92 AGRICULTURE. 



raised during that period of the year in which vegetation is 

 active by the removal of water by drainage. 



Many experiments have shown that, in retentive soils, 

 the temperature at 2 or 3 feet below the surface of the 

 water-table is, at no period of the year, higher than from 

 46 to 48, i. e. in agricultural Britain. This temperature 

 is little affected by summer heats, for the following short 

 reasons. Water, in a quiescent state, is one of the worst 

 conductors of heat with which we are acquainted. Water 

 warmed at the surface transmits little or no heat down- 

 wards. The small portion warmed expands, becomes lighter 

 than that below, consequently retains its position on the 

 surface, and carries no heat downwards.* To ascertain the 

 mean heat of the air at the surface of the earth over any 

 extended space, and for a period of eight or nine months, 

 is no simple operation. More elements enter into such a 

 calculation than we have space or ability to enumerate ; 

 but we know certainly that, for seven months in the year, 

 air, at the surface of the ground, is seldom lower than 48, 

 never much lower, and only for short periods : whereas, 

 at 4 feet from the surface, in the shade, from 70 to 80 

 is not an unusual temperature, and in a southern exposure, 

 in hot sunshine, double that temperature is not unfre- 

 quently obtained on the surface. Now let us consider 

 the effect of drains placed from 2 to 3 feet below the 

 water-table, and acting during the seven months of which 

 we have spoken. They draw out water of the temperature 

 of 48. Every particle of water which they withdraw at 

 this temperature is replaced by an equal bulk of air at a 

 higher, and frequently at a much higher, temperature. 

 The warmth of the air is carried down into the earth. 



* When water is heated from below, the portion first subjected to the 

 heat rises to the surface, and every portion is successively subjected to 

 the heat and rises, and each, having lost some of its heat at the surface, 

 is in turn displaced. Constant motion is kept up, and a constant 

 approximation to an equal temperature in the whole body. The appli- 

 cation of superficial heat has no tendency to disturb the quiescence of 

 water. 



