There was another bridge on a similar principle erected near the same place in 1805, called the "Market Street Bridge," which consisted of three arches, the centre one being 195 feet span, and the other two 150 feet span, with a breadth of roadway of 35 feet.

339. One of the earliest forms of bridges used in America for railway purposes is that shown by Plate XLIII., called the "Lattice," and which was patented, in 1835, by Mr. Ithiel Town. It was usually formed of planks 12 inches wide by 3 inches thick, fastened together by oak treenails 2 inches diameter. This form of bridge answered very well for moderate spans and slow traffic, but owing to the great number of joints, and, above all, the shrinking of the timber, it was unsuited for long spans or for heavy traffic. Many of these bridges had to be strengthened by the addition of arched ribs on each side. For short spans and the light traffic of country roads, they have some advantages, owing to their simple form, and to the ease with which they can be constructed by unskilled workmen.

A lattice bridge of 200 feet span on this principle was erected, in 1850, over the river Nore, in Ireland, on the Waterford and Kilkenny Railway. The lattice bars, which

* Cresy's ' Ency. of Civil Engineering.' were of Archangel deals, were 7 1/2 inches by 2 1/2 inches, and were fastened with iron pins instead of oak treenails.

In 1838 a lattice bridge, 2900 feet in length, was erected over the James River at Richmond, by Moneure Robinson, for the purpose of carrying the Richmond and Petersburgh Railway. The trusses, with spans varying from 130 to 153 feet, were supported on eighteen granite piers.

A similar bridge, 2200 feet in length, was erected by the same engineer across the Susquehannah, the spans of which were each 220 feet.

340. An improvement on the lattice form of bridge is that known as Long's Frame Bridge, shown by Figs. 106 to 109.

Fig. 106.

This form was frequently applied to spans of 100 to 200 feet. Fig. 106 shows part of the elevation of a Long's Frame Bridge, on the Western Railway, over the Connecticut River, as designed for a span of 180 feet, measured from centre to centre of the piers. The top string, which is shown to an enlarged scale by Figs. 108 and 109, consisted of three rows or beams of timber, each 8 inches square, so placed that, by leaving a small space between each, the breadth of the string was increased to 28 inches, to correspond with the bottom string, which was built of six planks, each 12 inches deep, the four inner ones being 5 inches thick, and the two outer ones 4 inches thick each; the whole together forming a beam 12 inches deep by 28 inches broad. The planks of the upper string were bolted together at intervals of 7 feet, and those .of the lower string at intervals of 2 feet.

Fig. 107.

Fig. 108.

FIG. 109.

Short wedge-shaped blocks of hard wood were let into the top and bottom strings to the depth of about 1 inch, as shown in Fig. 109. Into the sides of these blocks were mortised the ends of the diagonal braces; these braces were all 8 inches square, and the number abutting on each wedge-shaped block was always three, arranged two on one side and one on the other, so that in crossing each other the single one, which formed the counter-brace, passed between the other two. Cross pieces of timber, about 7 inches by 5 inches in scantling, were fixed to the top and bottom strings at each wedge-block, to take the ends of the vertical iron rods, which were in pairs, as shown by Figs. 108 and 109.

The transverse beams which supported the roadway, rested upon the bottom string pieces, and were placed one on each side of the abutting blocks throughout the whole length of the bridge.

The platform which carried the roadway was stiffened by horizontal beams placed diagonally between the bottom strings, as shown by Fig. 107.*

In its original form the Long's Frame Bridge had vertical wooden ties to connect the top and bottom strings instead of the iron rods shown in Fig. 106, and the braces were framed into the heads and feet of these ties, and were capable of adjustment by means of wedges.

The improved form shown by Fig. 106, is better known as the Howe Bridge, and the improvement chiefly consisted in the substitution of iron rods for the wooden ties, by which a certain amount of camber could be given.

One of these bridges was erected over the James River at

* Spons' Dict, of Engineering,' by Oliver Byrne.

Richmond, on the line of the Danville Railway, the length of which was about 1800 feet, the general span of the openings 121 feet, and the width of the bridge from out to out 18 feet. The number of trusses were two, and their depth 20 feet. The upper strings consisted of two pieces 9 inches by 12 inches, and four pieces 5 inches by 9 inches, having a total sectional area of 396 square inches. The bottom strings consisted of four pieces 6 inches by 12 inches, and four pieces 5 inches by 12 inches, having a sectional area of 528 square inches. The area of the bottom string should always be made greater than that of the upper string on account of the numerous joints. The end main braces - eight in number, four at each end of the bridge - were 8 inches by 9 inches, giving a sectional area of 576 square inches. The counter-braces were 7 1/2 inches by 7 inches; the vertical tie-rods were 1 5/8 inch diameter at the abutments and 11/2 inch diameter at the centre of the bridge.