HOW THEY AFFECT THE SOIL. 317 



The less this difference, the more nearly does the invariable 

 stratum approach the surface. Here we have the reason 

 why, in the intertropical torrid zone, where the temperature 

 scarcely varies above two or three degrees in. the whole year, 

 the invariable curve is not found more than forty centimetres 

 beneath the surface. 



In the temperate zone, lying between the torrid and the 

 frigid zones, the same phenomena assume, apparently, a more 

 complex character ; the isogeothermal lines inflect as diversely 

 as the isothermal, and the former are far from running parallel 

 with the latter. And this is easily understood, even without 

 any experiment, for there is no relation between the ever- 

 varied composition of the terrestrial strata, and the much 

 more uniform composition of the atmosphere. 



In the frigid zone, the soil remains constantly frozen for 

 an insignificant depth, whatever may be the temperature of 

 the encircling atmosphere. In some regions a stratum 

 of ice and snow eternally reposes on the surface of the 

 soil. 



Unfortunately the observations, which require to be under- 

 taken on an uniform plan and under well-weighed conditions, 

 at various points over the whole globe, are as yet far too 

 few to assist us in defining any general currents of heat cr 

 cold, whether variable or constant, as prevailing in the lower 

 strata of the gaseous ocean of our planet. 



