26 LAND DRAINAGE 



40. Movements of soil water. Mention has been 

 made of three conditions in which water may exist in the 

 soil. There are three movements of soil water one 

 each for these three conditions. Movements of gravita- 

 tional water are known as gravitational movements ; those 

 of capillary water, as capillary movements; while move- 

 ments of hygroscopic water are known as thermal move- 

 ments. These movements are very distinct as regards 

 their causes, characteristics, and agricultural importance. 

 For the present, gravitational movements have been 

 sufficiently discussed in paragraph (37). 



41. Capillary movements. Agriculturally, capillary 

 water is preeminent. Its control, Which is very desirable, 

 is accomplished largely through the control of its move- 

 ments, and this control depends largely upon a knowledge 

 of their causes and character and the factors modifying 

 them. Movements of capillary soil water are due chiefly 

 to surface tension, and entirely so after the soil particles 

 are invested by capillary water. 



42. Surface tension. When the rubber membrane of 

 a toy balloon is inflated with gas, it assumes a spherical 

 form, due to the pull of the membrane in its effort to 

 contract. This lessens the volume of the gas, and be- 

 cause the pull is uniform, the balloon assumes a spherical 

 form. (If some part of the rubber membrane were 

 weaker or stronger than the rest, the form of the balloon 

 would not be exactly spherical.) If a part of the inclosed 

 gas were permitted to escape, the volume of the balloon 

 would be reduced, but its form would not be changed. 



The dew-drop, suspended from the blade of grass, does 

 not go to pieces for the reason that the molecules of 

 water covering the surface of the mass act in much the 

 same way as the rubber membrane of the toy balloon. 



