SLUICES. A contrivance used in hydraulic engineering for the purpose of closing, or regulating the passage of water from one level to another, is properly speaking called a sluice, although occa when the door itself is upon binges, or is made to work by machinery, the closing door is called a vane, or a valve, and the leading lasaage only is called the sluice. By extension, the word sluicing has also been applied to the operation of allowing a largo body of water to escape through a sluice with greet velocity, for the purpose of removing the light alluvial matters which may accumulate in the outer harbours of a sea-port, or in any analogous position. Properly speaking a lock-gate is a species of sluice ; but the usages to which that class of machinery is applied are of so great importance as to have rendered it advisable to treat of its construction and details under the head of CANAL.
As was said under that article, the discharge through a sluice of the ordinary form,(that is to say, of a rectangular section, and without any provision for the effect of the contraction of the fluid vein,) is usually calculated by the formula o=rns a/2;in, in which m=a coefficient of 0'625, when only one sluice is used, and of when two sluicee are used and their streams can meet within a short distance of their points of outlet. This formula, it must also be added, only applies when the sluice discharges into the open air, and there is no head of water on the down side, and it becomes Q=r1rs N/21 (11-11) when there is such a head ; u representing the depth of water on the up side, and u' the depth on the down side ; in both forroube s represents the area, and p, the accelerating force of gravity. By forming the commencement of the sluice in the manner described by Venturi in his remarks on the effect of ajutuges, the discharge of single sluices may be increased to such an extent as to make the value of m= ; but unless the mouth of the sluice be executed in ashlar masonry, or in cast iron, it would ho difficult to effect this improvement ; and in practice it is rarely attempted. Canal sluices, or sluices for the irrigation or drainage of land, rarely work under great heads of water, and they do not therefore require any very extraordinary precautions in the con struction of their channels, or of their machinery ; but in the sluices used for scouring harbours, and occasionally in those used for drawing off the water for town supplies, the velocity of efflux renders it necessary to build sonic portions with particular care. For instance, it Is by no means rare to meet with acouring-aluices working with a head of water equal to 16 or 18 feet, and the variable head usually allowed upon the outlet of town reservoirs is equal to 14 feet ; under these circumstances the water rushes out with such force as to he able to tear np the surfaces it comes immediately in contact with, and it is therefore indispensable that the tail walls of open sluices, or the materials of closed ,ones, should be of the most resisting character. The apron at the tail end, and the leading passage of the sluice up to the valves, are in almost all cases executed either in solid masonry, or in iron pipes, to which the framework of the valves is fixed.
In small land sluices, discharging water into tidal rivers, the outlets are frequently closed by flaps hung in such a manner as to open when the head of water on the inside exceeds that upon the outside, but to close directly the outside head preponderates. These flaps, however,
are easily liable to derangement by the interposition of extraneous bodies ; and they are not therefore used when it is important to shut out external waters under all circumstances. In the latter cases valves working in close grooves, and raised by machinery worked by hand, or self-acting, must be resorted to ; and if the area of the water-way should be large—as in the case of scouring-sluices—doors must also be employed. Valves, in fact, are the most suited for closed channels, or aqueducts ; turning doors for large areas of water : the former description of machinery is well illustrated in the various works mentioned under WATER SUPPLY; and the latter, in Sganzin's 'Coors de Construction,' and Minard's Travaux Hydrauliques des Ports de ;tier.' The question as to the advantages of scouring-sluices for the pur pose of removing accumulations in harbours, is one upon which opinions are much divided : for although when the water from the scouring reservoirs is allowed to escape with considerable velocity, it is, able to act very energetically upon the materials it may meet in the confined part of the channel ; yet directly the stream passes the heads of the jetties it must lose its velocity, and consequently its transport ing power. Unless, therefore, there should exist a littoral current able to carry forward the matters scoured out from the inner harbour, a bar must eventually be formed in the zone of still water caused by the interferences of the respective forces of the sluice-waters and of the tides. At the present day, the tendency of modern engineering is to abandon the use of scouring-sluices, and to trust entirely to dredging for the removal of the alluvial matters carried into the ports exposed to that inconvenience. But as there are many occasions in which it might still be advantageous to resort to the use of sluices, it may be as well to add that the principles to be observed in their construction are ;-1st. To make the impounding reservoirs of such forms as to insure the prompt discharge of the whole quantity of water they may contain, for which purpose a semicircular basin, with the outlet upon the centre of the chord line, is the most advantageous ; 2nd. To make the outlets as large as possible, and to continue the outfall channel as far as may be necessary to conduct the stream, with its full force, against the obstacle to be removed; and 3rd. To place the outlet of the sluices as near to the head of the jetties as may be. The scouring sluices of Dover, Calais, Dunkirk, Ostend, and the great sluices of Catwyck, through which the waters of the old Rhine are dis charged into the German Ocean, are perhaps the most remarkable works of their several descriptions in existence. The reader will find much information on this subject, as also upon the construction of sluices, sluice-gates, valves, &c., in Wiebeking, Allgerneine Wasser baukunst ; ' Rennie, On Harbours ; ' in Weale's Quarterly Papers on Architecture ; ' Shn's Public Works of Great Britain; ' D'Aubuiason's I fydrauliq u (3; &c.