4 Prof. W. Thomsou on the Secular Cooling of the Earth. 



of the "Uniformitarian" school, who in other respects have taken 

 a profoundly philosophical view of their subject, have argued in 

 a most fallacious manner against hypotheses of violent action in 

 past ages. If they had contented themselves with showing that 

 many existing appearances, although suggestive of extreme vio- 

 lence and sudden change, may have been brought about by long- 

 continued action, or by paroxysms not more intense than some 

 of which we have experience within the periods of human history, 

 their position might have been unassailable, and certainly could 

 not have been assailed except by a detailed discussion of their 

 facts. It would be a very wonderful, but not an absolutely in- 

 credible result, that volcanic action has never been more violent 

 on the whole than during the last two or three centuries; but 

 it i§ as certain that there is now less volcanic energy in the 

 whole earth than there was a thousand years ago, as it is that 

 there is less gunpowder in a "Monitor" after she has been seen 

 to discharge shot and shell, whether at a nearly equable rate or 

 not, for five hours without receiving fresh supplies, than there 

 was at the beginning of the action. Yet this truth has been 

 ignored or denied by many of the leading geologists of the pre- 

 sent day, because they believe that the facts within their province 

 do not demonstrate greater violence in ancient changes of the 

 earth's surface, or do demonstrate a nearly equable action in all 

 periods. 



9. The chemical hypothesis to account for underground heat 

 might be regarded as not improbable, if it was only in isolated loca- 

 lities that the temperature was found to increase with the depth ; 

 and, indeed, it can scarcely be doubted that chemical action exer- 

 cises an appreciable influence (possibly negative, however) on the 

 action of volcanoes ; but that there is slow uniform " combustion," 

 " eremacausis," or chemical combination of any kind going on at 

 some great unknown depth under the surface everywhere, and 

 creeping inwards gradually as the chemical affinities in layer 

 after layer are successively saturated, seems extremely improbable, 

 although it cannot be pronounced to be absolutely impossible, or 

 contrary to all analogies in nature. The less hypothetical view, 

 however, that the earth is merely a warm chemically inert body 

 cooling, is clearly to be preferred in the present state of science. 



10. Poisson's celebrated hypothesis, that the present under- 

 ground heat is due to a passage, at some former period, of the 

 solar system throughout hotter stellar regions, cannot provide 

 the circumstances required for a palaeontology continuous through 

 that epoch of external heat. For from a mean of values of the 

 conductivity, in terms of the thermal capacity of unit volume, of 

 the earth's crust in three different localities near Edinburgh, 

 which I have deduced from the observations on underground 



