PRESIDENTIAL ADDRESS — SECTION A. 



37 



to relieve the pressure on it, have produced in it a tendency to 

 plasticity, and would have caused intense horizontal thrusts in 

 the more superficial crustal layers — all of which are phenomena 

 the occurrence of which must be regarded as extremely probable 

 from other lines of evidence. 



There is now a general consensus of opinion that we must 

 probably look to two sources for the origin of supply of the 

 earth's internal heat. The first, and in some ways, the more 

 important, is that which has arisen from the almost certain fall- 

 ing together of the earth's materials from a much more widely 

 disseminated state ; the second, the discovery and further in- 

 vestigation of which we owe chiefly to the researches of Strutt 

 and Joly, arises from the presence of uranium and its child 

 radium in the rocks of the earth's crust, and perhaps to some 

 slight extent in the interior of the earth. 



Temp. 



3700 



50 



Dept h 



100 



IS'omis. 



Fig. 1. Kelvin's curve:^ ut cooling of a supposed homogeneous Earth, 

 originally at .3700° C. throughout: A, after 25,000,000 years; B, after 

 .50,000,000 : C, fall of temperature in the interval hetween A and B. 



It is worth while considering each of these for a moment. 



The heat developed in the original concentration .)f the 

 earth was enormous, and the temperatures that were generated 

 must have been correspondingly high except so far as the heat 

 was lost from the free surface of the growing earth by the pro- 

 cess of radiation. It is easy to show that the energy developed 

 by a pound of any material in falling from any large multiple of 

 the earth's radius on to its present surface would raise nearly 

 1.5,000 lbs. of water through one Centigrade degree. When the 

 earth was much smaller the heat generated was less ; in fact, for 

 the same mean density, the heat generated would be less in pro- 

 portion to the square of the radius. No doubt, additions to the 



