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



face ; and hence I infer that the general climate cannot be sensibly affected by 

 conducted heat at any time more than 10,000 years after the commencement of 

 superficial solidification. No doubt, however, in particular places there might 

 be an elevation of temperature by thermal springs, or by eruptions of melted 

 lava, and everywhere vegetation would, for the first 3,000,000 or 4,000,000 

 years, if it existed so soon after the epoch of consolidation, be influenced l^y the 

 sensibly higher temperature met with by roots extending a foot or more below 

 the surfjace. 



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, 



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



160,000 „ „ 1° „ 



„ 4,000,000 „ „ tV° „ 



„ 100,000,000 „ „ ^v° » 



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

 temperature under ground has gradually diminished from about ^^^th 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 increased in 

 that period from ^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 metamorphic 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 rendering 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 mathematical theory — (1), That 

 the earth was once all melted, or at least melted all round its surface, and cannot 

 possibly, or rather cannot with any probability, be supposed to have been ever a 

 uniformly 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 sensibly the same as if the actual 

 lowering of temperature experienced by the surface had been produced in an 



