834 GEOLOGICAL REFORM X 



3. The effect of a different distribution of land 

 and water in modifying the retardation caused by 

 tidal friction, and of reducing it, under some cir- 

 cumstances, to a minimum, does not appear to be 

 taken into account. 



4. During the Miocene epoch the polar ice was 

 certainly many feet thinner than it has been dur- 

 ing, or since, the Glacial epoch. Sir W. Thomson 

 tells us that the accumulation of something more 

 than a foot of ice around the poles (which implies 

 the withdrawal of, say, an inch of water from the 

 general surface of the sea) will cause the earth 

 to rotate quicker by one-tenth of a second per 

 annum. It would appear, therefore, that the earth 

 may have been rotating, throughout the whole 

 period which has elapsed from the commencement 

 of the Glacial epoch down to the present time, one, 

 or more, seconds per annum quicker than it 

 rotated during the Miocene epoch. 



But, according to Sir W. Thomson's calculation, 

 tidal retardation will only account for a retardation 

 of 22" in a century, or y%%- (say i) of a second per 

 annum. 



Thus, assuming that the accumulation of polar 

 ice since the Miocene epoch has only been suffi- 

 cient to produce ten times the effect of a coat of 

 ice one foot thick, we shall have an accelerating 

 cause which covers all the loss from tidal action, 

 and leaves a balance of 4 of a second per annum 

 in the way of acceleration. 



