236 EAETHQUAKES. 



it must have had a much greater influence in bygone 

 times. 



We might speak similarly with regard to the internal 

 heat of the earth. 



From the present heat-gradient of our globe it is pos- 

 sible to calculate how much heat flows from the earth 

 every year. 



This is equivalent to a quantity which would raise a 

 layer of water '67 centimetres thick, covering the whole 

 of our globe, from a temperature of 0° to 100° C. 



Similarly, we might calculate the quantity of heat 

 which would be lost when the average heat gradient, 

 instead of being 1° F. for fifty feet of descent, was 1° F. for 

 twenty-five feet of descent. 



We might also calculate how many years ago it was 

 since such a gradient existed. 



The general result which we should arrive at would 

 be that in past ages the loss of heat was more rapid than 

 it is at present. Now the contraction of a body as it 

 cools is for low temperatures proportional to its loss of 

 heat, and this law is also probably true for contraction as 

 it takes place from high temperatures. 



Contraction being more rapid, it is probable that 

 phenomena like elevations and depressions would be more 

 rapid than they are at present, and generally all changes 

 due to plutonic action, as has already been pointed out 

 by Sir William Thomson, must have been more active. 



We have, therefore, every reason to imagine that 

 earthquakes which belong to the category of phenomena 

 here referred to were also numerous and occurred on a 

 grander scale during the earlier stages of the world's 

 history than they do at present, and seismic and volcanic 

 energy, when considered in reference to long periods of 

 time, is probably a decreasing energy. 



