29 



one-half as great as that in lands underlaid by free water-bearing 

 sand. Therefore the same size of tile or capacity of main drain 

 will snffice for about double the area of clayey subsoils that 

 would be needful for sandy subsoils ; and, inasmuch as the drains 

 should be placed at about one-half the distance in clayey soil, it 

 follows that the amount or length of drain that may be discharged 

 through a two-inch tile for such lands is about four times as great 

 as in those having a free subsoil. Thus, while a thousand feet of 

 two-inch tile, laid forty-five feet apart in a free subsoil, will serve 

 one acre, four thousand feet of the same size, laid twenty-two and 

 one-half feet apart, will serve two acres of clay land. 



The relative capacities of different sizes of tiles are approxi- 

 mately as follows, that of a two-inch tile being indicated by 1, 

 to wit : — 



Capacity of two-inch tile equals, . 

 Capacity of two-and-one-half-inch tile equals, 

 Capacity of three-inch tile equals, . 

 Capacity of four-inch tile equals, . 

 Capacity of five-inch tile equals, . 

 Capacity of six-inch tile equals, 

 Capacity of eight-inch tile equals, . 



1.00 



1.50 

 2.50 

 5.00 

 7.60 

 12.50 

 25.00 



Thus for land requiring two-inch tile to carry the maximum 

 drainage from one acre, a main drain of four-inch tile will serve 

 five acres, one of six-inch tile twelve and one-half acres and one 

 of eight-inch tile twenty-five acres. These sizes may be considered 

 suitable for the areas stated, where the fall is not less than three, 

 or better, four inches per hundred feet in main drains or laterals. 

 Where the fall is twice as much, or not less than six inches per 

 hundred feet, the same sizes will suffice for fifty per cent larger 

 areas ; and in general for clayey soils, with the same inclination 

 as in free soils, tile of a given size will serve double the area. 



Larger tile than are absolutely necessary to carry the maximum 

 amount of water to be discharged through them make an inferior 

 though more costly system of drainage, — inferior, because the 

 velocity of flow is slower througli tile of larger bore, consequently 

 any silt which enters is more certain to be deposited in the tile 

 and to gradually fill it up. 



The flattest grade at which tile drains should be laid should be 

 sufficient to insure a perfect scour, — that is, the carrying along 

 by the water flowing therein of all silt which may enter at the 

 joints. With care in laying at true grade, a fall of three inches 

 in a hundred feet is as little as may be safely adopted for two-inch 

 tile, and where the topography or surface contour allows, steeper 



