THE LAW OF GRAVITATION. 483 



historically, the foundatiou of that sublime superstructure wliicli in 

 a subsequent age was reared by Newton, and which, by reason of the 

 magnitude of its proportions and the multiplicity of its details, all 

 pervaded and determined by the most admirable unity, now stands 

 and in all probability will ever stand, as the most imposing monument 

 ever erected by the human intellect. 



Although Kepler's theory, that bodies terrestrial mutually at- 

 tracted each other, met with ready reception, more than thirty years 

 elapsed after the publication of this work before the idea was enter- 

 tained, at least favorably, of accounting for the revolutions of the 

 heavenly bodies on the theory of the universality of the attraction of 

 gravitation. Kepler indeed, as we have remarked above, alludes to 

 such an hypothesis only however to expose, as he imagined, its fallacy. 

 The motions of the heavenly bodies being curvilinear, whilst the 

 motions of bodies under the influence of gravity were rectilinear, it 

 was taken for granted as a thing self-evident that the two phenomena 

 must be due to entirely different physical causes. Familiar as we are 

 with the fact, that by the two laws of motion above mentioned, the 

 hypothesis of an attractive force of the sun, combined with the 

 hypothesis of a tendency of the planets to move in a straight line in 

 virtue of an original impulse communicated to them, would satisfac- 

 torily and readily account for their curvilinear motion, it cannot but 

 be a matter of surprise that the truth should have remained so long 

 unrecognized. 



The credit of having been the first to generalize the idea of gravity, 

 and refer the revolutions of the heavenly bodies to the attraction of 

 matter for matter, appears to be due to Borelli, an Italian philosopher, 

 a pupil of Galileo. It is announced in a work which he published "^On 

 the Satellites of Jupiter,' in 1666, although, as we shall have occasion 

 to notice subsequently, Newton had conceived the same idea at least as 

 early as 1665. Both Newton and Huyghens, however, attributed to 

 Borelli the honor of having been the first to announce the important 

 truth. 



The idea, having been suggested, was at once accepted by many 

 with favor and immediately led to the investigation of a hitherto unex- 

 plored field in the department of mechanical philosophy. Whilst the 

 labors of others in this field were not unimportant, particularly those 

 of Wallis, the name which is especially deserving of honorable mention 

 in this connection is that of Huyghens. In a work published in 1672, 

 we meet for the first time with a scientific discussion of the doctrine 

 of Central Forces. His investigations were remarkably satisfactory 

 and complete as to the phenomena of circular motion, the attractive 

 force being at the center and contributing largely to the success of the 

 labors of subsequent inquirers. 



