tresses should be placed at the back of the walls, in the most favorable position behind the quoins to subserve the object in view. The bottom of the tail-bay is arranged, in all respects, like that of the head-bay. 716. The top of the side walls of the lock may be from one to two feet above the general level of the water in the upper reach; the top course of the masonry being of heavy large blocks of cut stone, although this kind of coping is not indispensable, as smaller masses have been found to suit the same purpose, but they are less durable. As to the masonry of the lock, in general, it is only necessary to observe, that those parts alone need be of cut stone where there is great wear and tear from any cause, as at the angles generally; or where an accurate finish is indispensable, as at the hollow quoins. The other parts may be of brick, rubble, beton, &c., but every part should be laid in the best hydraulic mortar. 717. The filling and emptying the lock chamber have given rise to various discussions and experiments, all of which have been reduced to the comparative advantages of letting the water in and off by valves made in the gates themselves, or by culverts in the side walls, which are opened and shut by valves. When the water is let in through valves in the gates, its effects on the sides and bottom of the chamber are found to be very injurious, particularly in high lift-walls; besides the inconvenience resulting from the agitation of the boat in the lock. To obviate this, in some degree, it has been proposed to give the lift-wall the form of an inclined curved surface, along which the water might descend without producing a shock on the bottom. 718. The side culverts are small arched conduits, of a circular, or an elliptical cross section, which are made in the mass of masonry of the side-walls, to convey the water from the upper level to the chamber. These culverts, in some cases, run the entire length of the side walls, on a level with the bottom of the chamber, from the lift-wall to the end of the tail-wall, and have several outlets leading to the chamber. They are arranged with two valves, one to close the mouth of the culvert, at the upper level, the other to close the outlet from the chamber, to the lower level. This is, perhaps, one of the best arrangements for side culverts. They all present the same difficulty in making repairs when out of o der, and they are moreover very subject to accidents. They are therefore on these accounts inferior to valves in the gates. 719. It has also been proposed, to avoid the inconveniences of culverts, and the disadvantages of lift-walls, by suppressing the latter, and gradually increasing the depth of the upper level, to the bottom of the chamber. This method presents a saving in the mass of masonry, but the gates will cost more, as the head and tail gates must be of the same height. It would entirely remove the objection to valves in the gates, as the current through them, in this case, would not be sufficiently strong to injure the masonry. 720. The bottom of the canal below the lock should be protected by what is termed an apron, which is a covering of plank laid on a grillage, or else one of brush-wood and dry stone. The sides should also be faced with timber or dry stone. The length of this facing will depend on the strength of the current; generally not more than from fifteen to thirty feet from the lock will require it. The entrance to the head-bay is, in some cases, similarly protected, but this is unnecessary, as the current has but a very slight effect at that point. 721. Locks constructed of timber and dry stone, termed composite-locks, are to be met with on several of the canals of the United States. The side walls are formed of dry stone carefully laid; the sides of the chamber being faced with plank nailed to horizontal and upright timbers, which are firmly secured to the dry stone walls. The walls rest upon a platform laid upon heavy beams placed transversely to the axis of the lock. The bottom of the chamber usually receives a double thickness of plank The quoin-posts and mitre-sills are formed of heavy beams. 722. Lock Gates. A lock gate (Fig. 173) is composed of two m ท α a Fig. 173-Repre- m, m, balance- leaves, each leaf consisting of a solid frame-work covered on the side towards the water with thick plank made water-tight. The frame usually consists of two uprights, of several horizontal cross pieces let into the uprights, and sometimes a diagonal piece, or brace, intended to keep the frame of an invariable form, is added. The upright, around which the leaf turns, termed the quoin or heel-post, is rounded off on the back to fit in the hollow quoin; it is made slightly eccentric with it, so that it may turn easily without rubbing against the quoin; its lower end rests on an iron gudgeon, to which it is fitted by a corresponding indentation in an iron socket on the end; the upper extremity is secured to the side walls by an iron collar, within which the post turns. The collar is so arranged that it can be easily fastened to, or loosened from two iron bars, termed anchor-irons, which are firmly attached by bolts, or a lead sealing, to the top course of the walls. One of the anchor-irons is placed in a line with the leaf when shut, the other in a line with it when open, to resist most effectually the strain in those two positions of the gate. The opposite upright, termed the mitre-post, has one edge bevelled off, to fit against the mitre-post of the other leaf of the gate. 723. A long heavy beam, termed a balance beam, from its partially balancing the weight of the leaf, rests on the quoin post, to which it is secured, and is mortised with the mitre post. The balance beam should be about four feet above the top of the lock, to be readily manœuvred; its principal use being to open and shut the leaf. 724. The top cross piece of the gate should be about on a level with the top of the lock; the bottom cross piece should swing clear of the bottom of the lock. The position of the intermediate cross pieces may be made to depend on their dimensions if they are of the same dimensions, they should be placed nearer together at the bottom, as the pressure of the water is there greatest; but, by making them of unequal dimensions, they may be placed at equal distances apart; this, however, is not of much importance except for large gates, and considerable depths of water. The plank may be arranged either parallel to the uprights, or parallel to the diagonal brace; in the latter position they will act with the brace to preserve the form of the frame. 725. A wide board supported on brackets, is often affixed to the gates, both for the manoeuvre of the machinery of the valves, and to serve as a foot bridge across the lock. The valves are small gates which are arranged to close the openings made in the gates for letting in, or drawing off the water. They are arranged to slide up and down in grooves, by the aid of a rack and pinion, or a square screw; or they may be made to open or shut by turning on a vertical axis, in which case they are termed paddle gates. The openings in the upper gates are made between the two lowest cross pieces. In the lower gates the openings are placed just below the surface of the water in the reach. The size of the opening will depend on the time in which it is required to fill the lock. 726. Accessory Works. Under this head are classed those constructions which are not a part of the canal proper, although generally found necessary on all canals: as the culverts for conveying off the water courses which intersect the line of the canal; the inlets of feeders for the supply of water; aqueduct bridges, &c. &c. 727. Culverts. The disposition to be made of water courses intersecting the line of the canal will depend on their size, the character of their current, and the relative positions of the canal and stream. Small brooks which lie lower than the canal may be conveyed under it through an ordinary culvert. If the level of the canal and brook is nearly the same, it will then be necessary to make the culvert in the shape of an inverted syphon, and it is therefore termed a broken-back culvert. If the water of the brook is generally limpid, and its current gentle, it may, in the last case, be received into the canal. The communication of the brook, or feeder, with the canal, should be so arranged that the water may be shut off, or let in at pleasure, in any quantity desired. For purpose a cut is made through the side of the canal, and the sides and bottom of the cut are faced with masonry laid in hydraulic mortar. A sliding gate, fitted into two grooves made in the side walls, is manoeuvred by a rack and pinion, so as to regulate the quantity of water to be let in. The water of the feeder, or brook, should first be received in a basin, or reservoir, near the canal, where it may deposite its sediment before it is drawn off. In cases where the line of the canal is crossed by a torrent, which brings down a large quantity of sand, pebbles, &c., it may be necessary to make a permanent structure over the canal, forming a channel for the torrent; but if the discharge of the torrent is only periodical, a moveable channel may be arranged, for the same purpose, by constructing a boat with a deck and sides to form the water-way of the torrent. The boat is kept in a recess in the canal near the point where it is used, and is floated to its position, and sunk when wanted. 728. Aqueducts, &c. When the line of the canal is intersected by a wide water-course, the communication between the two shores must be effected either by a canal aqueduct bridge, or by the boats descending from the canal into the stream. As the construction of aqueduct bridges has already been considered, nothing farther on this point need here be added. The expedient of crossing the stream by the boats may be attended with many grave inconveniences in water courses liable to freshets, or to considerable variations of level at different seasons. In these cases locks must be so arranged on each side, where the canal enters the stream, that boats may pass from the one to the other under all circumstances of difference of level between the two. The locks and the portions of the canal which join the stream must be secured against damage from freshets by suitable embankments; and, when the summer water of the stream is so low that the navigation would be impeded, a dam across the stream will be requisite to secure an adequate depth of water during this epoch. 729. Canal Bridges. Bridges for roads over a canal, termed canal-bridges, are constructed like other structures of the same kind. In planning them the engineer should endeavor to give sufficient height to the bridge to prevent those accidents, of but too frequent occurrence, from persons standing upright on the deck of the passage-boat while passing under a bridge. 730. Waste-Wier. Waste-wiers must be made along the levels to let off the surplus water. The best position for them is at points where they can discharge into natural water courses. The best arrangement for a waste-wier is to make a cut through the side of the canal to a level with the bottom of it, so that, in case of necessity, the waste-wier may also serve for draining the level. The sides and bottom of the cut must be faced with masonry, and have grooves left in them to receive stop-plank, or a sliding gate, over which the surplus water is allowed to flow, under the usual circumstances, but which can be removed, if it be found necessary, either to let off a larger amount of water, or to drain the level completely. 731. Temporary Dams. In long levels an accident happening at any one point might cause serious injury to the navigation, besides a great loss of water. To prevent this, in some measure, the width of the canal may be diminished, at several points of a long level, to the width of a lock, and the sides, at these points, may be faced with masonry, arranged with grooves and stop-planks, to form a temporary dam for shutting off the water on either side. 732. Tide, or Guard Lock. The point at which a canal enters a river requires to be selected with judgment. Generally speaking, a bar will be found in the principal water course at, or below, the points where it receives its affluents. When the canal, therefore, follows the valley of an affluent, its outlet should be placed below the bar, to render its navigation permanently secure from obstruction. A large basin is usually formed at the outlet, for the convenience of commerce; and the entrance from this basin to the canal, or from the river to the basin, is effected by means of a lock with double gates, so arranged that a boat can be passed either way, according as the level in the one |