324 Mr. G. H. Darwin. Tidal Friction of a Planet, fyc. [Jan. 20, 



they cannot be traced back to an origin almost in contact with the 

 present surfaces of their planets, as was shown in previous papers to 

 be probably the case with the moon and earth. 



The numerical values spoken of above exhibit a very striking 

 difference between the condition of the earth and moon and that of 

 these other planets, and it may therefore be admitted that their modes 

 of evolution have also differed considerably. 



The part played by tidal friction in the evolution of planetary masses 

 is then discussed. 



A numerical comparison is made of the relative efficiency of solar 

 tidal friction in reducing the rotational momentum and the rotation of 

 the several planets. It is found that the efficiency as regards the 

 rotation is nearly the same for Mars and for the earth, notwithstanding 

 the greater distance of the former from the earth. This point is 

 important with reference to the rapid revolution of the inner satellite 

 of Mars, and confirms the explanation of this fact, which has been 

 offered in a previous paper. 



The numbers expressive of the relative efficiency of solar tidal 

 friction are of course very much smaller for the more remote planets 

 than for the nearer ones, but they must not be supposed to represent 

 the total amount of rotation destroyed by solar tidal friction, because 

 the exterior planets must be presumed to have existed much longer than 

 the interior ones. Nevertheless the disproportion between the numbers 

 is so great that it must be held that the influence of solar tidal fric- 

 tion on Jupiter and Saturn has been considerably less than on the 

 nearer planets. 



The manner in which tidal friction and the contraction of a plane- 

 tary mass would work together is then considered, and it is found to 

 be probable that tidal friction was a more important cause of change 

 when the masses were Jess condensed than it is at present ; thus the 

 present rate of action of solar tidal friction is not to be taken as a 

 measure of what has existed in all past time. 



This discussion leads the author to assign a cause for the observed 

 distribution of satellites in the solar system. For if, as the nebular 

 hypothesis supposes, satellites are formed when instability is produced 

 by the acceleration of rotation accompanying contraction, then the 

 epochs of instability would recur more rarely if tidal friction were 

 operative than without it ; and if tidal friction were sufficiently 

 powerful, an epoch of instability would never occur. 



The efficiency of solar tidal friction diminishes as we recede from 

 the sun, and therefore planets near the sun should have no satellites, 

 and the number of satellites should increase for the remoter planets. 

 This is the observed condition of the solar system. 



This theoretical view is also shown to explain how the earth and 

 moon came to differ from the other planets in such a manner as to 



