236 A CENTURY'S PROGRESS IN ASTRONOMY. 



He then remarks that probably the formation 

 of the Moon was retarded, and in the case of 

 Mercury and Venus, solar tidal friction pre- 

 vented satellite formation. This explains why 

 Mercury and Venus have no satellites, the 

 Earth only one, Mars two, while the exterior 

 planets have each several satellites. 



The theory of tidal friction was extended in 

 1892 to the explanation of the double stars 

 by the American astronomer, See. See showed 

 by mathematical calculation the effects of tidal 

 friction in shaping the eccentric orbits of the 

 binary stars, the course of evolution being traced 

 from double stars, revolving almost in contact, 

 which the spectroscope reveals, to the tele- 

 scopic doubles. See's researches have done much 

 to supplement those of Darwin, who considers 

 that there are two types of cosmical evolution, 

 the Laplacian, and the " second " or lunar 

 type. 



Lowell, in his work on * The Solar System ' 

 (1903), adds six congruities to those remarked 

 by Laplace and his successors. These are, "All 

 the satellites turn the same face to their 

 primaries (so far as we can judge) ; Mercury, 

 and probably Venus, do the same to the Sun ; 

 one law governs position and size in the Solar 

 System and in all the satellite systems ; orbital 



