Chap. XXVIII.
DRAINING AND EMBANKING.
1559
^>Ln_n_n
HARTLEPOOL.
protect Us course by building jetties, or soeffectually to cover it that no obstructions canoccur.
After the main drain in a level has been cut,the rivers which run into it are embanked, andcatch-water drains are formed to collect thewater flowing from a higher level; the severalchannels must be kept open by dredging, scour-ing, and cutting down the aquatic plants andweeds. Attention must then be turned to thesubsoil draining, that the surface may be renderedvaluable for the purposes of agriculture.
Subsoil Drainmg. — Geology has assisted thisoperation very materially by rendering us ac-quainted with the quality and nature, as well assuccession of the soils beneath the surface tobe drained. The soils .which are imperviousare usually the most heavy, and the porous arethose of a lighter quality. Clays, when theyreceive water, will only part with it by evapo-ration when left in a natural state, and there-fore to make such a surface fit for agriculture,or to render it dry for any useful purpose, or toimprove the atmosphere above it, requires con-siderable ingenuity and a great expenditure.
^ _ Such a soil is fertile when drained, and
it is only necessary to prevent the waters from the neighbouring lands overspreadingit, or to contrive means to carry off what falls in rain, and is not required to nourishthe crops: it is not rendered wet by the action of the underground springs, and maybe effectually drained by penetrating through the clay, and letting off the water intothe under-stratum where this is of a porous quality, as chalk or such material. The surfaceof clay is often cut into drains, which are taken in a diagonal direction across the piece otland operated upon;, these are made at about 15 or 20 feet apart, and usually about 2 or
3 feet in depth; they are often filled in with loose stones, and again covered up. Draintiles are admirably adapted for grass land, as well as that under the plough; they areabout 13 or 14 inches in length, 4 inches in width at bottom, and a little more in height;they have an arched channel for the passage of the water, 3 inches in width at bottom, and
4 inches in depth : the drains are cut just large enough for their insertion, and not deeperthan is required to prevent their being disturbed in the ordinary process of cultivating thesoil above. When the tiles are put into the channels, and laid with a proper current todischarge into the drain prepared at their lower extremity, some straw is laid over them toprevent any loose earth from falling in or choking them up : in general, 1000 tiles are putto an acre, and the cost of draining that quantity of land varies from 31. to 41.
Another common practice is for the drainer to take out a spit of clay with a wedge-formed spade or tool, and then to deepen it by another of the same form but smaller, after"which he uses a scoop to remove any crumbs that may have fallen down : the drain whenfinished presents the section of a wedge 30 inches deep, 1 foot wide at top, and only 2 or3 inches at bottom, and indeed the narrower it is the stronger it is. It is then filled in withbrushwood to the height of 10 inches, and covered with long wheat-straw twisted intobands, which are carefully laid in by hand; after this the earth is thrown on, and the
5 g 4