TEMPERATURE AFFECTED BY DRAINAGE. 225 



quis of Tweeddale offered -$400 for the best experiments on the 

 difference of drained and iindrained land at different depths be- 

 low the surface. The temperature of the drained and undrained 

 lands was to be observed in pasture lands at the depth of ten 

 inches, and in arable lands at ten, twenty and thirty inches 

 below the surface, and in both cases to be compared with the 

 temperature of the air four feet above the soil. The drains were 

 to be two and one-half feet deep, and the drained and undrained 

 lands were to be as similar as possible both in chemical and me- 

 chanical composition. The results proved that the drained lands 

 at the depth of ten inches have an average temperature of two 

 to three degrees above those undrained. During a warm rain 

 the temperature of the undrained soil was the higher, as the 

 water does not percolate through the soil and pass off. At the 

 depth of twenty and thirty inches the effect of warm rain is not 

 felt and the temperature of the drained soil is uniformly higher, 

 and the great advantage of drainage becomes more strikingly 

 evident. In the winter and in periods of cold weather in the 

 spring and autumn, the drain serves a better purpose in elevat- 

 ing the temperature than during a long continued warm sea- 

 son. It may be worth while to remark that the ground is 

 heated in two modes : by the sun's heat from above, and the 

 external heat of the earth from below, and at some periods the 

 heat is increasing from the surface downwards, and at others is 

 passing from below upwards. Undrained land having the in- 

 terstices between its particles filled with water is less porous and 

 therefore a better conductor of heat to the outer air, and hence 

 its temperature falls rapidly when the air becomes cooler than 

 the land ; but on the other hand, when land is saturated with 

 moisture it is almost impossible to conduct the heat of the air 

 downwards, as fluids do not convey heat from particle to parti- 

 cle as do solids, but by a change of place, the heated particles 

 rising and the cold descending. Every house-wife knows that 

 she cannot boil her tea-kettle by placing the fire over it, and 

 every farmer should know that he cannot make moist land 

 warm by surface or sun heat. 



Besides the impossibility of conducting heat downwards 



through water, except in quantities so minute as to require nice 



experiments to detect its descent, the evaporation going on has 



a tendency to produce cold. Just as soon as a surface drop of 



29 



