Resisting Medium and a Repulsive Force. 463 



tional to the extent of surface. The tangential force of this 

 composition leads to precisely the same conclusions. The sun 

 alone exercises it : it does not depend on the sun's mass, which 

 is not operative in this case ; it is probably only caused by the 

 incandescence of the sun's surface; for this it is that distinguishes 

 the sun from the planets, the vicinity of which does not affect 

 the figure of the comets. 



Such is an idea of the repulsive-force hypothesis, so far 

 developed ; a few steps more, and its astronomical formula may 

 be attained. A repulsive force operating at all distances, but 

 evidently becoming more feeble, and that rapidly as the distance 

 increases ; emanating from an incandescent polar surface ; pro- 

 pagated with a velocity equal to that of radiant caloric ; in the 

 ratio of the surface and not of the mass ; and pressing on the 

 surfaces which it repels, in place of acting through all matter 

 like gravitation : — Is there such a force in Nature ? 



If such a force exists, says M. Faye, it is probably the same 

 repulsive force which is manifested in all material bodies under 

 the names of dilatation, of expansion, of elasticity, &c. As in 

 the case of the assumed astronomical force, the physical force 

 which presides over these phenomena is due to heat ; like it, its 

 action is repulsive, does not extend through bodies, decreases 

 rapidly with the distance, and relates to the surface and not to 

 the mass ; but there is a single difference between them : the 

 physical, unlike the astronomical force, cannot act at a distance ; 

 so say many physicists ; beyond molecular intervals it is imper- 

 ceptible. But this, M. Faye, in common with Fresnel and others, 

 -regards only as an a priori opinion, not justified by experi- 

 mental research : it ought to be proved that calorific repul- 

 sion, which acts from molecule to molecule in solids, liquids, and 

 gases, that is to say, at intervals successively increasing at an 

 enormous ratio, does not suddenly vanish at certain particular 

 distances. 



M. Faye says that for nearly three years he has worked on 

 this subject ; and in order to remind the Academy, he rapidly 

 recounts the results up to the present time, viz. that his theory 

 accounts for all the observed phenomena concerning the accele- 

 ration of cometary motions. Hereafter account must be taken 

 of the variation of volume in the nucleus of comets, which 

 doubtless will present some difficulties. He has also shown 

 that, varied as the figures of comets are (which are complicated 

 by the effects of perspective), they possess certain features in 

 common — features which in their unity constitute in some mea- 

 sure a normal figure, which can be separated from accidental differ- 

 ences. And he thinks that the coexistence of several tails may 

 he simply explained by the presence of substances having dif- 



