SEQUEL TO THE EPOCH OF NEWTON. 243 



Sect. 5. Application of the Newtonian Theory 

 to Comets. 



A FEW words must be said upon another class of 

 bodies, which at first seemed as lawless as the 

 clouds and winds ; and which astronomy has re- 

 duced to a regularity as complete as that of the 

 sun; upon Comets. No part of the Newtonian 

 discoveries excited a more intense interest than this. 

 These anomalous visitants were anciently gazed at 

 with wonder and alarm; and might still, as in 

 former times, be accused of "perplexing nations," 

 though with very different fears and questionings. 

 The conjecture that they, too, obeyed the law of 

 universal gravitation, was to be verified by show- 

 ing that they described a curve such as that force 

 would produce. Hevelius, who was a most diligent 

 observer of these objects, had, without reference 

 to gravitation, satisfied himself that they moved 

 in parabolas 33 . To determine the elements of the 

 parabola from observations, even Newton called 34 

 "problema longe difficillimum." Newton deter- 

 mined the orbit of the comet of 1680 by certain 

 graphical methods. His methods supposed the orbit 

 to be a parabola, and satisfactorily represented the 

 motion in the visible part of the comet's path. But 

 this method did not apply to the possible return 

 of the wandering star. Halley has the glory of 

 having first detected a periodical comet, in the case 



33 Bailly, ii. 246. 34 Principia, ed. 1. p. 494. 



R2 



