12 THE AGE OF THE EARTH 



must be considerably greater than the external. Geo- 

 logists have argued from the amount of folding to which 

 the crust has been subjected that cooling must have 

 taken place to a greater depth than 1 20 miles, as assumed 

 in Lord Kelvin's argument. Professor Perry's assump- 

 tion would involve cooling to a much greater depth. 



Professor Perry's conclusion that the age of the 

 habitable earth is lengthened by increased conductivity 

 is the very reverse of that to which we should be led 

 by a superficial examination of the case. Professor Tait, 

 indeed, in the letter to which I have already alluded, 

 has said : ' Why, then, drag in mathematics at all, 

 since it is absolutely obvious that the better conductor 

 the interior in comparison with the skin, the longer 

 ago must it have been when the whole was at 7,000 F. : 

 the state of the skin being as at present ? ' Professor 

 Perry, in reply, pointed out that one mathematician 

 who had refuted the tidal retardation argument 1 had 

 assumed that the conditions described by Professor Tait 

 would have involved a shorter period of time. And 

 it is probable that Lord Kelvin thought the same ; for 

 he had assumed conditions which would give the result 

 so he believed at the time most acceptable to the 

 geologist and biologist. Professor Perry's conclusion 

 is very far from obvious, and without the mathematical 

 reasoning would not be arrived at by the vast majority 

 of thinking men. 



The ' natural man ' without mathematics would say, 

 so far from this being ' absolutely obvious ', it is quite 

 clear that increased conductivity, favouring escape of 

 heat, would lead to more rapid cooling, and would make 

 Lord Kelvin's age even shorter. 



The argument can, however, be put clearly without 

 mathematics, and, with Professor Perry's help, I am 

 able to state it in a few words. Lord Kelvin's assump- 

 tion of an earth resembling the surface rock in its 

 relations to heat leads to the present condition of things, 

 namely, a surface gradient of i F. for every fifty feet, 

 in 100,000,000 years, more or less. Deeper than 

 1 Rev. M. H. Close in R. Dublin Soc., February, 1878. 



