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 
than 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 
