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



V degrees, and kept permanently at this lower temperature, the 

 scales used being as follows : — 



(1) For depth below the surface, — scale along OX, 10 quarter 

 inches, or a, represents 400,000 feet. 



(2) For rate of increase of temperature per foot of depth, — 

 scale of ordinates parallel to OY, 10 half inches, or b, represents 

 ^ of V per foot. If, for example, V==7000°, this scale will 

 be such that 10 half inches, or b, represents g~ of a degree 

 per foot. 



(3) For excess of temperature, — scale of ordinates parallel to 



Y, 10 half inches, or b, represents y-y-, or 7900°, if V= 7000°. 



Thus the rate of increase of temperature from the surface 

 downwards would be sensibly ~ of a degree per foot for the first 

 100,000 feet or so. Below that depth the rate of increase per 

 foot would begin to diminish sensibly. At 400,000 feet it 

 would have diminished to about ^ of a degree per foot. At 

 800,000 feet it would have diminished to less than ^ of its 

 initial value, — that is to say, to less than — of a degree per 

 foot ; and so on, rapidly diminishing, as shown in the curve. 

 Such is, on the whole, the most probable representation of the 

 earth's present temperature at depths of from 100 feet, where 

 the annual variations cease to be sensible, to 100 miles ; below 

 which the whole mass, or all except a nucleus cool from the 

 beginning, is (whether liquid or solid) probably at, or very nearly 

 at, the proper melting temperature for the pressure at each depth. 



17. The theory indicated above throws light on the question 

 so often discussed — Can terrestrial heat have influenced climate 

 through long geological periods ? and allows us to answer it very 

 decidedly in the negative. There would be an increment of 

 temperature at the rate of 2° F. per foot downwards near the 

 surface 10,000 years after the beginning of the cooling in the 

 case we have supposed. The radiation from earth and atmo- 

 sphere into space (of which we have yet no satisfactory absolute 

 measurement) would almost certainly be so rapid in the earth's 

 actual circumstances as not to allow a rate of increase of 2° F. 

 per foot underground to augment sensibly the temperature of the 

 surface; 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 



