DRAINING. As a certain quantity of moisture is essential to vegetation, so an ex cess of it is highly detrimental. In the re moval of this excess consists the art of Drain ing, which presents three principal features : 1. To drain land which is flooded by water coming over it from a higher level, and having DO adequate outlet below. 2. To drain land where springs rise to the surface, and where there are no natural channels for the water to run off. 3. To drain land which is wet from its impervious nature, and where the evapora tion is not sufficient to carry off all the water.
The first branch includes all those extensive operations where large tracts of land are re claimed by means of embankments, canals, sluices, and tunnels. Such works are gene rally undertaken by associations under the sanction of the government, or by the govern ment itself : the BEDFORD LEVEL is an exam ple of them. In Switzerland many marshes have been laid dry by tunnelling through solid rocky obstructions ; and in Holland vast tracts are protected from flooding by embankments.
Where the land is below the level of the sea at high water, it requires a constant removal of the water which percolates through the banks or accumulates by rains ; and this can only be effected by sluices and mills, as is the case in the fans in England. The water is collected in numerous ditches and canals, and led to the points where it can most conve niently be discharged over the banks. In hilly countries it sometimes happens that the waters which run down the slopes of the hills collect in the bottoms where there is no outlet, and where the soil is impervious. In that case, it may sometimes be laid dry by cutting a sufficient channel all round, to intercept the waters as they flow down, and to carry them over or through the lowest part of the sur rounding barrier. In draining a great extent of land, it is often necessary to widen and deepen rivers, and alter their courses ; and not unfrequently the water cannot be let off with out being carried by means of tunnels under the bed of some river, the level of which is above that of the land.
The draining of land which is rendered wet by springs arising from under the soil is a branch of more general application. The oh.
ject is to find the readiest channels by which the superfluous water may be carried off; and for this purpose an accurate knowledge of the strata through which the springs rise is indis pensable. Abundant springs which flow con tinually generally proceed from the outbreak ing of some porous stratum in which the waters were confined, or through natural crevices in rocks or impervious earth ; and these, as well as land-springs, are traced by geological means. Wherever water springs, there must be a pervious and an impervious stratum to cause it, and the water either runs over the impervious surface, or rises through the cre vices in it. When the line of the springs is found, the obvious remedy is to cut a channel with a sufficient declivity to take off the water in a direction across this line, and sunk through the porous soil at the surface into the lower impervious earth. The place for this channel is where the porous soil is the shallowest above the breaking out, so as to require the least depth of drain ; but the solid stratum must be reached, or the draining will be im perfect. When there is a great variation in the soil, and it is difficult to find any main line of springs, it is best to proceed experi mentally by boring in various parts ; whereby it will generally be easy to ascertain whence the water arises, and how it may be let off. When the drains cannot be carried to a suffi cient depth to take the water out of the porous stratum, it is often useful to bore numerous holes with an auger in the bottom of the drain through the stiffer soil, and the water will either rise through these bores into the drains and be carried off, or it will sink down through them if it lies above. This method is often advantageous in the draining of peat mosses. If the soil, whatever be its nature, can be drained to a certain depth, it is of no conse quence what water may be lodged below it. It is only when it rises so as to stagnate about the roots of plants that it is hurtful.