Draining, in Agriculture, the process of drawing off the water from bogs, marshes, and lands liable to be flooded by excessive rains, by means of drains or trenches cut to some depth below the surface, which drains serve to collect the waters and convey them off to some lower level. Until the middle of the last century little attention was paid to the draining of lands. The first person who treated the subject systematically appears to have been Dr. J. Anderson, of Edinburgh, who showed that the origin of swamps and morasses lay in the waters, attracted from the atmosphere by the summits of hills and mountains, percolating through the various porous strata of which they are composed, until they arrive at a stratum of clay, which, being impervious to the waters, they stagnate and accumulate, until by their increasing upward pressure they force their way through the soil of the valleys and lowlands at the foot of the hills, and form bog3 and marshes. The principle of his system of drainage consists in intercepting the waters in their progress below the surface from the hills to the low grounds, by a trench running along the base of the hill, and extending to the substratum of clay, which impedes the progress of the water; from this trench a drain is cut, to convey the waters to the nearest channel that will carry them away.
Mr. Elkington, about the same period, appears to have paid great attention to the subject in England, and for various improvements which he introduced in the practice, received from Parliament the sum of 1000f. In cases where the top soil is of considerable depth, and no water therefore rises into the ditch, after cutting five or six feet down, both Mr. Elkington and Dr. Anderson recommend to bore with a proper auger until the clay be reached, when the water will rise through the holes into the trench. These trenches should be made narrower as they descend, by spades of a proportionate size, but the lowest part ought to be more contracted than any other, so that the shoulders or edges of it may support stones or faggots, in order to cover the whole at a small expense without obstructing the currents of water. In many places hollow bricks or ridge tiles are substituted for stones or faggots, as being cheaper. When the land to be drained consists of a long level tract, without sufficient fall to carry off the water from the reservoirs or pits into which it is conveyed by the drains, or when there is a rising ground or an embankment between the drained land and the level which is to convey the water off, it must be elevated mechanically; for this purpose pumps, driven by windmills, are very extensively employed in Lincolnshire.
Where there is sufficient outfall, water may also be conveyed over an intervening obstacle by means of a syphon, provided that the height of the syphon do not much exceed 20 feet; but one objection to this method consists in the interruption to which the action of the syphon is liable, from the extrication of the air which all common water contains, and which begins to separate from that fluid as it rises under diminished pressure in the short leg of the syphon, till at length the angle of the syphon is filled with air, and the current of water is interrupted. To remove this defect, Mr. Cowen, of Carlisle, places a box at the upper angle of the syphon, into which all the air separated from the water rises; and the application of a forcing pump for a few minutes once or twice a day drives the air out of the box into the atmosphere. Fig. 1 represents Mr. Cowen's plan applied to the draining of a quarry, a a represent a part of the quarry; b a level lower down than the bottom of the quarry, which, in this case, is 200 yards off where the water is to be discharged; c the highest ground over which the water is to be conducted; d, e, and f, three distinct rises, over which the pipes pass (they are further apart than here shown); g g h h the lead pipe, composing the two legs of the syphon; i a common forcing pump, fixed below the surface of the water, having a hinge valve at j, Fig. 2, and an open working box, with a similar valve k.
These valves are opened by the force of the water flowing through the pump in its passage to the syphon; l the working handle of the pump; m is a close iron receiver, shown larger in Fig. 3, having inserted at the end n the ascending leg of the syphon g, and at the other end o the descending leg h; at the upper side of both ends are inserted two small air pipes p p, joining with the syphon pipe at the highest bends, as shown at d, e, and f, with a regular slope into the interior, to allow the air to ascend into it; g is a small valve fixed at the highest point of the receiver, to allow the whole of the air to escape through the valve when the water is forced up by the pump; for this purpose the pump i and the pipe g must be capable of supplying the water quicker than the pipe h will carry it off.