8 TROPICAL NATURE, AND OTHER ESSAYS. 



Influence of the Heat of the Soil. It is well known 

 that at a very moderate depth the soil maintains a 

 uniform temperature during the twenty-four hours ; 

 while at a greater depth even the annual inequalities 

 disappear, and a uniform temperature, which is almost 

 exactly the mean temperature of the locality, is con- 

 stantly maintained throughout the year. The depth at 

 which this uniform temperature is reached is greater as 

 the annual range of temperature is greater, so that it is 

 least near the equator, and greatest in localities near the 

 arctic circle where the greatest difference between 

 summer and winter temperature prevails. In the 

 vicinity of the equator, where the annual range of the 

 thermometer is so small as we have seen that it is at 

 Batavia, the mean temperature of about 80 Fahr. is 

 reached at a depth of four or five feet. The surplus heat 

 received during the day is therefore conducted down- 

 wards very slowly, the surface soil becomes greatly super- 

 heated, and a large portion of this heat is given out at 

 night and thus keeps up the high temperature of the air 

 when the sun has ceased to warm the earth. In the 

 temperate zones, on the other hand, the stratum of 

 uniform earth-temperature lies very deep. At Geneva 

 it is not less than from thirty to forty feet, and with us 

 it is probably fifty or sixty feet, and the temperature 

 found there is nearly forty degrees lower than at the 

 equator. This great body of cool earth absorbs a large 

 portion of the surface heat during the summer, and con- 

 ducts it downwards with comparative rapidity, and it 

 is only late in the year (in July and August) when the 

 upper layers of the soil have accumulated a surplus store 

 of solar heat that a sufficient quantity is radiated at 



