Chap. XXVII.

CANALS AND RIVER LOCKS.

1539

in giving an extra depth of water to canals is the greater ease with which the boats can bedrawn, the weeds at the bottom causing less inconvenience, and the evaporation being ofcourse less than in a shallower body of water; in summer also, when the boats can onlycarry half a load, two loads may be put into one boat, and the transport rendered lessexpensive.

The quantity of water expended by locks is found to be in direct proportion to theheight of the fall, and the time employed in going through them, and the expense of con-struction nearly in the same proportion; this is greater as the locks are least elevated,because they are more in number, but the increase is not in proportion to the number.

Fig. 2837.

TRANSVERSE SECTION.

Fig. 2838.

To render these relations evident, trace the curves AB, AC,and the line D C on the axis E F; the ordinates of the first markthe relation of the time employed in going through the total ofthe locks of different falls: those of the second mark the relationof the expense of construction, and those of the line D C markthe quantity of water which they will expend.

It may be concluded that when there is a great deal of water in a canal, the locks maybe high, because the saving in the expense is a great consideration, less time being requiredto go through them, and the wages of the lock-men reduced.

The Talus . —It is by no means advisable to make the walls of the lock chamber strictlyperpendicular: some inclination, or talus, is always preferable, particularly at the back,whence resistance is required againstthe thrust of water; and as its filtrationis another point of importance to beguarded against, a greater thickness isnecessary at the bottom than at thetop of the wall, that filtration beinggreater in proportion to its height.

The walls should, however, be per-pendicular towards the interior of thelock, at least in that part where theboats mount and descend; the lowerpart may have the same talus as thatusually given to the boats. The leastthickness for the walls at the level ofthe water is 4 feet 3 inches, in orderto allow for a course of beton in cementor of brick, as a protection against thefiltration of water: as a general rule, the thickness should be equal to half their height,and as the walls become more lofty, their foundations must be spread out in proportion.

Inverted arches have been introduced with considerable advantage to form the profile of thewall of lock-chambers; where there is a good foundation they may commence with aconsiderable batter or inclination, rising gradually in a curved form, finishing at the top witha perpendicular face; such $. wall resembles an arch, from its having the joints and beds ofthe several stones made perpendicular to their face, which also prevents them from slidingon each other : the triangle of earth, supported by the back of the upright retaining wall,may be regarded as a wedge, of which its own gravity is the acting power, and it is onlysupported by the re-action of the inclined plane of earth, and the back of the wall, each ofwhich acts perpendicularly to its plane. The breadth of such retaining wall must bealways proportionate to the height, and to be of equal specific gravity with the earth, itsbreadth should be more than £ of its height. The specific gravity of walls built of brickor stone is a trifle more than that of the earth they support, therefore the dimensions maybe slightly diminished.

Fig. 2839.

5 F 2