20

HISTORY OF PHYSICAL ASTRONOMY.

indulge its adherents with the miserable delusion that it revealed to themthe whole secret of the mechanism of the universe.

Borelli, in his theory of the Medicean stars, published in 1666, appearsto have speculated more judiciously on the physical theory of the planetsthan any of his predecessors. He remarks that the motions of the planetsround the sun, and those of the satellites round their respective primaries,must doubtless depend in each case on some virtue residing in the centralbody. lie seems to have arrived at pretty accurate notions of the motionof a body in a circular orbit. He remarks that bodies so revolving have atendency to recede from their centre of revolution, as in the case of awheel revolving on its axle, or a stone whirled by a sling. When thisforce is equal to the tendency of the body to the centre, a compensa-tion of effects takes place, and the body will neither approach nor recedefrom the centre of force, but will continually revolve round it.

Here, for the first time, an attempt is made to account for the motion of abody in a circular orbit, by means of a force directed continually to the centreof the circle. It must be admitted, however, that Borelli’s explanation isat once imperfect and indistinct. He does not aualyze the phenomenonof curvilinear motion into its constituent elements, but merely seeks toestablish the necessity of a constant central force by an appeal to expe-riment. He rightly asserts that the body tends continually to recedefrom the centre, but he gives no account of the origin of this centrifugalforce: nor does he explain by what means the motion of the body in itsorbit is continually kept up. His account of the last-mentioned part of thephenemenon is so obscure, that it is quite evident he had obtained only avery weak hold of the problem. After remarking that the compensatoryeffects of the two constant forces will maintain the body at a determinatedistance from the centre, he then says, “ therefore the planet will appearbalanced and floating on the surface.”*

Although Borelli’s speculations possessed much merit, still theywere not sufficiently clear to lead to any measurable results, anduntil a complete dynamical view of the problem of centripetal forcescould be obtained, it was obviously hopeless to attempt its mathema-tical solution. Without stopping here to notice the partial researchesof Hooke, Huygens , Wren, and Halleyf, we shall at once proceed to givesome account of the immortal discoveries of Newton. This illustriousphilosopher was born in the year 1642, at Woolsthorpe, in the county ofLincoln. Before attaining the years of maturity he made a multitude ofbeautiful discoveries in Analysis, and was even in possession of the methodof Fluxions when he was only twenty-four years of age. He was nowabout to enter upon a field of speculation which -was destined to offer amagnificent theatre for displaying the resources of that powerful instrumentof investigation. Pemberton states that Newton, having quitted Cam-bridge, for Woolsthorpe, in 1665, to avoid the plague, was sitting one dayin his garden, when he was led to reflect on the principle which causes allbodies to tend towards the centre of the earth. As this tendency did notappear to suffer any sensible diminution on the tops of the highest build-

* “ Ideoque planeta libratus apparebit et supernatans. ” Theories: Mediceorum Plane-tarum in causis physicis deduct®. Florentise, 1666.

f We shall have occasion to notice incidentally in the following pages the labours ofthese philosophers on the subject of centripetal forces. Newton commenced his re-searches at least as early as any of his contemporaries; nor does it appear, throughout allthis career, that he was indebted to one or other of them for any of his ideas.