Resisting Medium and a Repulsive Force. 467 



vibrations of its atoms; and if so, it is capable of performing the 

 same office for radiant heat, which always accompanies light in 

 the sunbeam. Surely the admission of the universal presence 

 of the sether of space as a medium for the transmission of phy- 

 sical forces, supported as it is by many facts that such forces are 

 .so propagated, is preferable to the bare assumption that, because 

 celestial attraction and repulsion appear to act at a distance, 

 therefore it is a fact that they do so act. It is clearly beginning 

 at the wrong end: this question relates to the simple fact, the 

 an sit of Aristotle ; and in such a case no assumption is jus- 

 tifiable. . 



M. Payees analyses relating to the acceleration of comets, seem 

 to have rested on two data concerning a resisting medium : in the 

 one case the resisting medium is regarded as gravitating but im- 

 moveable ; in the other case as an imponderable (or immaterial) 

 and revolving medium. 



It does not appear in this memoir why such a distinction was 

 adopted ; and it is not easy to imagine the grounds for such con- 

 ceptions concerning the sether of space. If it be a gravitating 

 material fluid, it must necessarily partake of the common motion 

 of the entire solar system ; and if so, it remains to revise the 

 data of the calculations, the resisting medium not being station- 

 ary, but spirally revolving around the sun with a velocity pro- 

 gressively increasing from the confines to the centre of the solar 

 system. " When the medium revolves," says M. Faye, " it ceases 

 to gravitate towards the sun, and its layers also cease to press 

 each other according to the law of density." This statement is 

 incomprehensible: atmospheres revolve with their respective 

 central bodies, as sether with the sun, and yet their layers press 

 on each other. If the medium possess any weight, however 

 inappreciable, it cannot lose this by revolving ; and if it be im- 

 ponderable or immaterial, as the older physicists considered the 

 sether to be, it could not revolve ; for motion depends on a com- 

 position of forces essentially material or physical, that is, of 

 attraction and repulsion, to the former of which the weight of 

 bodies is due. 



From these considerations it is evident that the notion con- 

 cerning the nature of sether held by our neighbours is very dif- 

 ferent from that which we entertain. Grove thinks (and most of 

 us agree with him) that sether is a highly elastic fluid having 

 weight, though the amount of it is beyond the reach of determi- 

 nation, its excessive tenuity rendering it inappreciable. He regards 

 this matter as the rarefied extensions of the atmospheres of celes- 

 tial bodies. In my 'Philosophy of Nature 5 a different view is ad- 

 vanced; but we both agree in its being a material elastic fluid; and 

 if so, it must follow the common law of fluids, and be more and 



