Prof. W. Thomson on the Secular Cooling of the Earth. 9 



or 4,000,000 years, if it existed so soon after the epoch of con- 

 solidation, be influenced by the sensibly higher temperature met 

 with by roots extending a foot or more below the surface. 



18. Whatever the amount of such effects is at any one time, 

 it would go on diminishing according to the inverse proportion 

 of the square roots of the times from the initial epoch. Thus, 

 if at 10,000 years we have 2° per foot of increment below ground, 



o 



At 40,000 years we should have 1 per foot. 



160,000 „ 1 „ 



„ 4,000,000 „ ^ „ 



„ 100,000,000 „ ^ „ 



It is therefore probable that for the last 96,000,000 years the 

 rate of increase of temperature under ground has gradually dimi- 

 nished from about jjjth to about -^th of a degree Fahrenheit per 

 foot, and that the thickness of the crust through which any 

 stated degree of cooling has been experienced has gradually in- 

 creased in that period from -i-th of what it is now to what it is. 

 Is not this, on the whole, in harmony with geological evidence, 

 rightly interpreted? Do not the vast masses of basalt, the 

 general appearances of mountain-ranges, the violent distortions 

 and fractures of strata, the great prevalence of ' met amorphic action 

 (which must have taken place at depths of not many miles, if 

 so much), all agree in demonstrating that the rate of increase of 

 temperature downwards must have been much more rapid, and 

 in renderiag it probable that volcanic energy, earthquake shocks, 

 and every kind of so-called plutonic action have been, on the 

 whole, more abundantly and violently operative in geological 

 antiquity than in the present age ? 



19. But it may be objected to this application of mathema- 

 tical theory (1) that the earth was once all melted, or at least 

 melted all round its surface, and cannot possibly, or rather can- 

 not with any probability, be supposed to have been ever a uni- 

 formly heated solid 7000° warmer than our present surface 

 temperature, as assumed in the mathematical problem ; and (2) 

 no natural action could possibly produce at one instant, and 

 maintain for ever after, a seven thousand degrees' lowering of 

 the surface temperature. Taking the second objection first, I 

 answer it by saying, what I think cannot be denied, that a large 

 mass of melted rock exposed freely to our air and sky will, after 

 it once becomes crusted over, present in a few hours, or a few 

 days, or at the most a few weeks, a surface so cool that it can 

 be walked over with impunity. Hence after 10,000 years, or, 

 indeed, I may say after a single year, its condition will be sen- 

 sibly the same as if the actual lowering of temperature experi- 

 enced by the surface had been produced in an instant and main- 



