COMETS. 



' 54. Of the identity of the paths while visible on each of these 

 appearances Halley entertained no doubt, and announced to the 

 world the discovery of the elliptic motion of comets, as the result 

 of combined observation and calculation, and entitled to as much 

 confidence as any other consequence of an established physical 

 law ; and predicted the re-appearance of this body, on its suc- 

 ceeding return to perihelion in 1758-9. He observed, however, 

 that as in the interval between 1607 and 1682 the comet passed 

 near Jupiter, its velocity must have been augmented, and conse- 

 quently its period shortened by the action of that planet. This 

 period, therefore, having been only seventy-five years, he inferred 

 that the following period would probably be seventy- six years or 

 upwards ; and consequently that the comet ought not to be expected 

 to appear until the end of 1758, or the beginning of 1759. It is 

 impossible to imagine any quality of mind more enviable than that 

 which, in the existing state of mathematical physics, could have 

 led to such a prediction. The imperfect state of science ren- 

 dered it impossible for Halley to offer to the world a demonstra- 

 tion of the event which he foretold. " He therefore," says M. de 

 Pontecoulant, " could only announce these felicitous conceptions of 

 a sagacious mind as mere intuitive perceptions, which must be 

 received as uncertain by the world, however he might have felt 

 them himself, until they could be verified by the process of a 

 rigorous analysis." 



Subsequent researches gave increased force to Halley's predic- 

 tion ; for it appeared from the ancient records of observers, that 

 comets had been seen in 1456 and 1378, whose elements were 

 identical with those of the comet of 1682. 



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