PLANETARY EVOLUTION 



Thus the moon appears to afford an example 

 of the universal death which in an unimagina- 

 bly remote future awaits all the members of 

 the solar system. It then becomes an interest- 

 ing question whether this cosmic death will be 

 succeeded by Dissolution, — that is, by the re- 

 diffusion of the matter of which the system is 

 composed, and by the reabsorption of the lost 

 motion or its equivalent. We shall find it diffi- 

 cult to escape the conclusion that such a disso- 

 lution must ultimately take place. 



If, along with the dissipation of molecular 

 motion already described, the planets are also 

 losing that molar motion to which is due their 

 tangential momentum, this loss of motion must 

 ultimately bring about their reunion with the 

 sun. Upon such a point direct observation can 

 help us but little ; but there are two opposing 

 considerations, of a force which none will deny, 

 and based on facts which none can dispute. 

 Two sets of circumstances are struggling for 

 the mastery, — the one set tending to drive the 

 planets farther and farther away from the centre 

 of the system, the other set tending to draw 

 them towards the centre. Let us see which set 

 must prevail in the end. 



Hitherto, in all probability, the first set of 

 circumstances has had the advantage. There 

 is little reason to doubt that all the planetary 

 orbits, both primary and secondary, are some- 



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