138 LAPLACE. 



cate sinister events. Eegiomoutanus and Tyclio Bralic proved by their 

 observations that they are situate bej'ond the moon ; Heveliiis, Dortel, 

 &c., made them revolve around the sun ; Newton established that they 

 move under the immediate influence of the attractive force of that body • 

 that they do not describe right lines ; that, in fact, they obey the laws of 

 Kepler. It was necessary, then, to prove that the orbits of comets are 

 curves which return into themselves, or that the same comet has been 

 seen on several distinct occasions. This discovery was reserved for 

 Halley ; by a minute investigation of the circumstances connected with 

 the apparitions of all the comets to be met with in the records of history, 

 in ancient chronicles, and in astronomical annals, this eminent philoso- 

 pher was enabled to prove that the comets of 1682, of 1007, and of 1531 

 were in reality so many successive apparitions of one and the same body. 



This identity involved a conclusion before which more than one as- 

 tronomer shrank. 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 plan- 

 ets? 



The answer to this question would introduce comets into the category 

 of ordinary planets, or would exclude them forever. The calculation 

 •was difficult ; Olairaut 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 tbe following year when the comet of 1082 would re-appear. 

 He designated the constellations, nay, the stars, which it would encoun- 

 ter in its progress. 



This was not one of those remote predictions which astrologers and 

 others formerly combined very skillfully with the tables of mortality, so 

 that they might not be falsified during their life-time : the event was 

 close at hand. The question at issue was nothing less than the creation 

 of a new era in coraetary 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, conducted 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 action of Saturn, forming a total 

 of 018 days, or more than a year and eight months. 



Never did aquestion of astronomy excite a more intense, a more legit- 

 imate curiosity. All classes of society awaited with equal interest the 

 announced apparition. A Saxon peasant, Palitzch, first perceived the 

 comet, ncnceforward, from one extremity of Europe to the other, a 

 thousand telescopes traced each night the path of the body through the 

 constellations. Tbe route was always, within the limits of precision of 

 the calculations, that which Clairaut had indicated beforehand. The 



