PROFESSOR W. THOMSON ON THE SECULAR COOLING OF THE EARTH. 159 



of places to establish any fair average estimate for the upper crust of the whole 

 earth. But ^th is commonly accepted as a rough mean ; or, in other words, it is 

 assumed as a result of observation, that there is, on the whole, about 1° Fahr. of 

 elevation of temperature per 50 British feet of descent. 



7. The fact that the temperature increases with the depth implies a continual 

 lossof heat from the interior, by conduction outwards through or into the upper crust. 

 Hence, since the upper crust does not become hotter from year to year, there 

 must be a secular loss of heat from the whole earth. It is possible that no 

 cooling may result from this loss of heat, but only an exhaustion of potential 

 energy, which in this case could scarcely be other than chemical affinity between 

 substances forming part of the earth's mass. But it is certain that either the 

 earth is becoming on the whole cooler from age to age, or the heat conducted out 

 is generated in the interior by temporary dynamical (that is, in this case, chemi- 

 cal) action. To suppose, as Lyell, adopting the chemical hypothesis, has done,* 

 that the substances, combining together, may be again separated electrolytically 

 by thermo-electric currents, due to the heat generated by their combination, and 

 thus the chemical action and its heat continued in an endless cycle, violates the 

 principles of natural philosophy in exactly the same manner, and to the same 

 degree, as to believe that a clock constructed with a self-winding movement may 

 fulfil the expectations of its ingenious inventor by going for ever. 



8. It must indeed be admitted that many geological writers of the " Uniformi- 

 tarian" 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 violence 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 incredible result, that volcanic action 

 has never been more violent on the whole than during the last two or three cen- 

 turies ; but it is 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 present 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. 



* Principles of Geology. 



