ON GEOLOGICAL TIME. 21 



reduce the relative motions of the earth and 

 moon to that condition'. However, it is satisfac- 

 tory to know that we do not need to base a con- 

 clusion on so excessively general terms of the 

 theory of energy as those. It is easy to see that 

 the mutual action between the moon and the earth 

 must tend, in virtue of the tides, to diminish the 

 rapidity of the earth's rotation, and increase 

 the moment of the moon's motion round the 

 earth. 



8. " The tidal spheroid," you must understand, 

 is not a reality, because the waters do not cover the 

 whole earth, as we are here on terra firmato know. 

 But there is a perfectly definite surface, being an 

 elliptic spheroid calculated by mathematical rule, 

 which is such that if it were the outer boundary of 

 a distribution of water over a globe perfectly 

 covered with water, this mass of water would 

 exercise to an extremely close approximation the 

 same force upon any distant particle of matter, 

 and experience the same reacting force, as our 

 lidally disturbed waters really do. That is what 

 is properly called the tidal spheroid. It averages, 



