RESISTING MEDIUM. 197 



ever, will perhaps be understood if it be con- 

 sidered that by moving more slowly the comet 

 will be more rapidly drawn towards the centre, 

 and that in this way a revolution will be described 

 by a shorter path than it was before. It appears, 

 that in getting round the sun, the comet gains 

 more in this way than it loses by the diminution 

 of its velocity. The case is much like that of a 

 stone thrown in the air ; the stone moves more 

 slowly than it would do if there were no air ; but 

 yet it comes to the earth sooner than it would do 

 on that supposition. 



It appears that the effect of the resistance of 

 the ethereal medium, from the first discovery of 

 the comet up to the present time, has been to 

 diminish the time of revolution by about two 

 days : and the comet is ten days in advance of 

 the place which it would have reached, if there 

 had been no resistance. 



2. The same medium which is thus shown to 

 produce an effect upon Encke's comet, must also 

 act upon the planets which move through the 

 same spaces. The effect upon the planets, how- 

 ever, must be very much smaller than the effect 

 upon the comet, in consequence of their greater 

 quantity of matter. 



It is not easy to assign any probable value, or 

 even any certain limit, to the effect of the resist- 

 ing medium upon the planets. We are entirely 

 ignorant of the comparative mass of the comet, 



