380 BOARD OF AGRICULTURE. [Pub. Doc. 



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 in- 

 clination 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 through 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 grades should be used. 

 Where the declivity is very great, as upon the springy sides 

 of many New England hills of hard-pan, the lateral drains 

 should as a rule run diagonally with the slope, instead of in 

 the direction of most rapid descent, so as to more effectu- 

 ally cut off springs and underground water veins, which 

 otherwise might appear at the surface between the laterals. 

 The larger the volume of the flow, and consequently the 

 greater the size of the drain to carry it, the flatter may be 

 the grade at which it may safely be laid. Thus, while the 

 fall of two-inch and three-inch drains should rarely be less 

 than three inches per hundred feet, a four-inch drain may with 

 equal safety have a fall of two and one-half inches only, and 

 six-inch and eight-inch drains of two inches per hundred 

 feet. It should be borne in mind, however, that an obstruc- 

 tion in a large or main drain is a much more serious matter 



