THE POTATO 33 



discharge as soon after a rain as do those laid 

 more shallow. However, the rate of increase does 

 not seem to be so great in the deep tile in reten- 

 tive clay soils. To this is probably due the fact 

 that deep drains continue their discharge when 

 shallow drains have become nearly or wholly dry. 

 Furthermore, deep drains are effective for a 

 greater distance on each side of themselves than 

 are shallow drains. Generally speaking, tile should 

 be laid deeper in sand, muck, and peat than in 

 clay. 



"In narrow wet ravines and in springy spots 

 the demands made upon single lines of tile may 

 require a diameter of five or six inches. With 

 the vast majority of laterals, particularly those in 

 the gridiron systems, this is larger than the diame- 

 ter actually required if the tile remain entirely un- 

 obstructed. However, there should be made an 

 allowance: (1) in sandy soils, for the sand that 

 finds its way into the tile while they are being laid, 

 and before they are properly blinded; (2) in peat 

 soils, for the uneven settling that may take place 

 and cause a tile to 'jog' slightly past the ad- 

 jacent one; and (3) in all soils, for the empty space 

 that should be maintained in the top half of the 

 tile to facilitate the entrance of water. Laterals are 

 most efficient as collecting drains when they are 

 less than half full. They lose this efficiency when 

 made to discharge under a pressure head. For 

 these reasons three-inch tile should be rarely laid 

 except in retentive clay soils and then in lines less 

 than five hundred feet long. Four inches is the 

 diameter most commonly used. 



"Areas requiring drainage are usually so nearly 

 level that it is necessary to make the best possible 

 use of every inch of available fall. The gradient 



