SOIL MOISTURE 259 



These materials are made into cylinders from three to thir- 

 ty-six inches in diameter, and from twelve to thirty inches 

 in length. For lateral drains, tiles four or five inches in 

 diameter are most used. The main outlet drain usually 

 requires tiles from eight to twelve inches. 



Placing the drains. Moderately heavy clay soils re- 

 quire laterals about four rods apart to carry off the rainfall. 

 If the subsoil is sandy, the laterals may be as much as eight 

 rods apart. Sometimes the marshiness of a piece of ground 

 is caused by seepage leading to it from some higher area. 

 In this case, there should be a line of tile at the edge of 'the 

 lower ground to receive the seepage. 



Several lines of parallel drains are more economical than 

 one central line into which diagonal laterals run. This is 

 because with parallel drains there is less area receiving 

 double drainage. Several parallel lines can often be car- 

 ried into one line of larger tile, and all discharge through 

 the same outlet, thus saving trouble and expense. 



Depth of tile. Tile should be placed deep enough 

 that the level of ground water will not stand too near 

 the surface, and yet not too deep to carry off the gravita- 

 tional water without allowing it to stay too long in the 

 soil. In clay subsoil the most common depth is about three 

 feet. In partially sandy subsoils, the depth may be four feet. 

 All tile meant to catch seepage should be as deep as four feet. 



Gradient, or fall. The larger tiles may be laid with 

 a fall of an inch to one hundred feet. Laterals should have 

 from two to three times this much fall. Lines for catching 

 seepage should have still greater ' slope ; as much as five 

 inches to one hundred feet, if this is possible. 



Cost of tiling. The cost of tiling will, of course, vary 

 with the size of tile used, the depth it is laid, and the 

 character of the soil. The average cost under middle west- 

 ern conditions is about as shown in the following table : 



