THE EVOLUTION OF SATELLITES. 121 



vision, announced as the result of bis observation tliat both Mercury 

 and Venus rotate only once in their respective years, aud that each of 

 them always presents the same face to the sun. These conclusions 

 have recently been confirmed by Mr. Percival Lowell from observations 

 made in Arizona, and are exactly conformable to our theoretical expec- 

 tation. While it is not easy to see how these astronomers can have 

 been mistaken, yet it is proper to note that others possessing appar- 

 ently equal advantages have failed to detect ,the markings on the 

 planets. Accepting, however, this conclusion, we have the planets 

 Mercury and Venus, the satellites of the earth, and Jupiter and Saturn 

 presenting evidence favorable to the theory of tidal friction, while the 

 case of the Martian system is yet more striking as an instance of an 

 advanced stage in evolution. 



It would need another article to discuss the various aspects of this 

 theory in relation to the histories. of the planets and of their satellites. 

 I may say, however, that it serves in great measure to explain the fact 

 that the earth is tilted over with reference to its orbit round the sun, 

 and that it throws light on the fact that the plane of the moon's orbit 

 is not coincident with that of the earth. The same cause may also be 

 proved to tend toward making the orbit of a satellite eccentric, and it is 

 this effect of tidal friction to which Mr. See has appealed. I shall not 

 here repeat his arguments, but in Section IV I will make some com- 

 ments on his theories. 



With respect to the efficacy of tidal friction as a factor in the evolu- 

 tion of the earth, it is not too much to say that if we postulate a planet 

 consisting partly or wholly of molten lava, and rapidly rotating about 

 an axis at right angles to its orbit around the sun, and if that planet 

 have a single satellite, revolving nearly as rapidly as the planet rotates, 

 then a system will necessarily be evolved in time closely resembling 

 our own. 



A theory reposing on true causation, which brings into quantitative 

 correlation the lengths of the present day and month, the obliquity of 

 the ecliptic, and the eccentricity and inclination of the moon's' orbit, 

 must, I think, have strong claims to acceptance. 



IV. 



There are in the heavens many pairs of closely neighboring stars 

 which revolve about each other under the influence of their mutual 

 gravitation. The fact that both members of a pair are visible seems to 

 indicate that they do not differ widely in mass, and it is also a striking 

 peculiarity of these binary systems that the orbit is commonly very 

 eccentric. The distinction is great between our solar system, with its 

 large central mass and infinitesimal planets moving in nearly circular 

 orbits, and these binary systems, and hence there is abundant reason 

 for supposing that the course of evolution has been very different in 

 the two cases. 



