TEMPERATURE OF SUBSOIL 179 



year. The position and temperature of this point vary 

 much in different climates, and in different soils. It is to be 

 regarded simply as a point of equilibrium. In hot climates 

 there is a warmer soil above this point, and a cooler soil 

 below it. In cold climates the soil is cooler above and warmer 

 below. 



The depth at which a constant temperature is reached is 

 greater the greater are the variations in annual temperature 

 at the surface. In the tropics a constant temperature is 

 usually found a few feet beneath the surface; in higher 

 latitudes the depth is generally very considerable. The dis- 

 tance from the surface at which variations of temperature can 

 be felt is however considerably affected by the conductivity 

 and specific heat of the soil or rock of the locality ; observa- 

 tions at different places thus frequently differ even when 

 these places are in nearly the same latitude. We have already 

 seen that in the case of an undrained Lancashire bog, the tem- 

 perature remains practically constant throughout the year at 

 a depth little exceeding one foot. 



The average annual range of temperature observed at 

 various depths of soil at the Observatories of Greenwich, 

 Brussels, and Edinburgh is shown below ; the degrees are 

 Fahrenheit. The soil at Greenwich is gravel. At Edinburgh 

 the temperatures are taken in a drill-hole bored into a 

 porphyry trap- tuff rock. 



Average annual Range of Temperature at various depths 



Greenwich. Brussels. Edinburgh. 



N 2 



