28 



integrating effects from freezing and to escape the danger of 

 being obstructed by roots entering at the joints. Lowering tlie 

 water table much below the greatest depth of root action dimin- 

 ishes the moisture raised by capillarity, and is therefore disadvan- 

 tageous. Subject to these considerations and to such variations 

 as the necessities of the grade of the drain and the inequalities of 

 the surface of the ground may involve, from three and one-half to 

 four and one-half feet is a fair average depth to adopt. Less may 

 be used where a low or deep outfall cannot be had, as in the case 

 of flat lands situated at a slight elevation above an adjacent pond 

 or stream which fixes the level at which the main drain may dis- 

 charge. 



The distance between drains is governed chiefly by the greatest 

 depth, within the limits already stated, at which they can be laid, 

 and by the permeability of the soil and subsoil to be drained 

 thereby. In clayey soils, through which water percolates but 

 slowly and with the greatest difliculty, the drains should be placed 

 at a distance of about six to seven feet for every foot of their 

 depth ; while for loamy soils, underlaid by sand, equally good 

 drainage may be secured if the drains are laid at nearly double 

 that distance apart, or ten to fifteen feet for each foot of depth, 

 depending upon the porosity of the underlying material. Thus in 

 clay or hard-pan drains three to four feet in depth should be laid 

 from twenty to thirty feet apart, and for soils underlaid by sand 

 the distance (for the same depth) may be forty or fifty and some- 

 times sixty feet, while in material of intermediate character or 

 porosity a distance of thirty to forty feet would be suitable. 



"Without discussing the various considerations affecting the sizes 

 of tile to be used, it may be said that one thousand feet laid forty- 

 five feet apart will drain an acre of land underlaid by a permeable 

 soil, and that the maximum amount of ground water collected and 

 discharged thereby would rarely exceed the full capacity of a two- 

 inch tile with round bore, laid at such a grade or fall that the water 

 flowing through the same will carry along such fine silt as may un- 

 avoidably enter at the joints, say not flatter than three to four 

 inches in a hundred feet. With a fall of six inches in a hundred 

 feet the same size of tile will carry the water collected by about 

 fifteen hundred feet of drains, and will therefore be sufficient for 

 an acre and one-half of land ; while with a fall of twelve inches in 

 a hundred feet two-inch tile will serve about two thousand feet of 

 drains, or two acres of porous land. 



In clayey lands less of the rainfall percolates into the soil, con- 

 sequently drains therein will receive a smaller quantity of water 

 per acre, which in ordinary practice may be assumed to be about 



