parallel drains. The former must occupy the lowest part of the natural hollows, and the latter must run in the line of the greatest slope of the ground. In the case of flat land, where a fall is obtained chiefly by increasing the depth of the drains at their lower ends, these lines may be disposed in any direction that is found convenient; but in undulating ground a single field may require several distinct sets of drains lying at different angles, so as to suit its several slopes. When a field is ridged in the line of the greatest ascent of the ground, there is an obvious convenience in adopting the furrows as the site of the drains; but wherever this is not the case the drains must be laid off to suit the contour of the ground, irrespective of the furrows altogether. When parts of a field are flat, and other parts have a considerable acclivity, it is expedient to cut a receiving drain near to the bottom of the slopes, and to give the flat ground an independent set of drains. In laying off receiving drains it is essential to give hedgerows and trees a good offing, lest the conduit be obstructed by the roots.
When a main drain is so placed that parallel ones empty into it from both sides, care should be taken that the inlets of the latter are not made exactly opposite to each other. Much of the success of draining depends on the skilful planning of these main drains, and in making them large enough to discharge the greatest flow of water to which they may be exposed. Very long main drains are to be avoided. Numerous outlets are also objectionable, from their liability to obstruction. An outlet to an area of from 10 to 15 acres is a good arrangement. These outlets should be faced with mason work, and guarded with iron gratings.
The distance and depth apart of the parallel drains is determined chiefly by reference to the texture of the soil. In an impervious clay the flow of the water is much impeded and the water-table can be controlled only by frequent lines of pipes. On such land it is customary to lay them about 3 ft. from the surface and from 15 to 21 ft. apart. In lighter soils the depth, and proportionately the distance apart, is increased, but the drains are rarely more than 4 ft. 6 in. below the surface, though they may be 75 or 100 apart. A fall of at least 1 in 200 is desirable.
There are various forms of under-drainage, some of them alluded to in the historical section below, but by far the commonest is by means of cylindrical or oval pipes of burnt clay about 1 ft. in length, sometimes supplemented by collars, though nowadays the use of these is being abandoned. Pipes vary in bore from 2 in. for the parallel to 6 in. for the main drains.
In constructing a drain, it is of importance that the bottom be cut out just wide enough to admit the pipes and no more. Pipes, when accurately fitted in, are much less liable to derangement than when laid in the bottom of a trench several times their width, into which a mass of loose earth must necessarily be returned. This is easily effected in the case of soils tolerably free from stones by the use of draining spades and the tile-hook which are represented in the accompanying cut. The tile-hook is an implement by means of which the pipes may be lowered from the edge of the trench and laid at the bottom. An implement, sometimes propelled by steam, known as the draining plough, can be used for opening the trenches. Draining can be carried on at all seasons, but is usually best done in autumn or summer. A thoroughly trustworthy and experienced workman should be selected to lay the pipes, with instructions to set no pipes until he is satisfied that the depth of the drains and level of the bottoms are correct. The expense of tile-drainage may vary from about £2:10s. per acre on loose soils to £10 an acre on the most tenacious soils, the rate of wages and the cost of the pipes, the depth of the trenches and the ease with which they can be dug, all influencing the cost of the process.
Drainage is not a modern discovery. The Romans were careful to keep their arable lands dry by means of open trenches or covered drains filled with stones or twigs. It is at least several centuries since covered channels of various kinds were used by British husbandmen for drying their land. Walter Blith (see Agriculture) about the middle of the 17th century wrote of the improvement which might be effected in barren land by freeing it from the excess of stagnant water on or near the surface by means of channels filled with faggots or stones, but his principles, never generally adopted, were ultimately forgotten. In the latter half of the 18th century, Joseph Elkington, a Warwickshire farmer, discovered a plan of laying dry sloping ground that is drowned by the outbursting of springs. When the higher-lying portion of such land is porous, rain falling upon it sinks down until it is arrested by clay or other impervious matter, which causes it again to issue at the surface and wet the lower-lying ground. Elkington showed that by cutting a deep drain through the clay, aided when necessary by wells or auger holes, the subjacent bed of sand or gravel in which a body of water is pent up by the clay, as in a vessel, might be tapped and the water conveyed harmlessly in the covered drain to the nearest ditch or stream. In the circumstances to which it is applicable, and in the hands of skilful drainers, Elkington’s plan, known as “sink-hole drainage,” by bringing into play the natural drainage furnished by porous strata, is often eminently successful.
Draining Implements.
During the subsequent thirty or forty years most of the draining that took place was on this system, and an immense capital was expended in such works with varying results. Things continued in this position until about 1823, when James Smith of Deanston, having discovered anew those principles of draining so long before indicated by Blith, proceeded to exemplify them in his own practice, and to expound them to the public in a way that speedily effected a complete revolution in the art of draining, and marked an era in agricultural progress. Instead of persisting in fruitless attempts to dry extensive areas by a few dexterous cuts, he insisted on the necessity of providing every field that needed draining at all with a complete system of parallel underground channels, running in the line of the greatest slope of the ground, and so near to each other that the whole rain falling at any time upon the surface should sink down and be carried off by the drains. A main receiving drain was to be carried along the lowest part of the ground, with sub-drains in every subordinate hollow that the ground presented. The distances between drains he showed must be regulated by the greater or less retentiveness of the ground operated upon, and gave 10 to 40 ft. as the limits of their distance apart. The depth which he prescribed for his parallel drains was 30 in., and these were to be filled with 12 in. of stones small enough to pass through a 3-in. ring—in short a new edition of Blith’s drain. Josiah Parkes, engineer
