2 9 4 



A Short History of Astronomy 



[CH. XI. 



and more generally treated by later writers, and a good 

 many additions have been made, few if any results of 

 fundamental importance have been added to those con- 

 tained in Clairaut's book. 



He next turned his attention to the problem of three 

 bodies, obtained a solution suitable for the moon, and made 

 some progress in planetary theory. 



Halley's comet (chapter x., 200) was "due" about 



\ 



1898 



FIG. 80. The path of Halley's comet. 



1758; as the time approached Clairaut took up the task 

 of computing the perturbations which it would probably have 

 experienced since its last appearance, owing to the influence 

 of the two great planets, Jupiter and Saturn, close to both 

 of which it would have passed. An extremely laborious 

 calculation shewed that the comet would have been retarded 

 about 100 days by Saturn and about 518 days by Jupiter, 

 and he accordingly announced to the Academy towards the 

 end of 1758 that the comet might be expected to pass its 



