Resisting Medium and a Repulsive Force. 461 



within which orbit the perihelion of this comet occurs. Again, the 

 comet of seven years likewise requires, no less imperiously, that 

 the density of such a ring should be well marked in the region 

 of the orbit of Mars, rapidly decreasing in such a manner as to 

 become imperceptible on approaching the orbit of Jupiter ; for 

 it is between these orbits that the motions of this remarkable 

 comet are accomplished. These conditions can only be recon- 

 ciled by adopting for the resisting medium a series of cosmical 

 rings more or less resembling those of Saturn, but separated 

 from one another by great intervals. 



Such, in the opinion of M. Faye, is the only form under which 

 the hypothesis of a resisting medium can be hereafter main- 

 tained. But he adds that nothing can be more indefinite than 

 such a hypothesis ; for the number of such rings, their respec- 

 tive limits, and the law of their interior density are completely 

 arbitrary. It is impossible, for example, to extract from it any 

 relation between 8n } or S£ an d 8<f> } on which, however, all the 

 memoir of M. Plana proceeds. 



In this manner, says M. Faye, the two first parts of his 

 thesis have been justified : it has been shown that an immoveable 

 medium is impossible ; that a revolving medium is an indefinite 

 hypothesis with which analysis can have no concern ; and that a 

 series of cosmical rings is so fanciful that such a hypothesis must 

 be ranked with the transparent crystalline heavens and with the 

 Cartesian vortices. And he lastly proceeds to the consideration 

 of his theory of a repulsive force. 



The several successive returns of the comet of three years have 

 taught us that the duration of its revolution is constantly dimi- 

 nishing, whilst the other elements of its orbit remain unaltered. 

 This is a most important fact ; and Encke, the author of this 

 great discovery, has concluded therefrom the existence of a force 

 which is constantly opposed to the motion of the comet, and 

 which therefore results in the comet's successive acceleration, 

 without affecting its other elements, save the eccentricity. But 

 this repulsive tangential force, is it real or apparent ? If it be a 

 real force, it may be asked what such a force can be which is 

 able to contend in the heavens with gravitation, and thus to 

 break the unity of astronomical science. Encke has declared 

 for an apparent force, originating in the resistance of a medium ; 

 it was, it may be supposed, merely for the conservation of this 

 threatened unity. It is doubtless an enlightened view, but 

 arbitrary ; for the unity of force is not, it is presumed, a scien- 

 tific dogma. Let us then reserve our judgment concerning 

 the nature of this repulsive force. 



In studying, says M. Faye, the astonishing forms which 

 comets present, their gigantic tails, the matter which they seem 



