THE COOLING OF THE EARTH n 



by Lord Kelvin. If the interior of the earth be fluid 

 or contain fluid in a honeycomb structure, the rate at 

 which heat can travel would be immensely increased 

 by convection currents, and the age would have to be 

 correspondingly lengthened. If, furthermore, such con- 

 ditions, although not obtaining now, did obtain in past 

 times, they will have operated in the same direction. 



Professor Tait, in his letter to Professor Perry 

 (published in Nature of January 3, 1895), takes the 

 entirely indefensible position that the latter is bound 

 to prove the higher internal conductivity. The obliga- 

 tion 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 

 another set of assumptions leading to entirely different 

 conclusions, our troubles are at an end, and we cease to 

 be terrified by an array of symbols, however unintelli- 

 gible to us. It would seem that Professor Tait, without, 

 as far as I can learn, publishing any independent calcula- 

 tion of the age of the earth, has lent the weight of his 

 authority to a period of ten million years, or half of 

 Lord Kelvin's minimum. But in making this suggestion 

 he apparently feels neither interest nor responsibility in 

 establishing the data of the calculations which he 

 borrowed to infer therefrom a very different result 

 from that obtained by their author. 



Professor 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 

 calculated are not really available. We obtain different 

 results by making different assumptions, and there is 

 no sufficient evidence for accepting one assumption 

 rather than another. Nevertheless, there is some 

 evidence which indicates that the interior of the earth 

 in all probability conducts better than the surface. 

 Its far higher density is consistent with the belief 

 that it is rich in metals, free or combined. Professor 

 Schuster concludes that the internal electric conductivity 



