38 ENGINEERING FOR LAND DRAINAGE. 



drain until capillary water has been supplied to the 

 soil. 



The material out of which the tiles are made does not 

 affect their efficiency as drains, nor the facility with 

 which soil water enters the drain. 



Plane of Saturation. 



It will be readily understood, from the explanation 

 just given regarding the action of a single drain upon 

 the soil, that as the lateral distance from the drain is 

 increased the plane of saturation rises, for the reason 

 that the water in passing toward the drain encounters 

 the resistance of the particles of the soil, which resist- 

 ance requires a certain head to overcome, or there can 

 be no movement of water. The angle which a line in 

 this plane makes with a horizontal passing through the 

 floor of the drain must vary with the degree of soil re- 

 sistance. Upon this depends the lateral distance to 

 which a single line of tile will drain the soil, and the 

 depth at which drains should be placed. This line of 

 saturation has been assumed from observations upon 

 the action of drains to be a curve of some kind, but 

 no definite investigations have been made upon the 

 subject until recently. 



In 1 89 1 observations were made at the Wisconsin 

 Agricultural Experiment Station in a tile-drained field 

 bordering upon a lake, a part of which was provided 

 with natural underdrainage alone, for the purpose of 

 determining the actual contour of the plane of satura- 

 tion. The following is a brief account of the method 

 used and the results obtained; 



