Resisting Medium and a Repulsive Force. 469 



direction in any way opposite to its motion, the friction must be 

 tantamount to an increase of gravity, which, by reducing the 

 orbit, must increase the centrifugal force, thereby accelerating 

 its motion. This cause alone may not account for the entire 

 alteration ; but the subject is as yet in its infancy, and cannot 

 be fairly condemned in the summary manner in which M. Faye 

 has dealt with it. He gives us, it is true, another hypothesis in 

 its place, but it does not seem to be in any way superior to that 

 of Encke. We know the effect which radiant heat has on bodies 

 when it impinges on them : if the surface be reflecting, accord- 

 to its degree the rays of heat are diverted ; if it be absorbing, 

 the heat assumes another phase and operates as an expansive 

 force ; if it be diathermanous, it passes freely through and 

 renews its course on the opposite side of the body. In the case 

 of comets, radiant heat can only be supposed to act in the two 

 last-mentioned ways : in the last, in respect to a comet's motion, 

 it must be perfectly inoperative ; in the second, a certain amount 

 of heat would be absorbed and enlarge the volume of the comet, 

 thereby pro tanto counteracting the condensing effect of the sun's 

 gravitating pressure. 



M. Faye would seem to imply that the repulsive force of the 

 sun is arrested by the surface of the comet, and thus is enabled to 

 forcibly act on it as by impact ; and agreeably to this notion he 

 asserts, what is contrary to our daily experience, that " the repul- 

 sive force of the sun does not come within our reach ; that it is 

 dissipated on the superior beds of the atmosphere, in consequence 

 of its incapacity of acting through all matter after the manner 

 of gravitation." If it could be supposed that comets are enve- 

 loped by an impervious skin, as M. Valz suggested, and that 

 this, moreover, had a good reflecting surface like some polished 

 metab, then the impinging and reflecting of an enormous mul- 

 titude of the sun's rays might by their aggregate for-ce produce 

 a sensible motion. But of what avail are suggestions of this 

 kind : hypotheses arrived at inductively from facts may be tole- 

 rated, though they may prove to be invalid ; but it is perfectly 

 illogical to assume a fact, and then make it the subject of an 

 argument. 



It may be that M. Faye's repulsive force emanating from the 

 sun may be well conceived, but imperfectly apprehended : it may 

 prove to be akin to centrifugal force, which is the correlative of 

 gravity, as set forth in the ' Philosophy of Nature.' But then 

 such a mode of repulsion is quite different from that other mode 

 of repulsion, which, as radiant heat, not only emanates from 

 incandescent, but also from all hot bodies, and which is trans- 

 mitted from body to body by the vibrations of ethereal atoms. 



Each mode of the physical forces can only be manifested by 



