218 SOILS 



DISTANCE BETWEEN UNDER-DRAINS 



This depends chiefly upon the nature of the 

 subsoil and the depth of the drains. The ease 

 with which water can pass through the subsoil to 

 the drains would naturally have much influence in 

 determining the distance apart of the laterals. 

 Water will pass to the drains through a sandy sub- 

 soil from ten to one hundred times more rapidly 

 than through a stiff clay subsoil. The coarser the 

 subsoil and the freer it is from hard-pan the more 

 readily does water move to the drains and the 

 farther apart they may be placed. In other words, 

 the more open the subsoil is the farther apart 

 should the drains be, for the excess water in such 

 soils quickly drains off. The movement of water 

 in compact subsoils is very slow. 



The depth of drains should be considered in 

 deciding their distance apart; the deeper they are 

 the lower do they make the water-table. Under- 

 drains do not lower the water-table of the entire 

 field to their own level. At the point where the 

 drains are placed it is lowered to that level, but 

 midway between two lines of drains it may be 

 several inches or even several feet higher, depend- 

 ing upon the openness of the soil and the ease with 

 which water passes through it laterally. The 

 water-table of an undrained field is a series of 

 curves or crescents, the lower ends of each curve 

 being on a level with the bottom of the drains. So 

 the deeper drains are placed the farther apart they 

 may be without danger of the water-table coming 

 too near the surface, midway between the lines of 

 drains. 



The common distances apart for laying tile 

 drains are 20 to 30 feet on deep, very compact clays ; 



