CERTAIN HARMONIES OF THE SOLAR SYSTEM. 49 



this is not due to chance. The theory of gravitation causes the infinity of this 

 unlikelihood to disappear, by showing us that, for the existence of the phenomenon, 

 it would be sufficient that the motions should have been very little different at 

 their origin.^ Then the attraction of the planet established between them a perfect 

 equality; but at the same time gave rise to a periodic oscillation of the axis of the 

 satellite directed toward the planet, the extent of it dependant on the primitive 

 difference of the two motions. The observations of Mayer on the libration of the 

 moon and those which MM. Bouvard and Nicollet made with reference to this 

 matter, at my request, have failed to make known this oscillation. The difference 

 on which it depends must, therefore, have been very small; which indicates, with 

 extreme probability, a special cause which first kept this difference within the very 

 narrow limits within which the attraction of the planet could establish an equality 

 between the mean motions of rotation and revolution, and which afterwards 

 destroyed the oscillation which this equality had originated. Both these effects 

 result from our hypothesis. For it will be understood that the moon in the state 

 of vapors, formed, because of the powerful attraction of the earth, an elongated 

 spheroid the major axis of which must be incessantly directed towards that planet, 

 from the facility with which vapors yield to the smallest force which animates 

 them. The terrestrial attraction continuing to act in the same manner when the 

 moon was in a fluid state, at length, in approximating incessantly the two motions 

 of this satellite, caused them to fall within limits such that their rigorous equality 

 began to be established. Afterwards this attraction must, little by little, have 

 annihilated the oscillation which this equality produced in the axis of the spheroid 

 directed towards the earth." 



" It is thus that the fluids which covered this planet" have destroyed, by their 

 friction and their resistance, the primitive oscillations of its axis of rotation, which 

 now is subjected but to the nutation resulting from the actions of the sun and the 

 moon. It will be readily seen that the equality of motions of rotation and revolu- 

 tion would present an obstacle to the formation of rings and of secondary satellites 

 from the atmospheres of those bodies. Accordingly, observation has thus far 

 indicated none such." 



(69) It is claimed that the other satellites of the planetary system resemble the 

 moon in the coincidence of their times of rotation and revolution; and thus pre- 

 senting always nearly the same side of any satellite toward its primary. This is 

 inferred from special vicissitudes of the light of the satellites recurring Avhen they 

 have again arrived at the same positions in their orbits around their respective 

 primaries. 



Nor is that all. Among the remarkable phenomena presented by satellites is 

 that of their seeming loss of light ; all Jupiter's satellites, having, at times, been 

 seen to transit the disk of the planet, appearing, in whole or in part, as dark instead 

 of brigJit spots; and that sometimes after having Jirst appeared tright and then 

 dusky. 



^ 111 this connexion, see, again, Note on p. 22. ^ The Earth. 



7 January, 1875. 



