IGO TEMPERATURE OF THE SOIL. 



ing air better than those whose roots are superficial, 

 because the temperature of the earth varies less the 

 deeper we penetrate it. 



" Many interesting observations have been made 

 with regard to the heat of the soil at different depths ; 

 but, for our purpose, it is necessary only to consider 

 such as extend to about three feet from the surface : 

 for the roots of most plants penetrate no deeper ; 

 those which do, are generally such as afi'ect a very 

 light soil ; and even great trees have more root-fibres 

 near the surface than at the depth of three feet. It 

 matters little to our investigations that, in our cli- 

 mate, a stratum of unvarying temperature exists at 

 a depth of from twenty to thirty feet ; that this 

 stratum is found at three feet below the surface in 

 some very uniform climates, and at much greater 

 depth in such extremely cold countries as Siberia. 

 We shall consider only the monthly variations in 

 the upper three feet of the soil. 



" That the temperature of the soil at this depth 

 is less variable than that of the air, is proved by 

 Mr. Muncke's observation at Heidelberg, which 

 shows that the diurnal variations are not felt beyond 

 three feet, nor the monthly beyond five. At Brus- 

 sels, M. Quetelet has shown that the annual varia- 

 tions present the following decrease, in descending 

 from the surface to a depth of three and one-fourth 

 feet : 



