ENGLISH AGRICULTURE. 95 



of drainage gradually deepen and mellow to such a degree 

 that the original character of the soil and even of the sub-soil 

 becomes completely altered. 



Besides, we must not forget the improvement in the 

 climate of a drained soil. The average temperature of such 

 a soil is considerably enhanced, due in a great measure to 

 the checking of evaporation. A wet soil, like a wet garment, 

 is cold. It is constantly chilling the roots of the plants and 

 the body of the soil. Not only is the cooling of the soil due 

 to evaporation, but it is also owing to the peculiar mode of 

 cooling of water and other fluids when exposed to a low temper- 

 ature. During the winter season, when wet land is exposed to 

 cold air, and when it radiates its heat out into a clear sky, the 

 upper film of water becomes cold, and as this film increases 

 in weight or in density it sinks into the lower layers of the 

 water-logged soil. Its place is taken by warmer water from 

 below, and thus there is a gradual cooling of the entire body 

 of the water within the soil to a very great depth. Notice 

 that this is different from evaporation. Evaporation no 

 doubt chills the soil, but the soil is chilled to a very consider- 

 able depth owing to " convection," which is the law of the 

 >oling of fluids. Similarly the wet soil is never able to 

 )btain the benefits of summer heat, because if a thoroughly 

 ret soil is exposed to the sun's heat, the upper film 

 >f water becomes warm, and remains at the surface or is 

 evaporated from the surface, but being the lightest, the lower 

 lyers of water cannot rise through it. The temperature of 

 bottom water remains at a low degree, the entire solar 

 leat being consumed in the evaporation of the surface water. 

 These appear to me to be the chief reasons why drainage 

 is beneficial. This is a question which I have frequently 

 sked in my capacity of examiner, and the answers which I 

 receive are very often too much restricted to one or two con- 

 siderations bearing upon plant food in the soil. I am told 





