POPULAR LECTURES AND ADDRESSES. 



resistance destroys. The tidal deformation of the 

 water on the earth tends, on the whole, therefore, 

 to retard the moon's angular motion in her orbit ; 

 but (by the accompanying augmentation of her 

 distance from the earth) to increase the moment 

 of Jier motion round the earth's centre and the 

 ultimate .tendency so far as the earth's rotation 

 is concerned must be to make the earth keep 

 always the same face to the moon. 



15. It -may be remarked, in passing, that the 

 corresponding tendency has probably already had 

 effect on the moon itself. The moon always turns 

 the same face to the earth. If the moon were now 

 a liquid mass, there would be enormous tides in it. 

 The friction in that fluid would cause the moon 

 to tend to turn the same face towards the earth : 

 and we find the moon turns the same face always 

 to the earth. It seems almost inevitable to our 

 minds, constituted as they are to connect possible 

 cause and real effect, and say that a possible cause 

 is a real cause ; and thus to believe the reason why 

 the moon turns always the same side to us is 

 because it was once a liquid mass which ex- 



