1542

THEORY AND PRACTICE OF ENGINEERING.

Book II.

each collar forming an angle, generally in a line withthe pointing sills ; but it is better that one should bepointed up, and the other down the stream, to resistthe thrust of the water on one side when the gates areclosed, and on the other to check the weight of thegate when in movement.

Counterforts are sometimes introduced in the lengthof the chamber to resist the thrust of the water, inwhich case the thickness of the side walls might bediminished, which would be a considerable saving ofmasonry; but on account of the filtration, whichcannot be too carefully guarded against, it is better tomake the walls of sufficient thickness, and to dispensewith the counterforts.

The wall which constitutes the fall in the lock isusually perpendicular, though a talus may be givenwithout any inconvenience to the boats: when thecanal is dry, and requires to be filled, the waterflows from the top of this fall on to the platform, andtends to injure it; to remedy this as much as possible,the fall of the lock should have a curve of uniformdescent; the direction of the running water then be-

Fig. 2844.

GATE ANCHOR,

Fig. 2845.

T-J 1 - ).

11

coming horizontal at the end of its fall, ~it has much less action against the plat- uform than when its direction is inclined.

When the pointed sills are of stone, it isadvisable to make the fall of the lock iconcave on the plan, in order that the Fig. 2846. scouring sluice platform.

key of the sill may have a sufficient section

to be solidly based. The thickness of the walls which form the fall is usually considerable,but this is unnecessary; the same as that of the side walls is quite sufficient.

The Platform. — This construction is of the greatest importance, being that part of thelock most difficult to keep in repair, and most often requiring it; they were formerlymade of timber, but this is to be avoided as much as possible, timber never uniting wellwith masonry, and a film of water almost invariably insinuating itself between the planks andthe masonry, between the heads of the piles, the force of which in relation to its weight isconsiderable, making the platform yield, disuniting the framework, and preventing theeasy working of the gates: the shock of water from the upper interval is, however, lessinjurious to timber than to stone, and if good stone cannot be procured, to resist thiseffect, a platform ofcross planks over thatof rubble might beformed on pieces oftimber encased in thewalls, but only in thatpart where the shocktakes place. Whereit is possible to obtainthem, large flagstonesare preferable for thesurface of the plat-form ; and in order thatthey may be solidlybedded, each row

should be worked with dovetails into a course of freestone placed upright, which would

Fig. 2847. section through platporm.

D, Piles. G, Stone. H, Sleepers.

K, Planking. L, Heel-post. Y, Planking.