GENERALIZATION OF PRINCIPLES. 77 



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 pro- 

 jectiles 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 errours which had vitiated Kepler's 

 reasonings. In the same way, though Descartes is 

 sometimes spoken of as the first promulgator 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 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 Journals of the Royal So- 

 ciety, that " there was read a paper of Mr. Hooke's, 

 explicating the inflexion of a direct motion into a 

 curve by a supervening attractive principle ;" and 

 before the publication of the Principia in 1687, 

 Huyghens, as we have seen, in Holland, and, in our 

 own country, Wren, Halley, and Hooke, had made 

 some progress in the true mechanics of circular 



