48 COSMOS. 



the twenty-five days immediately preceding and succeeding 

 the comet's perihelion passage. The value of the constant 

 is therefore somewhat different, because in the neighbour- 

 hood of the sun the highly attenuated, but still gravitating 

 strata of the resisting fluid, are denser. Olbers maintained 38 

 that this fluid could not be at rest, but must rotate directly 

 round the sun ; and therefore the resistance offered to retro- 

 grade comets, like Halley's, must differ wholly from that 

 opposed to those comets having a direct course, like Encke's. 

 The perturbations of comets having long periods of revolu- 

 tion, and the difference of their magnitudes and sizes, com- 

 plicate the results, and render it difficult to determine what 

 is ascribable to individual forces. 



The gaseous matter constituting the belt of the Zodiacal 

 light may, as Sir John Herschel 39 expresses it, be merely the 

 denser portion of this comet-resisting medium. Although it 

 may be shown that all nebulae are crowded stellar masses, 

 indistinctly visible, it is certain that innumerable comets fill 

 the regions of space with matter through the evaporation of 

 their tails, some of which have a length of 56000000 of miles. 



observed by Bessel, between the 12th and 22nd of October, 

 1835, (Schumacher Astron. Nachr. ,nos. 300, 302, 185, 232), 

 " may, indeed, in the case of some individuals of this class of 

 cosmical bodies, exert an influence on the translatory and 

 rotatory motion, and lead us to infer the action of polar forces 

 ( 201, 229,) which differ from the ordinary attracting force 

 of the sun;" but the regular acceleration observable for 

 sixty-three years in Encke's comet, (whose period of revolu- 

 tion is 3-i- years), cannot be regarded as the result of in- 

 cidental emanations. Compare on this cosmically important 

 subject, Bessel in Schum. Astron. Nachr., no. 289, s. 6, and 

 no. 310, s. 345-350, with Encke's Treatise on the hypothesis 

 of the resisting medium, in Schum., no. 305, s. 265-274. 



35 Olbers in Scnum. Astr. Nachr.. no. 268, s. 58. 



89 Outlines of Astronomy, 556, 557. 



