460 Irrigation and Drainage 



it to spread laterally and to underflow the low ground 

 through the sandy stratum covered by the closer 

 textured layer above, and to rise up through that 

 soil layer, both by hydrostatic pressure and by cap- 

 illarity, and thus keep it too wet for agricultural 

 purposes. But when tiles are placed at A and B, 

 at the foot of the high lands on both sides, the water 

 can more easily escape into the drain than it can flow 

 on through the sand stratum, and the result is, the 

 pressure which before was forcing the water beyond 

 A to the left and beyond B to the right may now be 

 so nearly all absorbed by the flow of water into the 

 tile drains that no more water reaches the flat land 

 between them than is needed to meet the demands of 

 vegetation and surface evaporation. The case is 

 exactly similar to what is shown in the lower portion 

 of the diagram ; here it is plain that if water is 

 allowed to discharge at C and D nearly as fast as the 

 pipes can bring it from the reservoir, there would 

 be little left to pass on and escape through openings 

 beyond, while if C and D are closed, the full pressure 

 would operate to increase the discharge at lower 

 openings, as at E. 



DRAINING SINKS AND PONDS 



It frequently occurs that low places are entirely 

 surrounded by such high lands as to make it difficult 

 to provide an outlet for the surface water which col- 

 lects in them, especially during the winter and early 

 spring, keeping them too wet for agricultural purposes. 



