7^ Rfjleciions on the 7icu> PLmet. 



others dark? The comet of 1770, therefore, might exift 

 lometimes in an opake, and fometimes in a phofphorefccnt 

 iL'jte ; and hence perhaps, and from the perturbative power 

 of the larger and more denfe bodies, the unfrequency of their 

 return may be explained. They come back, and we do not 

 fee them ; they are prefcnt, and we do not obferve them. 

 To draw concUifions, from the identity of the elements of tlie 

 orbit of a body, refpcfting the identity of the body ilfelf, 

 gives, as all aftronomcrs know, but a high degree of proba- 

 bility *, among the ninety-one comets, confirmed by only one 

 hvpothefis i, which certainly may be true taken d'lrettly but 

 not tnverjely. Two comets, however, whofe orbits have not 

 the identical fame elements, may be one and the fame body. 

 This, as far as I know, has never been explicitly aflerted by 

 anyone: but Lexcl mentioned it implicitly, when the fol- 

 lowing objection was made to him on account of the live 

 vears elliptical orbit of the comet of 1770 : Since the period 

 of ihe revolution of this comet is fo fliort, why was it not 

 feen oftencr, and long ago ? Lexel was of opinion, and per- 

 haps fo was the great Euler his mafter, vmder whofe infpec- 

 tion he laboured, that the influence and perturbation of the 

 large bodv of Jupiter, to which this comet approached very 

 near on the 27th of May 1767, and the 23d of Auguft 1779, 

 micht have entirely changed its orbit;}:. Burckhardl was 

 of the fame opinion in his memoir written on the fubje6t 

 of the prize already mentioned. 



But how frightful is the mere idea of calculating the per- 

 turbation of iuch a body ! Would not fuch a calculation 

 exceed the powers of our analviis ? The difficult theory of 

 the moon would be mere elementary calculation compared 

 with the variable orbit of fuch a body. It were to be wiflied 

 that fo ftriking a phienomenon would produce the neceflity 

 f)f giving to our calculation, in regard to perturbation, a new 

 direclion, that the theory of fo complex an approximation 

 might be more improved, and that the influence of the fuc- 

 ceflive integrations on negleded quantities might be better 

 and more accurately determined. But the geonietrician, who 

 can reprcfent all the co-ordinates of the motion of each ce- 

 lefiial bodv in fpeedily approximating I'eries of fines and co- 

 lines of the angle depending on its real motion, does not, 

 perhaps, exift on our earth. 



• Monar. Corrdpond. vol. iii. p-4i4 



f I'lie ib callfcl Hallcy's comet, the fifth and laft obferved return of 

 which took place in 1759, and which will appear again in 1S35. 



+ " M. l.exel penlc, que ion orbite pent avoir etc totalement changce 

 par l'a6lion de Jupiter." Pingre Cpmetograph. part ii. p. 90. See alfo 

 Mom. dc Paris 1776. p, 648. 



4 Many 



