104 TILLAGE MOVEMENTS OF SOIL WATER [chap. 



often water-logged because their surface is little higher 

 than the water in the river and the general water table 

 in the adjoining soil. In these cases tile drains are of 

 no value because of the want of fall ; open cuts and 

 ditches draw off the water best, and by exposing some 

 of the subsoil water both to aeration and evaporation, 

 lead to the improvement of the land. Another cause of 

 swampy water-logged land is the rising to the surface 

 of a spring or a line of soakage, such as is always 

 formed at the junction of a clay or other stiff soil 

 with an overlying pervious formation, " when the sand 

 feeds the clay," as the old rhyme runs. Such wet spots 

 can be drained by tiles or by an open ditch cutting the 

 springs or the line of soakage. Land lying on an 

 impervious subsoil at the foot of a slope is often very 

 wet because the water which has accumulated in the hill 

 and soaked downwards is forced to the surface by the 

 hydraulic pressure of the water above ; such seepage 

 water rising to the surface from the subsoil is character- 

 istic of many valley soils, and can best be dealt with by 

 a system of tile drains. But tile drains are most 

 generally employed and are of greatest value in dealing 

 with stiff impervious subsoils, which cannot get rid of 

 the rain falling upon them ; indeed, one of the prime 

 improvements effected in English agriculture was the 

 drainage of something like 3,000,000 acres of heavy 

 land between the years 1840-70. A great portion of 

 the work was unfortunately of little avail, because at 

 first there was a tendency to set the drains too deep, 

 at 4 feet instead of the 2 to 3 feet which have been 

 found to answer best. The benefits conferred by 

 drainage depend upon the lowering of the permanent 

 water table to the depth at which the drains are laid, 

 so that instead of constantly stagnant water a movement 

 of both water and air is established in the soil above 



