TEMPERATURE OF THE SOIL. 161 



Degrees. 



Air in the shade at the surface, 62 



Air at a depth of eight inches, 56 



Air at a depth of two feet, 54J 



Air at a depth of two feet six inches, 52^ 



Air at a depth of three feet three inches, .... 5U 



It further took nineteen days for the maximum and 

 minimum temperature of the year to penetrate 

 three and one-fourth feet; or, in other words, its 

 march to this depth was nineteen days behind that 

 of the surface. Prof. J. Forbes has shown how 

 much the rapidity of transmission, and the amount 

 of heat transmitted, are modified by the nature of 

 the soil ; and that, in the case of sand, the variation 

 was three degrees greater, and five days more rapid, 

 than in sandstone (terrain de gres). 



" The roots of most plants are found within a 

 foot of the surface; and it is the temperature at 

 this depth that should be compared with that of 

 the external air. The Brussels observations supply 

 us with the necessary data for every month in the 

 year, and the following table exhibits the differences 

 between two thermometers, one hung in the shade 

 two and one-half feet above the surface of the soil, 

 and the other sunk a foot below the surface, — both 

 on the south side of the observatory. 



Degrees. Degrees. 



January, . . . +41 July, .... — 2^ 



February, ... August, .... — 3i 



March, . . . — 3| September, ... Of 



April, .... — 4| October, . . . . +2 



Ma}% .... — 2i November, . ; . +4 



June, .... —2i December, . . . +4 



14* 



