Page:The American Cyclopædia (1879) Volume III.djvu/691

 CANAL 681 may be controlled from above, and which serve when opened to admit the passage of water, when the gates are shut and cannot be opened on account of the pressure of the water against them. A lock, aside from the gates, consists principally of three parts. The space included between the gates is called the cham- ber ; the part above the upper gates is the up- per or head bay, and that portion below the lower gates is the lower or tail bay. The bot- tom of the chamber and also of the bays is covered by planks running longitudinally and lying upon cross timbers supported by hy- draulic masonry. The chamber is terminated at its upper end by a vertical wall, called the breast or lift wall, because its height is equal to the lift of the levels. It is covered at the top with a framework of timber forming in the middle an obtuse angle, against which the lower ends of the upper gates rest when they are closed. This timber is called the mitre sill, and is represented in fig. 2. Formerly it was sometimes constructed of stone, but this has been found to wear the gates much faster than wood, and also to cost more for construction and repair. A recess called a gate chamber is made at either side in the walls of the head bay, of sufficient depth to allow the gates to swing out of the way of the boat. The posterior curved part of this recess, in which the gate post turns, is called the hol- low quoin, and the gate post is called the quoin post, or heel, and is made of a semi-cylindrical form, somewhat eccentric to the curve of the quoin, to facilitate rotation. At a short dis- tance above the upper gate chambers the bay walls are turned outward, sometimes in straight and sometimes in curved lines, forming the wing walls. The chamber walls, immediately above the lower gates, are also provided with recesses, similar to those in the upper bay. The lower gates are of similar construction to the upper, and revolve in quoins in the same manner ; and the parallel walls of the lower bay are termi- nated, like those in the upper bay, by outward- curving wing walls. When a boat in ascending a canal comes to a lock, the lower gates being open, it passes in, and the gates are immediate- ly closed behind it. Water is then allowed to flow through the valves in the upper gates, or FIG. 8. Lift Lock. sometimes through a sluice called a side cul- vert, discharging from the upper level into the lock. As this fills, the boat is lifted up, and the upper gates are gradually freed, so that they can be opened and the boat can pass through upon the higher level. (See fig. 3.) Were another boat to follow in the same direc- tion, the upper gates must be first closed, and the lock emptied through the lower gates. These being then opened, the boat can pass in, and the process be repeated. In this operation there would be a lock full of water discharged to a lower level, which might have been used for letting down a boat had there been one ready to pass in the opposite direction. Hence, when the supply of water is limited, there is economy in passing the boats alternately each way through the locks, besides thus expediting the passage of the greatest number. The com- mon lift of a lock is 5 or 10 ft., though it is occasionally much less, and is sometimes as rV till i 1 I 1 1 / 1 i i ' i ' i -T^i V I/I 1 FIG. 4. Diagram of a Weigh Lock. great as 18 ft. Where tolls are collected upon the cargo, weigh locks are provided by which the amount of freight is determined by sub- tracting the known or ascertained weight of the empty boat from the combined weight of boat and cargo. The lock should be situated at a suitable point along a level where the canal is conducted along the side of a hill, to allow of the convenient discharge of water from the lock. The gates at either end are ad- justed to open outwardly, like the upper gates of a lift lock. The bottom of the lock consists of a platform which is suspended from a system of levers placed overhead. Tide or guard locks are constructed at the point where a canal enters a river or bay, and the place must be selected with much care. A bar is usually found at the mouth of an affluent, and when, as is often the case, a canal follows this, its outlet should be placed below the bar. A large basin is gener- ally built at the outlet of the canal, and at the outlet of the basin is constructed a lock with double gates, arranged in such a manner as to allow the boats to be locked up or down, de- pending upon whether the tide has caused the water to be higher or lower in the river than in the basin. Small streams which are lower than the canal are conveyed under it through ordinary culverts. If the level of the canal is not much above that of the brook, the culvert is given the form of an inverted siphon, and is called a broken-back culvert. When the stream is large the canal is taken across it in an aqueduct. Gates called waste weirs are pro- vided in levels of much length to let off surplus water, and should be located at points where it can be discharged into natural streams. That they may also be used for draining the canal,