HALLEY'S COMET. 



periods, and frequent returns, the last whose periodicity has been 

 discovered. 



52. It is evident that the idea of the possible existence of 

 comets with periods shorter than those of the more remote planets,, 

 and orbits circumscribed within the limits of the solar system, 

 never occurred to the mind either of Xewton or any of his con- 

 temporaries or immediate successors. 



In the third book of his PRINCIPIA, he calls comets a species of 

 planets, revolving in elliptic orbits of a very oval form. But he 

 continues, "I leave to be determined by others the transverse 

 diameters and periods, by comparing comets which return after 

 long intervals of time to the same orbits." 



It is interesting to observe the avidity with which minds of a 

 certain order snatch at such generalisations, even when but slen- 

 derly founded upon facts. These conjectures of Newton were soon 

 after adopted by Yoltaire : " II y a quelque apparence," says he, 

 in an essay on comets, " qu'on connaitra un jour un certain 

 nonibre de ces autres planetes qui sous le nom de cometes tournent 

 comme nous autour du soleil, mais il ne faut pas esperer qu'on le& 

 connaissent toutes." 



And again, elsewhere, on the same subject: 



" Cometes, que 1'on craint a 1'egal du tonnerre, 

 Cessez d'epoitvanter les peuples de la terre ; 

 Dans une ellipse immense achevez votre cours, 

 Remontez, descendez pres de 1'astre des jours." 



53. Extraordinary as these conjectures must have appeared at 

 the time, they were soon strictly realised. Halley undertook 

 the labour of examining the circumstances attending all the 

 comets previously recorded, with a view to discover whether 

 any, and which of them, appeared to follow the same path. 

 He found that a comet which had been observed by himself, 

 by Newton, and their contemporaries in 1682, followed a path 

 while visible, which coincided so nearly with those of comets 

 which had been observed in 1607, and in 1531, as to render it 

 extremely probable, that these objects were the same identical 

 comet, revolving in an elliptic orbit of such dimensions, as to 

 cause its return to perihelion at intervals of 75 76 years, 



The comet of 1682 had been well observed by La Hire, Picard, 

 Hevelius, and Flamstead, whose observations supplied all the 

 data necessary to calculate its path while visible. That of 1607 

 had been observed by Kepler and Longomontanus ; and that of 

 1531, by Pierre Apian at Ingolstadt, the observations in both 

 cases being also sufficient for the determination of the path of the 

 body, with all the accuracy necessary for its identification. 



175 



