PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 673 



the greater will be the retiirdino; effect of the resistance. We can 

 also understand that the resistance of the medium Avill be in a 

 direct ratio to the velocity of the body that moves through it. 

 One of the objections which it is said that Newton made to the 

 now received theory of light, namely, that it is the vibratory or 

 undulating motion of an etherial fluid, was, that such a fluid 

 medium would deflect the planets from their normal and proper 

 orbits, and cause them to move by spiral paths to the sun. The 

 undulatory theory of light is now firmly established; and the 

 existence of a resisting medium cannot, therefore, be denied. 

 Indeed, it seems impossible to avoid the conclusion that the fact 

 of gravitation, itself, demonstrates the necessary existence of a 

 medium through which that force is communicated from one 

 .planet to another; for it is certain that force cannot be Qommuni- 

 cated except through a material medium. 



When it was found that Eucke's comet approached nearer the 

 sun with each revolution, — so that in thirty thousand years it must 

 fall into it, — the ablest astronomers in Europe, after exhausting 

 all other modes of explanation, finally concluded that the comet, 

 being exceedingly light, is deflected inward by the resistance of 

 the etherial air which it encounters in space; and that it is, there- 

 fore, actually pursuing a spiral path toward the sun. 



" Now it appears probable," said Dr. Nichol, "that this comet ts 

 approaching the, Sim: on every successive appearance, its orbit ap- 

 pears somewhat contracted; and there is reason to believe that the 

 contraction will go on until it is either absorbed in that luminary, or 

 altogether dissipated by its beams. And after searching earnestly 

 for some other cause, most inquirers arc inclined to refer this extra- 

 ordinary and hitherto unparalleled change, to a resisting medium or 

 ether occupying the planetary spaces. ' I cannot but express my 

 belief,' said Professor Airy, ' that the principal part of the theory, 

 viz., an effect exactly similar to that which a resisting medium 

 would produce — is perfectly established by the reasoning in 

 Encke's memoir;' and similar opinions have been oftered by other 

 great authorities. * * * * How singular is it that we 

 should have been guided to a truth so remote and difficult — one 

 concerning which the grander phenomena of our system are silent 

 — hy the motions of a wandering object, in comparison with Avhose 

 etherial nature, even one of these light flocculi or flakes of cloud, 

 which scarce stain the sky of a summer evening, is heavy and sub- 

 stantial !" 



[Am. Inst.J QQ 



