XL] GEOLOGICAL REFORM. 217 



In the present instance it appears to be admitted : 



1. That it is not absolutely certain, after all, whether the 

 moon s mean motion is undergoing acceleration, or the earth s 

 rotation retardation. 1 And yet this is the key of the whole 

 position. 



2. If the rapidity of the earth s rotation is diminishing, it is 

 not certain how much of that retardation is due to tidal friction, 

 how much to meteors, how much to possible excess of 

 melting over accumulation of polar ice, during the period 

 covered by observation, which amounts, at the outside, to not 

 more than 2,600 years. 



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 circumstances, 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 during, 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 retarda 

 tion will only account for a retardation of 22&quot; in a century, 

 or y 2 ^ (say -J-) of a second per annum. 



Thus, assuming that the accumulation of polar ice since the 

 Miocene epoch has only been sufficient to produce ten times the 

 effect of a coat of ice one foot thick, we shall have an accelera 

 ting cause which covers all the loss from tidal action, and leaves 

 a balance of ^ a second per annum in the way of acceleration. 



1 It will be understood that I do not wish to deny that the earth s 

 rotation may be undergoing retardation. 



