OTHER CONDITIONS AND PROBLEMS 157 



sufficiently deep to lay a six-inch board in the bottom so 

 that the upper surface of the board shall lie at the proper 

 depth and upon this the tile be laid and the earth intro- 

 duced about the tile. Boards thus used will resist decay 

 for many years. 



208. To remove excessive surface water. It some- 

 times happens that the topography of a tile-drained area is 

 marked by depressions which, in times of heavy rain, 

 become filled with surface water. This means that an 

 amount of water which, if distributed uniformly over 

 the drained area, would be removed in reasonable time 

 by the tile system, must, because of the relatively greater 

 quantity over a restricted low area, require an unusual 

 or extended time to make its way through the soil to the 

 drains. The result may easily be the drowning of a crop, 

 injury to the structure of the soil, the washing away of 

 plant-food, or the destruction of food by denitrification. 

 Again, a tile-drained area may lie subject to the overflow 

 from surface drainage from adjacent higher areas, or ad- 

 jacent slopes, and therefore to the same consequent ills 

 as those named above. Both Elliott and Waring suggest 

 provision for the quick removal of such surface water by 

 ducts, or other means, by which the surface water is con- 

 veyed below ground to the tile. Wherever, in an area 

 to be tile-drained, surface depressions occur, in which the 

 surface water may gather in large quantities, the system 

 should be planned so that a main or lateral crosses it. 

 If a lateral crosses the area, and the area is of consid- 

 erable size, the size of the tile in the lateral should be 

 larger than that used in the ordinary lateral of similar 

 length. 



To provide means for the passage of the water from the 

 surface to the tile, one of the schemes suggested is to lay 



