Distance Between Drains 



439 



ment of the water in cases like that represented in 

 Fig. 136 will be chiefly straight down through the 

 soil and clay until the sand is reached, when the 

 movement will be sideways toward the drains and 

 finally upward, the water entering them chiefly from 

 the under side. That is to say, the flow sidewise 

 through the clay toward the drains will be very slight 

 indeed. 



Since the resistance to flow of water increases as 

 the soil texture becomes more close, it is clear that 

 the more open the soil the farther apart the drains 

 may be placed. It is common to place lines of tile 

 in underdraining varying distances apart, from 30 feet 

 to 100 and even 200 feet. The reasons for these-wide 

 differences will be better understood after considering 

 the way the ground -water surface changes under a 

 tile -drained field following a rain. 



Fig. 137. The observed surface of the ground water in a tile-drained field 

 48 hours after a rainfall of .87 inches. 



In Fig. 137 is represented the observed slope of the 

 ground -water surface in a tile -drained field where 

 the lines are placed 33 feet apart and between 3 and 



