DRAINAGE, in agriculture, a method of improving the soil by withdrawing the water from it. Though practised by the Romans, and though the value of drainage was expounded by Walter Blithe in the middle of the 17th century, it was not till after the middle of the 18th cen tury that the importance of drainage began to be understood in Great Britain. The public at tention is said to have been then excited by the practice of Elkington, a farmer of Warwick shire, England. But it was James Smith of Deanston, Perthshire, Scotland, who about 1823 led the way in modern practice of thorough draining.
The successful practice of draining in a great measure depends' on a proper knowledge of the various strata of which the earth is com posed, as well as of their relative degrees of porosity or capability of admitting or rejecting the passage of water through them and likewise of the modes in which water is formed and conducted from the high or hilly situations to the low or level grounds. In whatever way the hills or elevations that present themselves on the surface of the globe were originally formed, it has been clearly shown, by sinking large pits and digging into them, that they are mostly composed of materials lying in a stratified order, and in oblique or slanting directions downwards. Some of these strata, from their nature and properties, are capable of admitting water to percolate or pass through them, while others do not allow it any passage, but force it to run or filtrate along their surfaces without penetrating them in any degree, and in that way conduct it to the more level grounds below. There it becomes obstructed or dammed up by meeting with impervious materials of some kind or other, by which it is readly forced up into the superincumbent layers, where they happen to be open and porous, soon rendering them too wet for the purposes of agriculture; but where they are of a more tenacious and impenetrable quality, they only become gradually softened by the stagnant water below them; by which the surface of the ground is, however, rendered equally moist and swampy, though somewhat more slowly than in the former case.
Where grounds are in a great measure flat, and without degrees of elevation sufficient to permit those over-proportions of moisture that may have come upon them from the higher and more elevated grounds to pass readily away and be carried off, and where the soils of the land are composed or constituted of such materials as are liable to admit and retain the excesses of moisture, they are exposed to much injury and inconvenience from the retention and stagnation of water. Such lands consequently require arti ficial means to drain and render them capable of affording good crops, whether of grain or grass.
Wetness of land, so far as it respects agri culture and is an object of draining, may gener ally depend on the two following causes:— first, on the water which is formed and collected on or in the hills or higher grounds, filtrating and sliding down among some of the different beds of porous materials that lie immediately upon the impervious strata, forming springs below and flowing over the surface, or stagnat ing underneath it; and, secondly, on rain or other water becoming stagnant on the surface, from the retentive nature of the soil or sur face materials, and the particular nature of the situation of the ground. The particular wetness which shows itself in different situations, in the forms of bogs, swamps and morasses, for the most part proceeds from the first of these causes; but that superficial wetness which takes place in the stiff, tenacious, clayey soils, with little inclination of surface, generally originates from the latter.
Drains.—The drains used in land drainage may be divided into two classes— open and covered drains. These again may each be sub divided into drains intended merely to act as water-courses and drains which, in addition to acting as water-courses, • are also intended to carry off the surplus water from the land through which they pass.