RELATIONS OF SOILS TO HEAT. 303 



reach to greater depths; while the annual variations do not 

 disappear in the temperate zone, e. g., at Paris, Zurich and 

 Brussels, at a less depth than seventy-five feet. At these 

 depths of constant temperature we find approximately the 

 same temperature as that which we can deduce from the ther- 

 mometric observations as the annual mean. From similar 

 causes the mean annual temperature of any place may be ap- 

 proximately deduced from the observation of the water of 

 wells and springs derived from moderate depths. For below 

 the level of constant annual temperature the latter begins to 

 ascend steadily as we progress downward, owing to the in- 

 terior heat of the earth. 



Change of Temperature with Depth. The following table 

 of observations made at Brussels illustrates the decrease of 

 annual range of temperature with depth : 



At feet : Average temperature. Annual range. 



3.25 7.2C. 10.5 C. 



15-6 i3-SC. 4-5C. 



30.8 16.4 C. 1.3 C. 



75.0 17.0 C. 0.0 C. 



It is interesting to compare with this record that of a well 

 sunk by Ermann at Yakutzk, Siberia, where the mean annual 

 temperature is 9.7 C. (14.6 F.). This temperature was 

 found a few feet below the surface. At 50 feet the tempera- 

 ture was 7.2 C. (19 F.) ; at 145 feet 5 Cent. (23 F.) at 

 350 feet .9 C. (30.8 F.) showing that the ground was be- 

 low the temperature of freezing water for some distance far- 

 ther down ; so that the search for liquid water was abandoned. 



We thus see that in the Arctic regions, owing to the pres- 

 ence of water in the form of ice, the melting of which impedes 

 the access of solar heat, the level of no variation is found at 

 the distance of a few feet below the surface, despite the great 

 variations in temperature between the short but hot summer 

 and the extremely cold winter. In the tropics, also, the annual 

 temperature-variation disappears at a less depth than 2 feet, 

 in consequence of the very slight difference between the two 

 seasonal extremes of temperature. 



Surface Conditions that influence Soil-Temperature. 

 Among these color has already been mentioned, and to a cer- 



