The drawing off of the pent-up waters which are the sources of springs is a department of draining which requires a considerable knowl edge of the different varieties of strata.
In the laying out of drains, the first point to be determined is the place of outfall, which should always afford a free and clear outlet to the drains, and must necessarily be at the low est point of the land to be drained. This should be ascertained by a leveling instrument; and where a large extent of work is to be done, a competent surveyor should always be employed. A simple form of level for this purpose is a length of •ordinary garden hose, with a glass tube in each end. When the hose is filled with water, the height in the two end tubes will be equal. Two persons with such a level can easily mark desired heights on a row of stakes, and so get any desired inclination. The outfalls should be as few as possible, and each one should be carefully set in brickwork and covered by a grid hinged at the top to pre vent the ingress of rats and moles, but capable of giving way to a sudden rush of water from drains above. The next point to be determined is the water level or water table. This may be ascertained by digging test holes in different parts of the field and noticing the height to which the water rises in them. In an undulat ing field the height will vary in different places, but the knowledge gained by means of the test holes of the nature of the soil and subsoil will indicate the depth of draining required. In general it will be found that in clay land the drains should be shallow and close together, and in sandy or light land deeper and wider apart; the looser nature of the lighter soil per mitting the rapid percolation of water, and allowing the drains to draw from a wider area. Drains in arable land must be beyond the in fluence of agricultural implements; not only out of the reach of plough or cultivator, but too deep to be displaced by their passing weight. Less than two feet six inches from the surface is unsafe, except in very stiff clay pasture land which is never disturbed. The depths usually adopted in practice for the various classes of soils are as follows: Stiff clay 2 ft. 6 in. to 3 ft.
Medium soils 3 ft. 3 in. to 3 ft. 6 in.
Light soils 3 ft. 6 in. to 4 ft. 6 in.
As to the distance apart, experience has es tablished a rule that in clay soils a drain will •draw) an area of from 5 to 6 times its depth, on medium soils 7 to 9 times, and on light soils 8 to 10 times its depth. Thus 3-foot drains in the first case might be 15 feet apart, in the second 24 feet, and in the third 40 feet apart. Submains may be necessary in an undulating field and in all cases they are desirable where the minor drains would other wise be more than 10 chains in length. The rate of fall of any drain must not be less than 1 in 200. All drains should be kept as far as possible from the roots of trees, and curves are to be avoided, except at the junction of minor with main drains • or submains, when the junction must form an acute angle, the smaller drain curving toward the larger drain and entering it from above. Two minor drains must not enter a large drain opposite to each other, or the stop page thus caused will result in the deposition of silt and consequent chokage of the drain.
Drains are cut from the lowest point upward and the pipes laid in each section day by day. To ascertain if the pipe bed is being laid per fectly true, three leveling staves or ((hotline rods are used. Each consists of a staff and crosshead, and by placing them at various points in the drain and sighting over the cross heads, uniformity of the bed is determined. The surface soil in cutting the drains ought to be laid on one side of the trench and the subsoil upon the other side, and each should be re turned carefully to its original position. The size of the pipes to be used is dependent upon a number of factors, such as character of soil and subsoil, gradient, rainfall and length of drains; but in practice it is found most profitable to use 2Y2-inch to 3-inch pipes for minor drains, and 6-inch, 8-inch or 10-inch for mains and sub-. mains. The number of pipes required per acre is found by dividing the area of an acre in square feet by the distance between the drains in feet, provided the pipes are one foot in length. The following table will be of value in this particular : Miners { Lengths aoo to 700 feet. 1i in. Lengths soo to 5,00o feet. 3 in. Lengths up to soo feet, a in.