MOTIONS OF THE SATELLITES. 275 



ments. Granting, however, that they were mistaken, then 

 is still the fact that the diameter of Venus is variously esti- 

 mated ; and that a very small change in the data would 

 make the fraction less instead of greater than that of the 

 Earth. But admitting the discrepancy, we think that this 

 correspondence, even as it now stands, is one of the strong- 

 est confirmations of the Nebular Hypothesis.* 



Certain more special peculiarities of the satellites must 

 be mentioned as suggestive. One of them is the relation 

 between the period of revolution and that of rotation. 

 No discoverable purpose is served by making the Moon go 

 round its axis in the same time that it goes round the 

 Earth : for our convenience, a more rapid axial motion 

 would have been equally good ; and for any possible inhab- 

 itants of the Moon, much better. Against the alternative 

 supposition, that the equality occurred by accident, the 

 probabilities are, as Laplace says, infinity to one. But to 

 this arrangement, which is explicable neither as the result 

 of design nor of chance, the Nebular Hypothesis furnishes 

 a clue. In his " Exposition du Systeme du Monde," La- 

 place shows, by reasoning too detailed to be here repeated, 

 that under the circumstances such a relation of movements 

 would be likely to establish itself. 



Among Jupiter's satellites, w^hich severally display these 

 same synchronous movements, there also exists a still more 

 remarkable relation. "If the mean angular velocity of the 

 first satellite be added to twice that of the third, the sum 



* Since this essay was published, the data of the above calculationa 

 have been changed by the discovery that the Sun's distance is three mil- 

 lions of miles less than was supposed. Hence results a diminution in his 

 estimated mass, and in the masses of the planets (except the Earth and 

 Moon). No revised estimate of the masses having yet been published, the 

 table is re-printed in its original form. The diminution of the masses to 

 the alleged extent of about one-tenth, does not essentially alter the rela 

 tions above pointed out. 



