40 POPULAR LECTURES AND ADDRESSES. 



Delaunay soon after verified this result, and about 

 the beginning of 1866 suggested that the true 

 explanation may be the retardation of the earth's 

 rotation by tidal friction. Using the hypothesis 

 that the cause of the discrepancy is retardation by 

 tidal friction, and allowing for the consequent 

 retardation of the moon's mean motion, Adams, 

 in an estimate which he has recently worked out 

 in conjunction with Professor Tait and myself, 

 found, on a certain assumption as to the propor- 

 tion of retardations due to the moon and the sun- 

 that 22 seconds of time is the error by which 

 the earth would in a century get behind a 

 thoroughly perfect clock rated at, the beginning 

 of the century. 



1 8. Thus the most probable result that phy- 

 sical astronomy gives us up to the present time 

 is that the earth is not an accurate chronometer, 

 but, on the contrary, is getting slower and slower, 

 if tested by a truly perfect clock a clock as good 

 as an astronomical clock ought just now to be, 

 and that is at least 200 times as good as astro- 

 nomical clocks are because astronomical clocks 



