22 ENGINEERING FOR LAND DRAINAGE. 



tides. It is commonly called moistm^e and is the 

 water which is left in the soil after it is drained. 



Speaking in general terms, about 50 per cent of the 

 volume of a soil is empty space, that is, it contains only 

 air and water. The results of experiments hereafter 

 given show that the volume of empty space ranges 

 from 37 per cent as found in sandy soils to 65 per cent 

 in soils composed largely of clay. This space is so 

 divided up by the very large number of grains of soil 

 that the spaces between the grains are extremely 

 small. When a soil is only slightly moist the water 

 clings to the soil grains in a thin film. The force 

 which holds water to the grain of soil is called surface 

 tension. The water is called capillary water. It is 

 like a soap-bubble with a grain of sand or clay inside 

 instead of air. Where the grains come together the 

 films are united into a continuous film of water through- 

 out the soil. If more water enters the soil, the film 

 thickens and there is less exposed water surface. If 

 the empty space is completely filled with water, gravity 

 alone will act with its greatest force. If the soil is 

 nearly dry, there will be a great deal of this exposed 

 water surface, a great amount of surface tension, and 

 hence gravity will have no effect. In other words, 

 there will be no hydrostatic water, since the force of 

 surface tension is able to retain the entire supply of 

 water about the particles of which the soil is composed. 



Hydrostatic water moves through the soil with 

 greater or less rapidity and freedom, according to the 

 resistance which the soil particles offer to its passage. 

 It moves upward only when forced to do so. It moves 

 downward and laterally in obedience to the laws of 



