Chap, XXVII.

CANALS AND RIVER LOCKS.

1537

ternately, one going up, the other down : in like manner, where there are contiguous locks,the boats will expend in their journey as many locksfull as there are contiguous locks inascending; when one boat follows another, it will expend two locksfull, whether the locksare contiguous or isolated. It must be remarked that the passage of those boats only canbe considered relatively to the locks which join the starting point: when the locks are notcontiguous, and their fall is equal, which happens in the lower intervals, it has no influenceon the expenditure of water, especially when the boats do not stop any length of time ; ingiving 640 feet length to each interval, it is evident, when two boats follow each other,they will never be together in the same interval, since whilst the second passes the lock, thefirst will have time to pass the interval and enter the following lock ; thus two boats cannotmeet in the smaller intervals, except when one ascends and the other descends, and in thiscase, as one takes a lockfull from the interval, whilst a second pours one into it, consequentlythe water does not diminish or increase in it. It must be observed that we can never haveabove a lockfull, more or less, in an interval, unless several boats remain in them together,which should be avoided when they are small; further, when the contiguous locks are distantfrom the starting point, it often happens that the lockfull is not immediately taken; butwhen there is no second quantity of water before the contiguous locks, it is always thestarting point which furnishes that of the canal above them.

Locks to contain several boats are successfully employed where water is abundant. In form-ing them the side walls may be dispensed with, the sides of the chambers only being laid withstone to a proper talus ; some of those chambers may contain four boats, others a large and asmall one; at the ingress and egress there usually are two pairs of gates, one small and theother large: l^cks to contain two boats are either double the length or the width of thechamber. The second plan is the least expensive, the length of the chamber walls in thiscase not being greater than for the more simple locks ; but the first has the advantage ofreceiving a third pair of gates in the middle of its length, so that one or two boats can passat pleasure ; the length of the chamber is also varied, by placing more than two pairs oflarge gates, enabling boats of different dimensions to pass together or separately, withoutexpending a greater quantity of water than would be necessary for each boat. To makethe chamber of a sufficient length for two ordinary-sized boats, a gate is placed in themiddle of its length, and another at the necessary distance for the larger boats ; therest of the length is then available for smaller: it is, however, seldom necessary tomake provision for the passage of boats of various sizes, as there is no advantagein two boats passing at once; when the chamber is constructed of masonry, twiceas much water is expended for the passage of two boats as for one, consequently twice asmuch time is employed in filling a lock for two boats as for a single one ; there is thereforeno gain either in time or the quantity of water, and the expense of construction is con-siderably augmented. *

The chambers of this kind of locks are sometimes formed by a slope of earth only, withouta facing of masonry ; but they expend much more water, consequently take a longer timeto fill, and are less advantageous on account of the time lost by the boats having to wait foreach other : should one arrive alone it must remain until a second comes up before it canpass the lock, or as much water must be expended for the one as would be required fortwo, and of course this inconvenience is increased when the chamber is made to containfour boats.

Form to be given to the Chambers of Locks. — The most convenient is the parallelogram, alittle wider than the boats that require to pass, and sufficiently long to admit of the gatesbeing moved with facility. The chambers of the canal of Languedoc are of an oval form,to give greater strength in resisting the banks contiguous to them; but as this causes anincrease of expense in construction as well as in the quantity of water necessary to fill it, itwill be useful to enquire if, in avoiding one inconvenience, a greater is not produced. Theoval chambers of the canal of Languedoc contain an area of 3636 feet, whilst if the sidewalls were parallel, they would only be 2248 superficial feet: thus the expenditure of waterin the oval chamber exceeds more than a third that of the parallelogram, the proportionbeino* about 5 to 3 : the inconvenience is considerably increased by want of water, whichfrequently occurs. Another result of the oval form is that the passage of the lock is alsolonger than in the rectangular ; in the same proportion the expense of the timber platformis also increased: it is, however, certain that a curved wall is stronger against a pressure ofearth than a straight one, and if the cost of masonry requisite to give the same strength to astraight wall is greater, the expense is compensated for by the diminution of the cost of thetimber platform, which is two-fifths stronger. It is very essential to prevent the filtrationof water through the side walls, and the best method to effect this is to place on their thick-ness a lining of beton, or of brick laid in cement, which will be impervious to water; butas this will destroy the bond, a greater thickness of wall is requisite; thus there are manycircumstances where it might be necessary to give to curved walls as great a thickness asto straight. The thickness of straight walls which support earth should be a third of theirheight, whilst those which resist the thrust of water should be one-half; if the walls of the

5 F