motions also of the progression of the apsides are very evident, but as for the motion of libration or latitude that cannot be so well made out by this way of pendulum; but by the motion of a wheel upon a point is most easy."

1. Ismaelis Bullialdi Astronomia Philolaica. - Paris, 1645, p. 23. Sir Isaac Newton admitted that Bullialdus here gives the true "proportion on gravity." - Letter to Halley, June 20, 1686, postscript.

2. Theoricae Mediaeorum Planetarum ex causis physicis deductae. A. Alphonse Borellio. - Florentiae, 1666.

3. Newton (in his posthumous work, De Systemate Mundi, 2, Opera, tom. iii. p. 180, and in his postscript in his letter to Halley, June 20, 1686, where he says "that Borelli did something") and Huygens have attached greater value to the views of Borelli. The last of these philosophers thus speaks of them:- "Refert Plutarchus in libro supramemorato de Facie in Orbe Lunae, fuisse jam olim qui putaret ideo manere lunam in orbe suo, quad vis recedeudi a terra, ob motum circularem, inhiberetur pari vi gravitatis, qua ad terram accedere conaretur. Idemque aevo nostro, non de luna tantum sed et planetis ceteris statuit Alphonsus Borellius, ut nempe primariis eorum gravitas esset solem versus; lunis vero ad Terram Jovem et Saturnum quos comitantur Multoque diligentius, subtiliusque idem nuper explicuit Isaacus Newtonus, et quamodo ex his causis nascantur Planetarum orbes Elliptici, quos Keplerus excogitaverat; in quorum foco altero Sol ponitur. Christiani Hugenii Cosmotheoros, finem. Opera, tom. ii. p. 720.

4. Angelo Fabroni, Lettere inedite d'uomini illustri, tom. i. p. 173.

5. Birch's Hist. of Royal Society, vol. ii. pp. 69-72.

6. Ibid., vol. ii. pp. 90-92.

7. This pendulum consisted of a wire fastened to the roof of the room, with a large wooden ball of lignum vitae at the end of it. - Waller's Life of Hooke, p. xii.