292 DISCUSSIONS IF COSMOLOGY. 



purely physical nature. The first is based on the age 

 of the sun's heat ; the second, on the tidal retardation 

 of the earth's rotation ; and the third, on the secular 

 cooling of the earth. Several years ago I ventured to 

 point out some difficulties which appear to me to beset 

 these arguments. They are as follows : — 



Argument from the Age of the Sun's Heat. — It will 

 be obvious that, if what has already been advanced 

 in regard to the origin of the sun's heat be correct, it 

 will follow that the argument for the recent age of the 

 earth, based upon the assumption that the sun could 

 have derived its store of heat only from the condensa- 

 tion of its mass, must be wholly abandoned, and that, 

 in so far as this argument is concerned, there is no 

 known limit to the amount of heat which the sun may 

 have possessed, or to the time during which it may 

 have illuminated the earth. 



Argument from Tidal Retardation. — It is well 

 known that, owing to tidal retardation, the rate of the 

 earth's rotation is slowly diminishing ; and it is there- 

 fore evident that, if we go back for many millions of 

 years, we reach a period when the earth must have 

 been rotating much faster than now. Sir William's 

 argument is* that, had the earth solidified several 

 hundred million of years ago, the flattening at the 

 Poles and the bulging at the Equator would have been 

 much greater than we find them to be. Therefore, 

 because the earth is so little flattened, it must have 

 been rotating, when it became solid, at very nearly 

 the same rate as at present. And as the rate of rota- 

 tion is becoming slower and slower, it cannot be 

 so many millions of years since solidification took 

 place. A few years ago I ventured to point out"f* 



* Trans. Geol. Soc. of Glasgow, vol. iii., p. 1. 



f " Nature," August 21, 1872. ' Climate and Time,' p. 335. 



