250 LAY SERMONS, ADDRESSES, AND REVIEWS. [xi. 



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, acording to Sir W. Thomson's calculation, tidal 

 retardation will only account for a retardation of 22" in 

 a century, or 2 2 (say ■§) 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 accelerating cause which covers 

 all the loss from tidal action, and leaves a balance 

 of -f- a second per annum in the way of acceleration. 



If tidal retardation can be thus checked and over- 

 thrown by other temporary conditions, what becomes 

 of the confident assertion, based upon the assumed uni- 

 formity of tidal retardation, that ten thousand million 

 years ago the earth must have been rotating more than 

 twice as fast as at present, and, therefore, that we 

 geologists are " in direct opposition to the principles 

 of Natural Philosophy " if we spread geological history 

 over that time ? 



II. The second argument is thus stated by Sir W. 

 Thomson :-— "An article, by myself, published in ' Mac- 

 millan's Magazine' for March 1862, on the age of the 

 sun's heat, explains results of investigation into various 

 questions as to possibilities regarding the amount of heat 

 that the sun could have, dealing with it as you would 



