50 STATEMENT AND EXPOSITION OF 



This — as has elsewhere been mdicated by the author of this paper — would seem 

 to be due to the absorption, and, possibly also, to the interference of light on a 

 scale such as Astronomy alone exhibits; of the light, viz., reflected from Jupiter 

 and meeting that of the satellite. 



(a) Aside from all that, however, the phenomenon, or rather phenomena, in 

 question would seem to be consistent with the conclusion of a coincidence in the 

 times of rotation and revolution; for the appearance of the satellite, in the course 

 of its transit, as a black spot has, within moderate intervals of succession, recurred 

 when the satellite had returned to a like position in its orbit around its primary.^ 



(b) Admitting the absorption already indicated; then, instructed by the revela- 

 tions of the spectroscope, we may regard it as probable that the satellite must be 

 colder than its primary.^ 



(c) This last would happen — indeed we would have a reason for it — if the satel- 

 lite, like the moon, had little or no atmosjihere. 



(d) All these analogies would be quite consistent with the hypothesis that all 

 these satellites (including the moon) had been similarly condensed from the nebulous 

 state, and then subjected to the stringent conditions which prevail in satellite 

 systems. The loss of atmosphere is one of the supposable consequences of those 

 stringent conditions; as indeed M. Laplace has intimated, when after stating the 

 distance at which the attractive force of the earth is in equilibrium with that of 

 the moon, he adds: "If at this distance, the primitive atmosphere of the moon had 

 not been deprived of all elasticity, it would be carried to the earth, which could 

 thus draw it to itself, (aspirer). This is, perhaps, the reason why the moon's 

 atmosphere is so nearly insensible."^ 



Of the Zodiacal Light, 



(70) As to the region of the zodiacal light; M. Laplace, in speaking of the 

 atmosphere of the sun, says: "The atmosphere at the equator cannot extend 

 beyond the point where the centrifugal force exactly balances gravitation ; for it is 

 manifest that beyond that limit the fluid must itself be dissipated. As respects the 

 sun, this point is at the distance from his centre of the radius of the orbit of a 

 planet which would complete its revolution in a time equal to that of the rotation 

 of the sun. The atmosphere of the sun, therefore, does not extend even to the 



' But the conclusion is not a necessary one. M. Secchi makes the time of rotation shorter than 

 that. 



- Some recent observations of Jnpiter seem to indicate that the planet itself is highly heated — 

 possibly even to the extent of being locally self-luminous. The color of the belts and its variations 

 together seem consistent with all this. [Witness the exquisitely beautiful chromo-lithographs accom- 

 panying the Earl of Rosse's paper in No. 5. of vol. XXXIY, of the Proceedings of the Royal Astro- 

 nomical Society ; and Mr. John Browning's very beautiful representations of similar phenomena in 

 No. 9 of the same volume. Also M, Taechini's very remarkable diagram of Jupiter's appearance ; 

 with his explanations (Comptes Revdus, tome LXXVI, p. 423).] 



' Conclusion of Chap. X, of Book lY, of the Systeme du Monde. For a discussion and an expla- 

 nation of the various phenomena here in question, see two communications, by the author of this 

 paper, to the Aslronomische NacJirichten , Nos. 1986 and 2012. 



