LAND DRAINAGE 



647 



the compact layer to receive the tile, and if its depth 

 exceeds half the diameter of the tile special care should 

 be taken to place the topsoil or some other porous material 

 on the tile and around the joints in order to insure the 

 entrance of water. 



549. Distance between drains. The interval be- 

 tween drains must be determined by the nature and the 

 wetness of the soil and the value of the crops produced. 

 In soil where drains must be installed at a depth of two 

 and a half feet or less, for general farming the interval 

 between drains must ordinarily be not more than fifty 

 feet. Where they may be placed deeper, the interval 

 may be correspondingly greater. 



The number of feet and rods of tile required when the 

 lines are laid regularly at a specified distance apart is 

 given in the following table : 



Under the influence of the drains the physical nature 

 of the surface soil and of the subsoil gradually changes 

 and undergoes improvement. Lines of seepage develop, 



