Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/358

Rh 332 A G R I C IT L T II R E [DRAINING persisting in fruitless attempts to dry extensive areas by a few dexterous cuts, he insisted on the necessity of pro viding 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. The distances between drains he showed must be regulated by the greater or less reteutiveness of the ground operated upon, and gave 10 feet as the minimum, and 40 feet as the maximum of these distances. The depth which he pre scribed for his parallel drains was 30 inches, and these were to be filled with 12 inches of stones small enough to pass through a 3-inch ring in short, a new edition of Blithe s drain. A main receiving-drain was to be carried along the lowest part of the ground, with sub-mains in every subordinate hollow that the ground presented. These receiving drains were directed to be formed with a culvert of stone work, or of tiles, of waterway sufficient to contain the greatest volume of water at any time requiring to be passed from the area to which they respectively supplied the outlet. The whole cultivated lands of Britain being disposed in ridges which usually lie in the line of greatest ascent, it became customary to form the drains in each furrow, or in each alternate, or third, or fourth one, as the case might require or views of economy dictate, and hence the system soon came to be popularly called furrow draining. From the number and arrangement of the drains, the terms frequent and parallel were also applied to it. Mr Smith himself more appropriately named it, from its effects, thorough draining. The sound principles thus promulgated by him were speedily adopted and extensively carried into practice. The great labour and cost incurred in procuring stones in adequate quantities, and the difficulty of carting them in wet seasons, soon led to the substitution of tiles and soles of burned earthenware. The limited supply and high price of these tiles for a time impeded the progress of the new system of draining ; but the invention of tile-making machines by the Marquis of Tweeddale and others, removed this impediment, and gave a mighty stimulus to this fundamental agricultural improvement. The substitution of cylindrical pipes for the original horse shoe tiles has still further lowered the cost and increased the efficiency and permanency of drainage works, rn The system introduced and so ably expounded by Smith 11 of Deanston has now been virtually adopted by all drainers. Variations in matters of detail (having respect chiefly to the depth and distance apart of the parallel drains) have indeed been introduced ; but the distinctive features of his system are now recognised and acted upon by all scientific drainers. 11. In setting about the draining of a field, or farm, or estate, the first point is to secure, at whatever cost, a proper outfall. The lines of the receiving drains must next be determined, and then the direction of the parallel drains. The former must occupy the lowest part of the natural hollows, and the latter must run in the line of the greatest ascent 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 undu lating 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 tlie ground, irrespective of the furrows altogether. When parts of a field are flat, and other parts have a con siderable 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 hedge-rows and trees a good offing, lest the conduit should be obstructed by roots. When a drain must of necessity pass near to trees, we have found it practicable to exclude their roots from it by the use of coal-tar. In our own practice, a drain carried through the corner of a plantation has by this expedient remained free from obstruction for now fourteen years. In this instance the tar was applied in the following manner : Sawdust and coal-tar being mixed together to the consis tency of ordinary mortar, a layer of this was laid in the bottom of the trench; the drain-pipes were then laid, and completely coated overwiththe same mixture to the thickness of an inch, and the earth carefully replaced in the ordinary way. 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. Indeed, we have found it expedient in such cages to have two receiving drains parallel to each other, each to receive the subordinate drains from its own side only. As these- receiving drains act also as ordinary drains to the land through which they pass, no additional cost is incurred by having two instead of one, provided they arc as far apart as the other drains in the field. 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 ten to fifteen acres is a gcod arrangement. These outlets should be faced with mason- work, and guarded by iron gratings. The depths of the parallel drains must next be deter- Depths, mined. In order to obtain proper data for doing so, the subsoil must be carefully examined by digging test-holes in various places, and also by taking advantage of any quarries, deep ditches, or other cuttings in the proximity, that afford a good section of the ground. We have already expressed an opinion that the drains should not be less than four feet deep ; but it is quite possible that the discovery at a greater depth than four feet of a seam of gravel, or other very porous material charged with water, underlying considerable portions of the ground, may render it expedient to carry the drains so deep as to reach this scam. Such a seam, when furnished with sufficient outlets, supplies a natural drain to the whole area under which it extends. When such exceptional cases are met with, they are precisely those in which deep drains, at Avide intervals, can be trusted to dry the whole area. When the subsoil consists of a tenacious clay of considerable depth, it is considered by many persons that a greater depth than three feet is unnecessary. The greater depth is, however, always to be preferred ; for a drain of four feet, if it works at all, not only does all that a shallower one can do, but frees from stagnant water a body of subsoil on Avhich the other has no effect at all. It has indeed been alleged that such deep drains may get so closed over by the clay that water will stand above them. If the surface of clay soil is wrought into puddle by improper usage, water can undoubt edly be made to stand for a time over the shallowest drains as easily as over the deepest. But the contraction which takes place in summer in good alluvial clays gradually estab lishes fissures, by which water reaches the drains. In such soils it is usually a few years before the full effect of draining is attained. This is chiefly due to the contraction and con sequent crackir.g of clay soils in summer just referred to, and partly, as Mr Parkes thinks, to the mining operations of the common earth-worm. Both of these natural aids to drainage operate with greater force with drains f our feetdeep than when they are shallower. The tardy percolation of water through