BY G. E. SMALLEY, ESQ. 



335 



There is nothing inconceivable in the idea that there is an 

 invisible, almost imponderable aether pervading all parts of space 

 on the contrary, assured as we are, that " Nature abhors a 

 vaccuum," we may fairly think that no part of space is abso- 

 lutely void, though we can well understand that such astherial 

 medium would, by gravitation, be condensed in the immediate 

 vicinity of substantial bodies. 



It remains, however, for us to examine whether this supposi- 

 tion of Encke's about the " ^Etherial medium," does account for 

 the fact, that the comet which bears his name has been con- 

 tinually decreasing the size of its orbit and its period of revolu- 

 tion from the time of its first appearance up to the present time. 



Now I have already remarked that, however attenuated the 

 material of a comet may be, it must still be subject to the Laws 

 of Universal Gravitation ; and, though I may be treading on 

 familiar ground, yet I will just observe that when a body moves 

 in an ellipse under the action of a force tending towards a fixed 

 centre, any diminution in its velocity, from whatever cause it 

 may arise, will have the effect of diminishing the dimensions of 

 the ellipse, and therefore of diminishing the time of revolution. 

 But this is just the effect that would be produced by an aetherial 

 medium, and I venture for a few moments to draw attention to 

 a principle, well known as it may be, upon which this fact 

 depends. 



