OUR CHIEF TIME-PIECE LOSING TIME. 43 



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 hundred 

 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 gai n 

 is explained, the other half remains to be interpreted ; 

 in other words, the moon travels further 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, expended. The 

 doing of work may show itself in a variety of ways 



