Hi.] PERCOLATION OF WATER 77 



The downward movement of rain water through 

 soils is known as "percolation," and is distinguished from 

 " flow " by the fact that the water is supposed to have 

 free surfaces, so that surface tension comes into play. 

 It takes place under the action of gravity through the 

 pore space proper, and also through the cracks, the 

 worm tracks, the passages left by decayed roots, and 

 other adventitious openings in the soil. The percola- 

 tion proceeds until the zone is reached where the pore 

 space is completely filled ; this is known as the " water 

 table," and is the level at which water stands in the 

 wells. Above the water table the soil will be more or 

 less in the state represented in the diagram showing 

 sands and soils in which percolation has ceased ; though 

 there will be most probably a more irregular distri- 

 bution, with zones which contain an excess of water 

 travelling downwards with greater or less rapidity, 

 according to the texture of the soil. It is these 

 temporarily saturated zones which cause the ordinary 

 tile drains to run, although situated many feet above 

 the permanent water table. A soil in which percola- 

 tion has ceased, though it may still contain much 

 water, will not part with it to a drain ; the water 

 cannot break away from the elastic film and run 

 off down the drain, unless it be present in such an 

 excess that the surface tension is insufficient to hold 

 it against gravity. But in a clay soil, percolation is 

 so slow that the upper few feet of soil may become 

 saturated by the winter rains and remain so for 

 months, if percolation has to proceed all the way down 

 to the water table ; by the introduction of drains, the 

 percolating column is shortened to the distance between 

 the surface and the drain. In a coarse-grained sandy 

 soil percolation is very rapid, the land dries quickly 

 after rain, and retains a minimum of water by surface 



