252 ISOGEOTHERMAL LINES. SECT. XXVI. 



whole quantity of heat which the earth receives annu- 

 ally from the sun is such as would be sufficient to melt 

 a stratum of ice covering the whole globe 46 feet deep. 

 Part of this heat is radiated back into space ; but by far 

 the greater part descends into the earth during the 

 summer, toward the zone of uniform temperature, 

 whence it returns to the surface in the course of the 

 winter, and tempers the cold of the ground and the at- 

 mosphere in its passage to the ethereal regions, where 

 it is lost, or rather where it combines with the radiation 

 from the other bodies of the universe in maintaining 

 the temperature of space. The sun's power being 

 greatest between the tropics, the caloric sinks deeper 

 there than elsewhere, and the depth gradually dimin- 

 ishes toward the poles ; but the heat is also transmitted 

 laterally from the warmer to the colder strata north and 

 south of the equator, and aids in tempering the severity 

 of the polar regions. 



The mean heat of the earth above the stratum of 

 constant temperature is determined from that of springs ; 

 and if the spring be on elevated ground, the temperature 

 is reduced by computation to what it would be at the 

 level of the sea, assuming that the heat of the soil 

 varies according to the' same law as the heat of the 

 atmosphere, which is about 1 of Fahrenheit's ther- 

 mometer for every 333-7 feet. From a comparison of 

 the temperature of numerous springs witk that of the 

 air, Sir David Brewster concludes that there is a par- 

 ticular line passing nearly through Berlin, at which the 

 temperature of springs and that of the atmosphere 

 coincide ; that in approaching the arctic circle the tem- 

 perature of springs is always higher than that of the air, 

 while proceeding toward the equator it is lower. 



Since the warmth of the superficial strata of the earth 

 decreases from the equator to the poles, there are many 

 places in both hemispheres where the ground has the 

 same mean temperature. If lines were drawn through 

 all those points in the upper strata of the globe which 

 have the same mean annual temperature, they would 

 be nearly parallel to the equator between the tropics, 

 and would become more and more irregular and sinuous 

 toward the poles. These are called isogeothermal lines. 



