462 Dr. Boase on M. Faye's Memoir on the Existence of a 



to dart towards the sun, but which soon returns to be con- 

 founded with the tail, &c, all the world will naturally say that 

 such things come to pass because the sun exercises a repulsive 

 action upon the atmosphere of the comets. Some will have it 

 that this repulsive action is due to electricity, and others to 

 magnetism, without once reflecting that these forces, so precise 

 when applied to terrestrial phenomena, are vague and little 

 understood in relation to celestial bodies. Others, again, have 

 spoken of an apparent repulsion ; it was the idea of Hooke and 

 of Newton. Bessel, after a very profound study of certain phe- 

 . nomena, which he has, however, too much generalized, can only 

 . see in these cometary forms the effects of polar forces analogous 

 to magnetism. But to form an opinion concerning the nature 

 . of such a force, one single order of facts is not sufficient ; for it 

 entails the necessity of guessing. We therefore again ask, what 

 is this repulsive force ? 



Such was the state of the case, M. Faye says, when he took up 

 the question : — on one side a repulsive tangential force, indicated 

 by the motions of comets ; on the other a radial repulsive force, 

 indicated by their tails : on one hand, Encke with the ancient 

 hypothesis of a resisting medium for the explanation of the 

 former force; on the other, Bessel with his polar forces to 

 account for the latter. A discussion, short but memorable, took 

 place between these great astronomers. Bessel, who did not 

 believe more than myself in a resisting medium, referred every 

 thing to his radial forces ; Encke, on the other hand, pointed out 

 to him that this was impossible. So it would seem that the 

 . resisting medium adopted by the one is a physical impossibility ; 

 the play of forces, imagined by the other, in consideration of a 

 * single fact arbitrarily generalized, is still more inadmissible. 

 These two forces, real or apparent, are both repulsive; can 

 they be reduced to a single force ? If any celestial body exer- 

 cise this singular action, it can be no other than the sun itself. 

 ' But can it be imagined that a force emanating from the sun can 

 act on any body in any other direction than that of the radius 

 vector ? Yes, certainly, the mechanician answers, if the body 

 moves from right to left, and if the force is not instantaneously 

 propagated like that of gravitation, but with an enormous velo- 

 city, indicated by the disproportion of the composing forces. 

 So, then, the repulsive force exercised by the sun and endowed 

 with a successive propagation, after the manner of luminous and 

 calorific radiations, will furnish the two composing forces, the 

 one radial and the other tangential, which are required to ex- 

 plain both the forms and the motions of comets. In studying the 

 radial force under this point of view, it will be readily seen that 

 this ought to be a force independent of the mass, and propor- 



