OPINIONS OF THE ANCIENTS RESPECTING COMETS. 319 



so many successive apparitions of one and the same 

 body. 



This identity involved a conclusion before which more 

 thiin one astronomer shrunk. It was necessary to admit 

 that the time of a complete revolution of the comet was 

 subject to a great variation, amounting to as much as 

 two years in seventy-six. 



Were such great discordances due to the disturbing 

 action of the planets ? 



The answer to this question would introduce comets 

 into the category of ordinary planets or would exclude 

 them for ever. The calculation was difficult : Clairaut 

 discovered the means of effecting it. While success 

 was still uncertain, the illustrious geometer gave proof 

 of the greatest, boldness, for in the course of the year 

 1758 he undertook to determine the time of the fol- 

 lowing year when the comet of 1682 would reappear. 

 He designated the constellations, nay the stars, which it 

 would encounter in its progress. 



This was not one of those remote predictions which 

 astrologers and others formerly combined very skilfully 

 with the tables of mortality, so that they might not be 

 falsified during their lifetime : the event was close at 

 hand. The question at issue was nothing less than the 

 creation of a new era in cometary astronomy, or the 

 casting of a reproach upon science, the consequences of 

 which it would long continue to feel. 



Clairaut found by a long process of calculation, con- 

 ducted with great skill, that the action of Jupiter and 

 Saturn ought to have retarded the movement of the 

 comet ; that the time of revolution compared with that 

 immediately preceding, would be increased 518 days by 

 the disturbing action of Jupiter, and 100 days by the 



