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



bute to the medium a density proportional to —# because in 



ceasing to gravitate towards the sun, the beds of this medium 

 will cease to mutually press on one another in that direction ; it 

 therefore becomes requisite to attribute to the medium a constant 

 density in space. Unfortunately, in this case, the beautiful 

 agreement above alluded to between calculation and observation 

 will disappear ; for instead of 34"-6, the formula of M. Plana 

 for this hypothesis will not give more than 14"; and, moreover, 

 there still remains to apply to this medium a velocity of trans- 

 lation equal and contrary to that which transports the masses of 

 the solar system in indefinite space. 



It is a long time ago, says M. Faye, since I first advanced 

 this objection : no one has ever been able to answer it ; and, 

 notwithstanding, the hypothesis of an immoveable medium is 

 persisted in. Can it be then that in itself the immobility or 

 the movement of the medium is a thing of no consequence? 

 This is what we are going to see. 



Let us accept, then, continues M. Faye, the very different 

 hypothesis of a medium revolving around the sun. It is then 

 evident that such a medium can only resist the motion of comets 

 in virtue of the excess of velocity. This excess, positive at peri- 

 helion, becomes negative at aphelion; consequently if the medium 

 resists in the one case, it will push in the other. This circum- 

 stance alone is sufficient to show that the analysis of this new 

 problem cannot be identified with that of the first. It indeed 

 involves more than this ; for what then will become of the law 

 of density ? No one can tell. When the medium was regarded 

 as immoveable, it was admitted that its beds, in gravitating the 

 one on the other and on the sun, would be mutually com- 

 pressed in such a manner that the density would progress- 

 ively increase towards the interior, according to the law approxi- 

 mately represented by -^ But when the medium revolves, it 



ceases to gravitate, not only towards, but on the sun ; its beds 

 cease to press upon each other and the law of density becomes 

 a perfectly indefinite problem. 



M. Faye then gives some analyses for the periodical comet of 

 seven years which bears his name ; the result of which is 2' 40", 

 instead of 34"*6 as given by observation. From this enormous 

 discordance, he says, it must be concluded that a continuous 

 ring of constant density is inadmissible. It is necessary, there- 

 fore, that the density of the medium should vary according to a 

 peculiar law. Thus Encke's comet requires that this density 

 should go on rapidly decreasing outside the orbit of Mercury, 



