construction, to preserve them from accidents, owing to the great pressure of water against their sides. The expense of construction is otherwise in their favor; that is, the expense will increase with the total number of locks, the height to be ascended being the same. The smallest lifts are seldom less than five feet, and the greatest, for ordinary canals, not over twelve; medium lifts of seven or eight feet are considered the best under every point of view. This is a point, however, which cannot be settled arbitrarily, as the nature of the foundations, the materials used, the embankments around the locks, the changes in the direction. of the canal, caused by varying the lifts, are so many modifying causes, which should be carefully weighed before adopting a definitive plan.

The lifts of a flight should be the same throughout; but in isolated locks the lifts may vary according to circumstances. If the supply of water from the summit level requires to be economized with care, the lifts of locks which are furnished from it may be less than those lower down.

705. Levels. The position and the dimensions of the levels must be mainly determined by the form of the natural surface. Those points are naturally chosen to pass from one level to another, or as the positions for the locks, where there is an abrupt change in the surface.

A level, by a suitable modification of its cross section, can be made as short as may be deemed desirable; there being but one point to be attended to in this, which is, that a boat passing between the two locks, at the ends of the level, will have time to enter either lock before it can ground, on the supposition, that the water drawn off to fill the lower lock, while the boat is traversing the level, will just reduce the depth to the draught of the boat.

706. Locks. A lock (Fig. 170) may be divided into three distinct parts:-1st. The part included between the two gates, which is termed the chamber. 2d. The part above the upper gates, termed the fore, or head-bay. 3d. The part below the lower gates, termed the aft, or tail-bay.

707. The lock chamber must be wide enough to allow an easy ingress and egress to the boats commonly used on the canal; a surplus width of one foot over the width of the boat across the beam is usually deemed sufficient for this purpose. The length of the chamber should be also regulated by that of the boats; it should be such, that when the boat enters the lock from the lower level, the tail-gates may be shut without requiring the boat to unship its rudder.

The plan of the chamber is usually rectangular, as this form is, in every respect, superior to all others. In the cross section

Fig. 170-Represents a plan M, and a section N, through the axis of a single lock laid on a beton foundation.-A, lock-chamber. B, fore-bay. C, tail-bay. a, a, chamber-walls. b, b, recesses or chambers in the side walls for upper-gates. c, c, lower-gate chambers. d, d, lift wall and upper mitre sill. e, e, lower mitre sill. h, h, tail walls. o, o, head walls. m, m, upper wing, or return walls. n, n, lower wing walls. D, body of masonry under the fore-bay

b

M

D

of the chamber, (Fig. 171,) the sides receive generally a slight

B

A

Fig. 171-Represents a section of Fig. 170, through the chamber.

A, A, chamber walls.

B, chamber formed with an inverted-arch bottom.

batter; as when so arranged they are found to give greater facility to the passage of the boat than when vertical. The bottom of the chamber is either flat or curved; more water will be required to fill the flat-bottomed chamber than the curved, but it will require less masonry in its construction.

708. The chamber is terminated just within the head gates by a vertical wall, the plan of which is usually curved. As this wall separates the upper from the lower level, it is termed the lift-wall; it is usually of the same height as the lift of the levels. The top of the lift-wall is formed of cut stone, the vertical joints of which are normal to the curved face of the wall; this top course projects from six to nine inches above the bottom of the upper level, presenting an angular point, for the bottom of the head-gates, when shut, to rest against. This is termed the mitre-sill. Various degrees of opening have been given to the angle between the two branches of the mitre-sill; it is, however, generally so determined, that the perpendicular of the isosceles triangle, formed by the two branches, shall vary between one fifth and one sixth of the base.

As stone mitre-sills are liable to injury from the shock of the gate, they are now usually constructed of timber, (Fig. 172,) by

Fig. 172-Represents a plan of a wooden mitresill, and a horizontal section of a lock-gate (Fig. 173) closed.

a, a, mitre-sill framed with the pieces b and c, and firmly fastened to the side walls A, A. d, section of quoin posts of lock-gate. A e, section of mitre posts.

framing two strong beams with the proper angle for the gate when closed, and securing them firmly upon the top of the liftwall. It will be well to place the top of the mitre-sill on the lift-wall a little lower than the bottom of the canal, to preserve it from being struck by the keel of the boat on entering, or leaving the lock.

709. The cross section of the chamber walls is usually trapesoidal; the facing receives a slight batter. The chamber walls

are exposed to two opposite efforts; the water in the lock on one side, and the embankment against the wall on the other. The pressure of the embankment is the greater as well as the more permanent effort of the two. The dimensions of the wall must be regulated by this pressure. The usual manner of doing this, is to make the wall four feet thick at the water line of the upper level, to secure it against filtration; and then to determine the base of the batter, so that the mass of masonry shall present sufficient stability to counteract the tendency of the pressure. The spread, and other dimensions of the foundations, will be regulated according to the nature of the soil, in the same way as in other structures.

710. The bottom of the chamber, as has been stated, may be either flat or curved. The flat bottom is suitable to very firm soils, which will neither yield to the vertical pressure of the chamber walls, nor admit the water to filter from the upper level under the bottom of the lock. In either of the contrary cases, the bottom should be made with an inverted arch, as this form will oppose greater resistance to the upward pressure of the water under the bottom, and will serve to distribute the weight of the walls over the portion of the foundation under the arch. The thickness of the masonry of the bottom will depend on the width of the chamber, and the nature of the soil. Were the soil a solid rock, no bottoming would be requisite; if it is of soft mud, a very solid bottoming, from three to six feet in thickness, might be requisite.

711. The principal danger to the foundations arises from the water which may filter from the upper to the lower level, under the bottom of the lock. One preventive for this, but not an effectual one, is to drive sheeting piles across the canal at the end of the head-bay; another, which is more expensive, but more certain in its effects, consists in forming a deep trench of two or three feet in width, just under the head-bay, and filling it with beton, which unites at top with the masonry of the head-bay. Similar trenches might be placed under the chamber were it considered necessary.

712. The lift-wall usually receives the same thickness as the chamber walls; but, unless the soil is very firm, it would be more prudent to form a general mass of masonry under the entire head-bay, to a level with the base of the chamber foundations, of which mass the lift-wall should form a part.

713. The head-bay is enclosed between two parallel walls, which form a part of the side walls of the lock. They are terminated by two wing walls, which it will be found most economical to run back at right angles with the side walls. A recess, termed the gate-chamber, is made in the wall of the head

bay; the depth of this recess should be sufficient to allow the gate, when open, to fall two or three inches within the facing of the wall, so that it may be out of the way when a boat is passing; the length of the recess should be a few inches more than the width of the gate. That part of the recess where the gate turns on its pivot is termed the hollow quoin; it receives what is termed the heel, or quoin-post of the gate, which is made of a suitable form to fit the hollow quoin. The distance between the hollow quoins and the face of the lift-wall will depend on the pressure against the mitre-sill, and the strength of the stone; eighteen inches will generally be found amply sufficient.

The side walls need not extend more than twelve inches be yond the other end of the gate-chamber. The wing walls may be extended back to the total width of the canal, but it will be more economical to narrow the canal near the lock, and to extend the wing walls only about two feet into the banks, or sides. The dimensions of the side and wing walls of the head-bay are regulated in the same way as the chamber walls.

The bottom of the head-bay is flat, and on the same level with the bottom of the canal; the exterior course of stones at the entrance to the lock should be so jointed as not to work loose.

714. The gate-chambers for the lower gates are made in the chamber walls; and it is to be observed, that the bottom of the chamber, where the gates swing back, should be flat, or be otherwise arranged not to impede the play of the gates.

715. The side walls of the tail-bay are also a part of the general side walls, and their thickness is regulated as in the preceding cases. Their length will depend chiefly on the pressure which the lower gates throw against them when the lock is full; and partly on the space required by the lock-men in opening and shutting gates manoeuvred by the balance beam. A calculation must be made for each particular case, to ascertain the most suitable length. The side walls are also terminated by wing walls, similarly arranged to those of the head-bay. The points of junction between the wing and side walls should, in both cases, either be curved, or the stones at the angles be rounded off. One or two perpendicular grooves are sometimes made in the side walls of the tail-bay, to receive stop-planks, when a temporary dam is needed, to shut off the water of the lower level from the chamber, in case of repairs, &c. Similar arrangements might be made at the head-bay, but they are not indispensable in either case.

The strain on the walls at the hollow quoins is greater than at any other points, owing to the pressure at those points from the gates, when they are shut, and to the action of the gates when in motion; to counteract this, and strengthen the walls, but