Resisting Medium and a Repulsive Force. 459 



since read this memoir in the August. Number of the Comptes 

 Rendus of the French Academy, I found that there was not the 

 slightest foundation for the supposed similarity of opinions; buk 

 its contents are so interesting, that I was induced to make 

 copious extracts therefrom : and I am inclined to think that they 

 will be generally acceptable; for even those, most averse to 

 speculations cannot refuse their attention to the opinions of such 

 an illustrious man. 



M. Faye commences his memoir by briefly and clearly stating 

 the points of the thesis which he proposes to prove, which are, — 

 1st, that the hypothesis of a resisting medium, as formulated by 

 geometers, is unacceptable ; 2nd, that if it be corrected so as to 

 render it more rational, it becomes too indefinite for directing 

 analysis ; 3rd, that the theory of the repulsive force is the only 

 one that can be scientifically constituted. 



The hypothesis of a resisting medium, says M. Faye, applies 

 very well as far as the motions of the periodical comets of three 

 and seven years ; it gives a clear and precise idea of the nature 

 of their acceleration. According to M. Plana, this- theory gives 

 29"*5 for the variation of the second comet's eccentricity, a 

 result very near 34"-6 assigned by observation. Since M. Plana, 

 M.. Axel Moller advances a step further by introducing, after the 

 suggestion of M. Valz, the variation of volume which a comet, 

 supposing it compressible and not permeable to the surrounding 

 medium, ought to experience when it penetrates into the gradu- 

 ally denser strata of this medium. The diminution of eccentri- 

 city is then equal to 32", that is, almost identical with the 

 result of observation. But could we approach still nearer to 

 34"' 6, the value of which is not indeed definitively fixed, I would 

 still not the less persist in regarding these forniulse and calcula- 

 tions as purely empirical, inasmuch as it has not yet been proved 

 that a ponderable medium, elastic or not, can exist around the 

 sun without revolving round it. 



This idea of a heavy (or gravitating) and immoveable, medium, 

 says M. Faye, is no novelty ; it may be traced back to the materia.. 

 Ccelorum of the ancients, which was supposed to fill the world 

 after the manner of the extension of the atmosphere of a central 

 body ; and since the time of Newton, its existence has only been 

 upheld for the purpose of conserving the conception of gravita- 

 tion, as the one governing force of the universe. But this 

 singular hypothesis ought to have vanished when Laplace made 

 known the definite limits which mechanics imposes on the 

 atmospheres of celestial bodies. 



In vain we suppose, in order to evade the objection, that this 

 medium is imponderable ; for then we must have recourse to the 

 aether of physicists. But in this case we must no longer attri- 

 2H2 



