THEOKIES OF TEMPERATURE CHANGES. 261 



those which look to a progressive change in one direction, one idea may be 

 mentioned, which for some time had great weight with geologists. It is to 

 the effect that the earth being itself a cooling body, this diminution of tem- 

 perature of the whole mass of the globe has also been manifested in its 

 effect on climate. The idea might seem, at first thought, a plausible one, for 

 since the theory — if such it may be called — of a cooling earth is one almost 

 universally accepted by geologists, it seems natural that it should be con- 

 sidered as connected with an analogous condition of the surface. 



The question whether climate has during the geological ages been affected 

 by the earth's cooling is one, no doubt, of considerable difficulty and compli- 

 cation ; but it appears to be one capable of being solved by the aid of mathe- 

 matics. More than one eminent man has attacked it, beginning witli Fourier, 

 and the result is most decidedly against the possibility of accounting for 

 climatic changes on the surface by reference to the condition of the interior 

 of the globe. 



Sir William Thomson has expressed himself most emphatically on this 

 point. He says: "Underground heat, though certainly greater in the earlier 

 geological times, cannot, as I have shown elsewhere,* have ever sensibly 

 influenced the climate. Ten, twenty, thirty times the present rate of aug- 

 mentation of temperature downwards could not raise the surface temperature 

 of the earth and air in contact with it by more than a small fraction of a 

 degree Fahrenheit. The earth might be a globe of white-hot iron covered 

 with a crust of rock 2,000 feet, or there might be an ice-cold temperature 

 within 30 feet of the surface, yet the climate could not on that account be 

 sensibly different from what it is, or the soil be sensibly more or less genial 

 than it is for the roots of trees or smaller plants. Yet underground heat is 

 the hypothesis which has been most complacently dealt with by geologists 

 to account for the warmer climates of ancient times."! 



It is evident that the idea of connecting the phenomena of the internal 

 heat of the globe with terrestrial cliuuates, whether of the present, or of past 

 geological ages, must be entirely abandoned, as it has been by most writers 

 on this subject. The hypothesis cannot be allowed to stand as one even of 

 the possible theories of climatic change. 



There is a cause to which can be ascribed the gradual refrigeration of the 



* See Sir \V. Tliomsou's great paper " On tUe Secular Cooling of the Earth," in the Transactions of the Eoyal 

 SoL-iety of Edinliurgh, Vol. X.XIII. p. lii?. 



+ TiiiiiaMctiona of the Geological Society of Glasgow, Vol. V. p. 250. 



