X GEOLOGICAL REFORM 335 



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 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 ' Macmillan's Magazine ' for March 1862, on the 

 age of the sun's heat, explains results of investiga- 

 tion 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 neces- 

 sarily 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 ilium • 

 inated the earth for five hundred million years." ^ 

 * I/^. ciL p. 20. 



