Chap. XXI11.

TIM 15EIt HUIDGES

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The platform that carries the road is suspended from these five arches by perpendiculariron rods, which hook into eyes fixed on the arches, at a distance of 8 feet apart in the

ARCH OF DELAWARE BR1DGI

three middle, and 16 in the two outer arches; stirrup-irons at the lower ends of theserods receive the joists ; diagonal braces unite the platform with the tie-beams, and preventany motion to which it would be subjected ; stone piers and abutments carry thisingenious piece of carpentry, and the whole is roofed, to protect it from the Avealher.Three arches on one side are each 200 feet opening, the fourth 180, the fifth 160 feet, andthe versed sine of the largest 27 feet.

The Bridge over the Susquehanna, in Columbia , constructed in a similar manner, was com-pleted in 1834; there are twenty-nine timber arches, each of 200 feet span, supported ontwo abutments and twenty-eight stone piers. The water-way of this magnificent bridgeis 5800 feet, and its whole length, including piers and the abutments, 1\ mile. There arethree sets of timber arches, which allow of two passage ways for road and railway carriages,the whole breadth being 30 feet.

The Timber Bridge over the Portsmouth rivers in America , built after the designs of Mr.Illudget, has but one arch, with a span of 250 feet. The timbers are placed at adistance apart equal to twice their own depth : there are three concentric ribs; the middlecarries the floor of the bridge; they were selected from crooked timbers, so that the fibremight run nearly in the direction of the curves, and are connected together by piecesof hard and incompressible wood, with wedges driven between, the ribs being mortisedto receive them ; thus the three ribs are kept at a regular and parallel distance fromeach other. Each rib is formed of two pieces, laid side by side, about 15 feet in length ;they are all disposed in such a manner as to break joint, the end of one piece of timbercoming in the middle of the length of the other, which is near it; their ends all abutwith a square joint against each other, and are neither scarfed nor mortised, the twopieces of timber being held together by transverse dovetail keys and joints : all thetimbers are admirably united, and freely exposed to the action of the air; any piece mayalso be removed, in case of its requiring reparation, without injury to the rest of thestructure.

The only defect in its construction is that of mortising through the principal ribs so fre-quently for the purpose of admitting the wedges, which renders them not so strong, andmore susceptible of injury by compression ; the keys and wedges effectually fill up the holescut to receive thorn, therefore it is supposed that the strength is not so much impaired. Thisbridge would receive additional strength by the introduction of braces, which wouldconnect the three ribs together, and if of iron more firmness would be given to it.

Richmond Bridge , in America , is a fine example of lattice-work, as adapted by Mr. Townto a span of 78 feet up to 150 feet; and as there is no lateral thrust, and materials ofsmall scantling are required, this system has been much adopted on the railroads : thelattice framing is composed of fir planks, about 3 inches in thickness and 12 inches wide,arranged so as to. lie across each other at right angles; at the points of intersection theyare united by oak treenails 1£ inch in diameter, which pass entirely through them. Thedepth of the lattice-work is proportioned to the span, and is about 9 feet 6 inches when theformer is 78 feet.

In the bridges constructed upon this system there are but two ribs, one under each sideof the roadway, connected together at every 12 feet by cross timbers, between which arediagonal braces. The lattice frames are usually placed in recesses prepared for them instone abutments: at Philadelphia a lattice bridge was constructed, which extended 1100feet, resting on ten stone piers ; and on the New York and Haerlem railway another, 736feet in length, with only four stone piers.

One of the chief points to be considered in the construction of large timber arches is thelightness and elasticity of the material; for when a heavy load is placed near one of the

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