October 30, 1896.] 



SCIENCE. 



631 



tore, the rate at wliicli heat can travel 

 would be immensely increased by convec- 

 tion currents, and the age would have to be 

 correspondingly lengthened. If, further- 

 more, such conditions, although not obtain- 

 ing now, did obtain in past times, they will 

 have operated in the same direction. 



Prof. Tait, in his letter to Prof. Perry 

 (published in Nature of January 3, 1895), 

 takes the entirely indefensible position that 

 the latter is bound to prove the higher in- 

 ternal conductivity. The obligation is all 

 on the other side, and rests with those who 

 have pressed their conclusions hard and 

 carried them far. These conclusions have 

 been, as Darwin found them, one of our 

 ^ sorest troubles ;' but when it is admitted 

 that there is just as much to be said for an- 

 other set of assumptions leading to entirely 

 different conclusions our troubles are at an 

 end, and we cease to be terrified by an ar- 

 ray of symbols, however unintelligible to 

 lis. It would seem that Prof. Tait, with- 

 out, as far as I can learn, publishing any 

 independent calculation of the age of the 

 earth, has lent the weight of his authority 

 to a period of 10 million years, or half of 

 Xiord Kelvin's minimum. But in making 

 ihis suggestion he apparently feels neither 

 interest nor responsibility in establishing 

 the data of the calculations which he bor- 

 rowed to obtain therefrom a very different 

 result from that obtained by their author. 



Prof. Perry's object was not to substitute 

 a more correct age for that obtained by 

 Lord Kelvin, but rather to show that the 

 data from which the true age could be cal- 

 culated are not really available. We ob- 

 tain different results by making different 

 assumptions, and there is no sufficient evi- 

 dence for accepting one assumption rather 

 than another. Nevertheless, there is some 

 evidence which indicates that the interior 

 of the earth in all probability conducfcs bet- 

 ter than the surface. Its far higher density 

 is consistent with the belief that it is rich 



in metals, free or combined. Prof. Schus- 

 ter concludes that the internal electric con- 

 ductivity must be considerably greater than 

 the external. Geologists 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 120 

 miles, as assumed in Lord Kelvin's argu- 

 ment. Prof. Perry's assumption would in- 

 volve cooling to a much greater depth. 



Prof. Perry's conclusion that the age of 

 the habitable earth is lengthened by in- 

 creased conductivity is the very reverse of 

 that to which we should be led by a super- 

 ficial examination of the case. Prof. Tait, 

 indeed, in the letter to which I have al- 

 ready alluded, has said : " Why, then, drag 

 in mathematics at all, since it is absolutely 

 obvious that the better conductor the inte- 

 rior 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?" Prof. Perry, in reply, pointed 

 out that one mathematician who had refuted 

 the tidal retardation argument* had as- 

 sumed that the conditions described by 

 Prof. 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. 

 Prof. 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 con- 

 ductivity, favoring 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 Prof. Perry's 



*Eev. M. H. Close in B. Dublin Soc, February 

 1878. 



