50 LIGHT SCIENCE FOR LEISURE HOURS. 



minuteness of the discrepancy about which all the coil 

 has been made. 



Suppose that, just in front of our moon, a false moon 

 exactly equal to ours in size and appearance (see note 

 at the end of this paper) were to set off with a motion 

 corresponding to the present motion of the moon, save 

 only in one respect namely, that the false moon's 

 motion should not be subject to the change we are 

 considering, termed the acceleration. Then one hun- 

 dred years would elapse before our moon would fairly 

 begin to show in advance. She would, in that time, 

 have brought only one-one-hundred-and-fiftieth part of 

 her breadth from behind the false moon. At the end 

 of another century, she would have gained four 

 times as much ; at the end of a third, nine times as 

 much : and so on. She would not fairly have cleared 

 her own breadth in less than twelve hundred years. 

 But the whole of this gain, minute as it is, is not left 

 unaccounted for by our modern astronomical theories. 

 Half the gain is explained, the other half remains to 

 be interpreted ; in other words, the moon travels fur- 

 ther by about half her own breadth in twelve centuries 

 than she should do according to the lunar theory. 



But in this difficulty, small as it seems, we are not 

 left wholly without resource. We are not only able 

 to say that the discrepancy is probably due to a 

 gradual retardation of the earth's rotation-movement, 

 but we are able to place our finger on a very sufficient 

 cause for such a retardation. One of the most firmly 

 established principles of modern science is this that 

 where work is done, force is, in some way or other, 



