354 HISTORY OF MECHANICS. 



retical Mechanics, as, a little previously, Dynamics had eclipsed and 

 superseded Statics. 



The laws of variable force and of curvilinear motion were not much 

 pursued, till the invention of Fluxions and of the Differential Calculus 

 again turned men's minds to these subjects, as easy and interesting ex- 

 ercises of the powers of these new methods. Newton's Princvpia, of 

 which the first two Books are purely dynamical, is the great exception 

 to this assertion ; inasmuch as it contains correct solutions of a great 

 variety of the most general problems of the science ; and indeed is, 

 even yet, one of the most complete treatises which we possess upon 

 the subject. 



We have seen that Kepler, in his attempts to explain the curvilinear 

 motions of the planets by means of a central force, failed, in consequence 

 of his belief that a continued transverse action of the central body was 

 requisite to keep up a continual motion. Galileo had founded his theory 

 of projectiles on the principle that such an action was not necessary ; 

 yet Borelli, a pupil of Galileo, when, in 1666, he published his theory 

 of the Medicean Stars (the satellites of Jupiter), did not keep quite 

 clear of the same errors which had vitiated Kepler's reasonings. In the 

 same way, though Descartes is sometimes spoken of as the first pro- 

 mulgator of the First Law of Motion, yet his theory of Vortices must 

 have been mainly suggested by a want of an entire confidence in that 

 law. When he represented the planets and satellites as owing their 

 motions to oceans of fluid diffused through the celestial spaces, and 

 constantly whirling round the central bodies, he must have felt afraid 



ml O * 



of trusting the planets to the operation of the laws of motion in free 

 space. Sounder physical philosophers, however, began to perceive the 

 real nature of the question. As early as 1666, we read, in the Jour- 

 nals of the Royal Society, that " there was read a paper of Mr. Hooke's 

 explicating the inflexion of a direct motion into a curve by a super- 

 vening attractive principle ;" and before the publication of the Prin- 

 cipia in 1687, Huyghens, as w r e have seen, in Holland, and, in our own 

 country, Wren, Halley, and Hooke, had made some progress in the 

 true mechanics of circular motion, 2 and had distinctly contemplated 

 the problem of the motion of a body in an ellipse by a central force, 

 though they could not solve it. Halley went to Cambridge in 1684, 3 

 for the express purpose of consulting Newton upon the subject of the 

 production of the elliptical motion of the planets by means of a central 



" Newt. Princip. Schol. to Prop. iv. 3 Sir D. Brewster's Lift of Newton, p. 154. 



