80 OUTLINES OF NATURAL PHILOSOPHY. 



for a limited portion of time, by considering 

 the orbit as an ellipsis, the elements of which 

 are continually changing. 



This is the method of LA GRANGE, and is followed 

 in the Mecanigue Celeste, Part u. Chap. 9. 



ij&mfe.sdi 



Dr HALLEY, when he predicted the return of the 

 comet of 1 682, took into consideration the action 

 of Jupiter, and concluded that it would increase 

 the periodic time of the Comet a little more than a 

 year ; he therefore fixed the time of the re-appear- 

 ance to the end of the year 1758, or the beginning 

 of 1 759. 



He professed, however, to have made this calculation 

 hastily, or, as he expresses it, levi calamo. Synop- 

 sis of the Astronomy of Comets. 1 



CLAIRAUT, on calculating with great 

 care and labour the effects both of Jupiter and 

 Saturn, found that the return of the Comet 

 would be retarded 511 days by the former, and 

 100 by the latter ; in consequence of which he 

 foretold that its return to its perihelion would 

 be on the 15th of April 1759- 



He said at the same time, that he might be out a 

 month in his calculation. The Comet actually 

 reached its perihelion on the 13th of March, just 

 33 days earlier than was predicted 5 thus affording 



