Chap. XXVII.

CANALS AND RIVER LOCKS.

1543

comprise the whole thickness of the masonry, and be very difficult to displace. When onlylarge rubble stone can be obtained, the platform should be concave, like an invertedvault: if the lock is on a solid foundation, it will be only necessary to give it a thicknessof 27£ inches to 3 feet 3 inches: when the ground is of a doubtful quality, it is better tomake a wide platform of from 4 feet 3 inches to 5 feet 3 inches in thickness, on which thewalls of the lock and platform are constructed: if the ground have no consistence, pilingmust be adopted.

Down stream there should be a guard platform as deep as is practicable, and as thispart of the lock is most subject to injury, it must be made perfectly solid, for which pur-pose the plan of the masonry of the platform should be concave, tangential to the directionof the wing-walls, forming a species of arch with them as abutments. The last courseshould be cut in a double section, making a hollow arc above; the key-stones will thenbe perfectly closed, and, independently of the cramps, this course, which holds together allthose of the platform, will be as solid as it is possible to make it.

The Projection of the pointed Sills .—M. Belidor has given dimensions for this partof a lock, but they are too small for general purposes. He remarks first, that if the gatesform a right angle, the projection should be equal to half the opening, the greatestthat could naturally be given them, and this angle would be the most advantageousfor shutting the gates against each other, because the thrust of the water acting perpen-dicularly against them, each gate would push the other at right angles to the length of

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Fig. 2848.

POINTED SILL.

the timber, in which direction they have the greatest strength ; but, on the other hand, bygiving them an obtuse angle, there is the advantage of having the leaves narrower, andconsequently stronger. If this angle be extremely obtuse, that is to say, if the gatesmerely formed a straight line, they would certainly have the least width that could begiven them, but they would not support themselves, a very essential defect; a meanshould be adopted either between the pointed sill having the greatest projection andthat which has the least, or between the greatest width of gate and the least, or betweenthe greatest angle the gate forms with the lock and the least. In the first case the pro-jection of the pointed sill would be a quarter of the width, in the second nearly the third,and in the third about the fifth. M. Belidor has determined upon the latter, but theangle is not sufficiently projecting, as it prevents the timbers from resting against eachother, and giving it a greater projection, increases the width of the gates, the solidity ofwhich may be maintained by additional strength; thus it appears a selection must bemade between the other two methods, and possibly the most judicious would be a mean,that is to say, to give to the projection of the pointed sill between a third and a quarterof the width of the lock : either might be adopted without any inconvenience.

Pointed sills are sometimes made of timber, and frequently of stone; the latter is very

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