218 LAY SERMONS, ESSAYS, AND REVIEWS. [xi. 



If tidal retardation can be thus checked and overthrown by 

 other temporary conditions, what becomes of the confident 

 assertion, based upon the assumed uniformity of tidal retarda 

 tion, that ten thousand million years ago the earth must have 

 been rotating more than twice as fast as at present, and, there 

 fore, that we geologists are &quot; in direct opposition to the principles 

 of Natural Philosophy&quot; if we spread geological history over 

 that time ? 



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

 &quot;An article, by myself, published in Macmillan 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 with a stone, or a piece of matter, only taking into 

 account the sun s dimensions, which showed it to be possible 

 that the sun may have already illuminated the earth for as many 

 as one hundred million years, but at the same time rendered 

 it almost certain that he had not illuminated the earth for five 

 hundred millions of years. The estimates here are necessarily 

 very vague ; but yet, vague as they are, I do not know that it 

 is possible, upon any reasonable estimate founded on known 

 properties of matter, to say that we can believe the sun 

 has really illuminated the earth for five hundred million 

 years.&quot; x 



I do not wish to &quot; Hansardize &quot; Sir William Thomson by 

 laying much stress on the fact that, only fifteen years ago, he 

 entertained a totally different view of the origin of the sun s 

 heat, and believed that the energy radiated from year to year 

 was supplied from year to year a doctrine which would have 

 suited Hutton perfectly. But the fact that so eminent a 

 physical philosopher has, thus recently, held views opposite to 

 those which he now entertains, and that he confesses his own 

 estimates to be &quot;very vague,&quot; justly entitles us to disregard 

 those estimates, if any distinct facts on our side go against them. 



1 Loc. cit , p. 20. 



