Chaf. XXVIII.

DRAINING AND EMBANKING.

1557

of the pool that receives it; the frames and sluice-doors are of iron; the latter, are workedby rack and pinion-wheels within a copper groove.

In forming the channels for these lets-off, it is necessary to guard against the tearingup of the beds by the rush of water, by conducting it over a watercourse lined with

Fig. 2900.

Fig. 2899.

Fig. 2901.

masonry : a platform of timber is laid on piles over soft ground to receive stone ; thesection given to it varies according to the nature of the situation, or quantity of water to bedischarged.

Stop Gates are sometimes placed at the bottom of the canal, lying partially afloat, so thatany motion in the canal water makes them rise and shut against a vertical frame in themasonry of the abutments on each side: others are so contrived that they can be drawnacross the canal from a recess, and so made to run in a groove, but the most simplemethod is the common lock-gate, or the sluices made in the sides of the canal, connectedwith drains as first described.

Reservoirs for supplying the Canal with Water must be placed sufficiently low to collectall the flood waters that may be required, and at the same time high enough to enable allthe water they contain to be drawn into the summit of the canal: the basin should bemade narrower towards its lower extremity, in order to contract the base of the embank-ment as much as possible, and the bottom and sides must be impervious to water. Thedimensions of the embankment depend upon its height and the materials to be used; ingeneral, the top is 3 or 4 yards higher than that of the water when the reservoir is full, andthe breadth of the top is then about 5 yards ; the outside slopes at the rate of three hori-zontal to one perpendicular, and the inside two to one. It is not only highly importantfor the engineer to take into consideration the best position for the reservoir, but also tobe assured that he has the means of a regular as well as abundant supply of water on alloccasions.

Aqueducts are of course requisite where rivers are to be crossed, as are weirs where thewaste water is to be discharged; several of these have already been described in the historyof canals.

CHAP. XXYin.

DRAINING- AND EMBANKING.

When a level is to be drained, or the water carried off from the surface of a swamp orfenny district, the first point for the engineer to ascertain is the situation of the lowestoutfall, in discovering which nature frequently affords him considerable assistance: theruns of water in flood times may be noticed, as well as the direction in which the severalaquatic plants lie, these always pointing down the stream, and inclining with the currents, andwhen the flood waters are draining off, their motion and progress may be accurately measured.After the surfaces have been levelled, and the most available outlet decided upon, a maindrain should be set out, from which oblique branches should be cut, pointing in the direc-tion of the current; into these all the minor cuts and ditches are to be collected, so that,by intersecting the whole watery waste in an equal and regular manner, it may be effectu-ally drained. The fall should be greatest at the most remote part, diminishing as theQuantity of water increases; large and deep rivers run sufficiently fast when the fall is onefoot per mile; for smaller rivers double that is requisite; ditches and ordinary drains re-quire 8 feet per mile. ,

When water is drained from the surface, it should be made to pass away gradually, sothat the sides and bottoms of the drains are not injured by friction : it should be in con-stant motion, that the channel may be kept clear and increase in velocity as it proceeds.When the surface is a dead level, the fall must be obtained by cutting the drain at thelower end deep enough to obtain one, which is always preferable to increasing the width ;if there be a large body of water, less fall is needed, for, as above observed, the small drainrequires a fall eight times as great as that of a large river, and the water flows over straightand smooth beds with greater rapidity than when the channel winds and rocks andshallows intervene to obstruct it.

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