172 



soil, whether black or brown, was more than a degree warmer than the' 

 light soil at four inches deep. 



Influence of Topography. The degree of inclination of the land sur- 

 face and the direction of the slope, whether facing east, west, north 

 or south, may exert a marked influence upon the temperature of the 

 soil and particularly upon its diurnal range. The temperature of a 

 stiff red clay soil, upon a level plateau, and upon a south exposure 

 sloping about 18°, was found in the surface three feet to make a dif- 

 ference in temperature of from a little more than 3° F., in the sur- 

 face foot, to a little less in the second and third feet. 



Influence of chemical changes. When heavy dressings of farmyard 

 manure are ploughed in, and when heavy crops are turned under for 

 green manure, the fermentation which is set up in these materials re- 

 sults in a measure of heat which warms the soil in the same way that 

 a manure heap heats when fermenting. Indeed all the steps in the for- 

 mation of nitrates in the soil result in the evolution of some heat. 



Influence (f rains. Heavy rains which fall upon a field and penetrate 

 the soil may exert very marked effects upon its temperature on ac- 

 count of the relatively high specific heat of the water as compared 

 with that of the soil. 



If the atmosphere is warmer than the deeper soil, and if rains fall 

 which result in heavy percolation, a large amount of heat is conveyed 

 rapidly and deeply into the soil with the water and the temperature of 

 the ground, two to four feet below the surface, may thus be very ma- 

 terially raised. 



Influence of evaporation. There is no factor, except the direct sun- 

 shine and the direct radiation of heat away from the earth into space, 

 which exerts so strong an influence on the temperature of the soil as 

 the evaporation of moisture from its surface ; and the chief reason why 

 an undrained clay soil is colder than one well drained is the cooling 

 effect associated with the larger evaporation of soil moisture. 



To evaporate a pound of water from the surface of a square foot of 

 soil, by means of the heat contained in the soil, makes it imperative 

 that 966.6 heat units be expended to do the work and this, if with- 

 drawn from a cubit foot of saturated clay soil, would lower its tem- 

 perature some 10.3° F. 



The difference in temperature shown by the wet and dry bulb ther- 

 mometers measures, in one way, the cooling effect of evaporation ; the 

 wet bulb often reading as much as 15 or even 20 degrees lower than 

 the dry one, under otherwise identical conditions. 



