196 COSMICAL ARRANGEMENTS. 



It will perhaps appear remarkable that a body 

 so light and loose as we have described this 

 comet to be, should revolve about the sun by 

 laws as fixed and certain as those which regulate 

 the motions of those great and solid masses, the 

 Earth and Jupiter. It is however certain from 

 observation, that this comet is acted upon by 

 exactly the same force of solar attraction, as the 

 other bodies of the system ; and not only so, but 

 that it also experiences the same kind of disturb- 

 ing force from the action of the other planets, 

 which they exercise upon each other. The effect 

 of all these causes has been calculated with great 

 care and labour ; and the result has been an 

 agreement with observation sufficiently close to 

 show that these causes really act, but at the 

 same time a residual phenomenon (as Sir J. 

 Herschel expresses it) has come to light : and 

 from this has been collected the inference of a 

 resisting medium. 



This medium produces a very small effect upon 

 the motion of the comet, as will easily be sup- 

 posed from what has been said. By Encke's 

 calculation, it appears that the effect of the re- 

 sistance, supposing the comet to move in the 

 earth's orbit, would be about 1 -850th of the sun's 

 force of the body . The effect of such a resistance 

 may appear, at first sight, paradoxical ; it would 

 be to make the comet move more slowly, but 

 perform its revolutions more quickly. This, how- 



