238 MEAX HEAT OF THE EAETH. SECT. XXV, 



Sir John Herschel at the Cape of Good Hope, show that the 

 direct heating influence of the solar rays is greatest at the equator, 

 and that it diminishes gradually as the latitude increases. At 

 the equator the maximum is 48f , while in Europe it has never 

 exceeded 29. 



M. Pouillet has estimated with singular ingenuity, from a 

 series of observations made by himself, that the whole quantity 

 of heat which the earth receives annually 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, towards 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 atmosphere 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 main- 

 taining the temperature of space. The sun's power being 

 greatest between the tropics, the heat sinks deeper there than 

 elsewhere, and the depth gradually diminishes towards 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 tem- 

 pering 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 tem- 

 perature of numerous springs with that of the air, Sir David 

 Brewster concludes that there is a particular line passing nearly 

 through Berlin, at which the temperature of springs and that of 

 the atmosphere coincide ; that in approaching the arctic circle 

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

 while, proceeding towards the equator, it is lower. 



Since the warmth of the superficial strata of the earth de- 

 creases from the equator to the poles, there are many places in 

 both hemispheres where the ground has the same mean tempera- 

 ture. If lines were drawn through all those points in the upper 

 strata of the globe which have the same mean annual tempera- 



