Distance Between Drains 



441 



is desired to place the lines of tile close enough 

 together, so that after 6 days following an inch of 

 rain the water shall nowhere stand within 3 feet of 

 the surface, and that the tiles are placed 4 feet deep. 

 Since in the sandy subsoil of the upper part of the 



Fig. 139. Diagram of influence of distance between drains on 

 depth of drainage. 



field the mean gradient is 1 foot in 175, the lines 

 of tile may, under such conditions, be placed twice 

 this distance apart, or 350 feet, for then halfway 

 between them the water would only stand 1 foot above 

 the drains and hence 3 feet below the surface. But 

 in the lower part of the field, where the soil is finer 

 and where the observed mean gradient is 1 in 27, 

 the lines of tile could only be placed 54 feet apart 

 to ensure the same conditions. 



It was pointed out, in connection with Fig. 133, 

 that the slope of the ground water toward the lake 

 was at the rate of 1 foot in 24.4 early in the season, 

 and later 1 foot in 35.86 feet, which would call for 

 placing the lines of tile 50 to 72 feet apart. Refer- 

 ring to the diagram, Fig. 139, it will be readily under- 

 stood that when there is a drain at A and C only, 

 the soil undrained must be highest at B, but if an 



