42 POPULAR LECTURES AND ADDRESSES. 



in the moon's motion are very accurately known. 

 The moon's rotation round the earth is as it were 

 a clock hand going round in about 29 days, and 

 the earth is as it were the hand of another clock 

 going round in 24 hours. The only timekeeper 

 by which we can at present test the accuracy of 

 the earth's motion is the moon. Imperfect as 

 the moon is, an error has, you see, been discovered 

 in the earth as a time-keeper, on reference to the 

 moon. Consider that fact, and see whether it 

 justifies the statement I have referred to by Play- 

 fair in his Illustrations of Hutton's theory, that 

 there is nothing in the motions of the heavenly 

 bodies that tends to their own dissolution or to 

 a permanent alteration of the existing state of 

 things. For instance, no resistance tending to 

 stop the progress of the earth ! 



19. Now, if the earth is losing angular velocity 

 at that great rate, at what rate might it have 

 IK-CM rotating a thousand million years ago? It 

 must have been rotating faster by one-seventh 

 part than at present, and the centrifugal force 

 imi-t have turn LM'eater in llie latio of the square 



