PiiOFESSOit Gokdon on the Temperature of the Earth. 141 



There is this remarkable result too, namely, the mean temperature is 

 greater as the depths are greater. The variation of temperature at the 

 different depths throughout the five years of experiment, was shown by 

 means of diagrams, containing the mean temperature for each week of 

 the year, taken by the mean of five years, as the best mode of disposing of 

 irregular fluctuations. 



The upper curves (of the thermometer at three feet depth,) follow each 

 other with singular regularity. 



At increasing depths the curves systematically separate from each 

 other, showing difference of conducting power in soils. 



In reference to the thermometric range, these experiments confirmed 

 the theory that the ranges of temperature may be represented by the 

 ordinates of a logarithmic curve, of which the corresponding depths are 

 the abscisses; and that the retardations, the epochs of maxima and minima, 

 increase uniformly with the depth. 



The geometrical expression of the first law being log. A = A + B p, 

 when A = thermometric range at depth p in French feet, and A and 13 

 are constant quantities determined by these experiments as follows : — 



Mean Value of A. Mean Value of B. 



Trap Rock, 1-105 -0545 



Sand Soil, 1-174 -0477 



Sandstone Rock, 1-060 -0311 



So that by means of this formula, we can calculate the range for any 

 depth. 



Diagrams illustrating the agreement of the experiments with this law, 

 were shown. 



The depths for which the annual range of temperature is reduced to 

 one-hundredth of a degree of Centigrade, calculated by this formula, 

 would be in 



Trap Rock, 57-3 Paris feet. 



Sandy Soil, 666 



Sandstone Rock, 96*9 



That is to say, at these depths respectively, there occurs a stratum that 

 does not vary in temperature. 



Another diagram was exhibited, showing the progress of heat down- 

 wards in these different soils, from which it appears that the greatest 

 cold at the depth of 24 feet occurred in — 



Trap Rock, 13th July. 



Sandy Soil, 29th June. 



Sandstone, 3d May. 



Such is the nature of the results obtained by Professor Forbes's experi- 

 ments, confirming and confirmed by those of Quetelet at Brussels, of 



