But to proceed : the secondary drain, which supplies the whole of a field through which it passes, should be inter rupted at every fall of four inches at farthest, by small sluices, or penstocks, and have small branch-drains cut to the right and left, in such manner as may cause the water to branch out into the whole expanse of its level. The turf cut from the surface of each drain, Ought to be placed, face downwards, between it and the land it is to overflow ; being made firm and level, by beating with the flat of a spade. As the penstocks are situated just below the lines of the b ranch drains above described, they keep up the water, so as to fill, and to cause their overflowing into the next inferior talus or slope, as shown in fig. 1 and 2, where A is the main drain, taken from the water head or river, B ; the drain C, C, C, C, shows the secondary drain, which, being on a declivity, would carry off all the water, were it not kept up at the places where the catch-drains, or branches, D, D, D, D, proceed laterally from it, by the sluices E, E, E, E. By this means, any particular level, either 1, 2, 3, 4, may be irrigated at pleasure, without wetting the others ; the water being kept on by the sluice above, and carried away by the sluice appertaining to each level re spectively. Or, if other meads at some distance are to be watered, the secondary channel, having all its sluices open, will convey it to them without interruption, when all its sluices are opened.
It is evident, that in this manner the whole of the water is carried down to the lowest level : hence it becomes a matter of no small importance to ascer tain, that the whole shall either be ab sorbed or be carried off ; so as not to in jure the last level, which might otherwise be subjected to very considerable injury, were the inundation to be too long sup ported. The judicious computer will be cautious not to allow so much to remain as may rot his grass ; in lieu of causing it to vegetate vigorously. This, in some situations, presents a serious diffi culty; for if the water is debarred free access to the lowest levels, they will be less fruitful than the others, which, ex clusive of the great fecundity derived from first receiving the fluid, receive ab solutely a larger portion of moisture. The greatest care is therefore requisite, to in sure that the tail, or spent-water, shall be carried off. Where the declivity is con siderable, and that the stream, or any other water-course, offers itself to receive such tail-water, at a due level beneath, there is no difficulty ; but where the stream takes another course, and the descent is trifling, some artificial means must be resorted to. Perhaps no more simple or efficacious plan can be hit upon, than that of forming a fish-pond, of a suit able extent and depth, to receive the tail-water ; whereby the apprehended damage may be avoided, and a useful store be created.
We shall Show what we may term a truly ingenious device, whereby water may be laid upon lands that are above the level of the stream : it consists merely of an air-vessel, A, fig. 3, into which the water descends forcibly from the stream, B, and by compressing the air in the up per part, C, is itself forced to ascend through the conducting pipe, D, with such force as to rise to a level, E, far above that at which it formerly stood. This is
the principle of the common fire-engine, which we are all sensible, can, when exert ed, throw water to a great height. By such means, the tail-water may also be forced up to such a level as may cause it to return into the stream.
Where the stream runs through the lands that are watered, and that its de clivity is moderate, it will sometimes be found difficult to restore the tail-water to its level. To effect this with as little ex pence as possible, wooden pipes should be laid from the lowest level of the land along the bank of the stream, but carried horizontally on a bank, to such extent as may suffice to convey the tail-water to the surface of the surface. This, how ever, is not applicable to all situations ; for where the stream is very slow, its de clivity would be very trifling. Where that happens, the air vessel will be found a good plan, provided the height to which the water is to be returned, be not con siderable. In many situations, a water wheel might answer well ; observing, that in deep slow waters, that are broad, and under the speculator's own management, it will be best to throw a weir across, and then to let the whole body of the stream rush through a narrow slip, so as to turn a wheel placed immediately in the line of the water's run. By this device, the cur rent may be made to pass that particular spot with sufficient velocity to turn a wheel ; whereby water might either be raised out of the river, to supply a main drain, or the tail-water might he restor ed to the stream : in either case, one or more pumps would be necessary. (See fig. 4.) The second mode of laying water over the land is by means of ridges, whose centres are occupied by small horizontal drains, out of which the water, furnished by the main drain, is allowed to flow to the depth of about an inch down each side of the pitch. These ridges should be from four to six feet measurement for each face ; the drain being about a foot broad, and four inches deep ; thus the whole breadth of a pitch, declining each way equally, might occupy a base of about ten feet at the utmost. The de clivities ought not to exceed an inch to the foot; in loose soils, not more than half an inch ; else the finer parts will be washed away, and the drains, formed by the junctions of the ridges, will be filled up, whereby the water will be detained, and prevented from passing into the next level. Fig. 5, shows the profile, or section of a range of ridges on the same level, and fig. 6, displays an inclined plane, whereon ridges are formed in regular succession, the catch.drains being a little higher than the branch drains of the next lower level, so that the latter may be fill ed from the former : the water thus gra dually descending, until the whole is gra dually absorbed by the successive ridges; or the surplus is carried off by a large catch-drain, made to direct it into some other succession of ridges, as seen in the ground.plan, fig. 7.