Chap. XXVII.

CANAL AND RIVER LOCKS.

1545

former, being supported throughout their height, are not subject to much wear, althoughthey are of larger scantling than the other timbers of the gate, which is necessary,as they sustain the entire framework: thehorizontal rails resist the weight, and asthat weight is greater where the rails areplaced below the level of the water, it wouldseem natural that their dimensions shouldvary in proportion to the weight. Todetermine these dimensions it must be re-collected that the thrust of water againstvertical surfaces is equal to the weight of aprism of water having its surfaces as a base,and its height half that of the water. It mustnext be considered that the rails of the gateare at least 26 inches apart, and 38 inchesfrom centre to centre, so that, on account ofthe casing of plank in the first instance,

12 inches of height support 26 inches ofwater, and in the second 38 inches. Theweight supported by each rail will be foundby multiplying their length, the interval fromone to the other, the height of the waterabove the centre of the rail, and the wholeby 62 pounds, the weight of a cube foot of

water; the product of these measures will be Fig. 2851.

the number of pounds which the rails oughtto support throughout their wholelength.

Timbers from 4 to 5 inchessquare would be sufficient forsmall gates, and for larger from8 feet 6 inches to 10 feet 6 inchesof fall, with a width of 17 feetbetween the hanging-posts, therails would be sufficiently strongif from 7 to 8 inches square, puttingsix rails in the height. They aregenerally from 9 to 10 inches atleast, which is double the strengthrequired; it is true that the gatesare more durable, but the weightis greater, which is sometimesinjurious to the collar and themasonry to which it is attached,requiring more reparations thanlighter gates.

The frames or styles of gates #should be at least 5 inches inthickness more than the rails,and the joint covered by a fillet,as well as the edge of the planks,

which are affixed perpendicularly Fig. 2852.

to the rails, and mortised into the

styles, increasing the strength of the rails and the framework by their greater thickness.Braces are also introduced between the rails, which aid materially in strengtheningthem, and by their inclined position transfer the stress to the hanging-post. °

Great gates should always have a line of braces placed diagonally, and making an an<rlewith the lower rail; all the braces above should have the same effect, and consequentlythe same inclination : those below resting on the lower rail tend to depress it, and, evenwhen properly framed and pinned into the rails, their inclination towards the hanging-postrenders them insufficient to sustain the lower rail; but they may be made useful bygiving them an inclination in a contrary direction, and uniting them by pins to the rails,producing the effect of a St. Andrew’s cross, without which no framework of carpentry isperfectly solid.

Instead of inclining the braces below the diagonals on the side of the strutting-post, a barof iron is sometimes placed diagonally from the collar to the lower end of the strutting-post,which is an excellent contrivance ; or the planks may be placed diagonally, inclining them

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