Draining Basins. 307; 



with sides sloping gently to a flat bottom which can carry 

 a considerable volume of water slowly without being eroded. 



383. Intercepting the Underflow from Higher Lands. In 

 a very large number of cases lands require drainage be- 

 cause of the underflow of water from the adjacent higher 

 land in the manner indicated in Fig. 127. In such cases, 



FIG. 127. Showing how lines of tile may be placed at A and B to inter- 

 cept the underflow from the higher land. 



when drains are laid along the foot of the hill below the 

 ground water surface, as represented at A and B, much of 

 the seepage water will rise into the drain and be conveyed 

 away rather than flow on under the flat land beyond. When 

 such corrections as these are made it may even be unneces- 

 sary to underdrain the flat land or when the drains at the 

 foot of the hill do not fully correct the evil the cost is 

 made relatively less. 



384. Draining Basins Without Outlets. There frequently 

 occur sinks or ponds entirely surrounded by rims too high 

 to permit drainage outlets to be constructed across them. 

 Such cases must be met in special ways. 1. Occasionally 

 such basins are underlaid with gravel or sand which is 

 well drained and the water is retained on the surface only 

 by a comparatively thin stratum of clay subsoil. When this 

 is true, one or more wells may be sunk through the clay 

 into the sand or gravel, as represented in Fig. 128, and 

 filled with cobblestone and gravel. Into this underdrains 

 may be led from various directions to collect the water 

 and bring it to the subterranean outlet thus provided. 



2. Where several acres must be drained the above 

 method would hardly be practicable even if the under- 

 drainage conditions were favorable. It is possible, how- 



