ON GEOLOGICAL TIME. 69 



pocket-watch), the only chronometer by which we 

 can check the earth is one which goes much worse 

 the moon. The marvellous skill and vast labour 

 devoted to the lunar theory by the great physical 

 astronomers Adams and Delaunay, seem to have 

 settled that the earth has really lost in a century 

 about ten seconds of time on the moon corrected 

 for all the perturbations which they had taken into 

 account. M. Delaunay has suggested that the 

 true cause may be tidal friction, which he has 

 proved to be probably sufficient by some such 

 estimate as the preceding. 1 But the many dis- 

 turbing influences to which the earth is exposed 

 render it a very untrustworthy time-keeper. For 

 instance, let us suppose ice to melt from the polar 

 regions (20 round each pole, we may say) to the 

 extent of something more than a foot thick, enough 

 to give I ' i foot of water over those areas, or '066 

 of a foot of water if spread over the whole globe, 

 which would in reality raise the sea-level by only 

 some such almost undiscoverable difference as J 

 of an inch, or an inch. This, or the reverse, which 



1 It seems hopeless, without waiting for some centuries, to arrive 

 at any approach to an exact determination of the amount of the actual 

 retardation ef the earth's rotation by tidal friction, except by exten- 

 sive and accurate observation of the amounts and times of the tides 

 on the shores of continents and islands in all seas, and much 

 assistance from true dynamical theory to estimate these elements 

 all over the sea. But supposing them known for every part of the 

 sea, the retardation of the earth's rotation could be calculated by 

 quadratures. 



