FORMATION OF SOILS 83 



because much of the heat of the sun is in these cases 

 consumed in evaporating water. Parkes found that 

 an undrained peat bog, 30 feet deep, had a uniform 

 temperature of 46° below a distance of one foot from 

 the surface. In the middle of June he found the 

 temperature 47" at seven inches below the surface, 

 while the drained portion had a temperature of 66° at 

 this depth, and a temperature of 50° at two feet below 

 the surface. Draining is the only cure for a cold, wet 

 soil. 



The temperature of the subsoil is practically constant 

 throughout the year at a certain distance from the 

 surface ; this distance will be a few feet in the tropics, 

 but becomes very considerable in northern latitudes, 

 where there is a wide difference in the summer and 

 winter temperature. Between the point of constant 

 temperature and the surface the changes of season are 

 felt, but the maximum and minimum temperatures in 

 the subsoil always occur after they have been reached 

 at the surface. At a certain depth the seasons are 

 reversed, and the maximum temperature occurs in the 

 subsoil while it is winter at the surface. At Greenwich 

 Observatory, in a well-drained gravel, the variations of 

 day and night are slightly felt at three feet from the 

 surface. At 25"6 feet the maximum temperature 

 usually occurs in the latter part of November, and the 

 minimum in the first week in June ; the difference 

 between the two is about 3°. It follows from what 

 has been stated that the soil and subsoil are generally 

 warmer than the air in autumn, and cooler than the 

 air in spring. 



5. Formation of Soils. — All soils have been produced 

 by the disintegration of rocks, through the prolonged 

 3 



