THE CONTROL OF SOIL-MOISTURE 



213 



Every regular system consists of two parts, the laterals and 

 the main drain. The laterals are usually constructed of 

 three 3- or 4-ineh tile, seldom smaller. These laterals should 

 always enter the main at an angle of about 45 degrees. This 

 causes a joining of the currents with no loss of impetus and 



Fig. 38.-~ISratural (1) and interception (3) systems for laying tile 

 drains. 



allows the more rapidly moving lateral streams to speed up 

 the flow in the main drain. The size of the main depends on 

 the rainfall, the area drained, and the slope. It, of course, 

 must be larger near the outlet than at any other point. The 

 following practical table from Elliott ^ indicates the influence 



^Elliott, G. 0., Engineering for Land Drainage; p. 108, New York, 

 1912. 



