The author states, that his attention has been long directed to the extensive construction of the brick and stone bridges usually erected over and on the line of railways. and the apparent want of durability in the timber-bridges, o hick have in some instances been substituted ; as well as to the cast-iron bridges, which have generally been constructed in situations where the height between the top of the rails and level of the roads which they span, were so limited, as not to admit of a stone or brick arch. In the latter case, cast-iron girders have been employed, but their great weight has rendered them expensive, and has obliged the abutment piers for supporting them to be very substantial.
In order to obviate these objections, the author has intro duced combinitions of cast and wrought iron in forms which he contends may be advantageously adopted ffir occupation bridges, or even for carrying the railway. and that they- ;nay be constructed at a less cost than stone, brick, or even timber bridges. These bridges consist of long it ndina I and segmental girders of cast-iron, abutting against each other at the ends, secured together by bolts and nuts through the flanges, and resting on masonry abutments; a system of wrought-i•on tie trussing is then applied, and struts are placed at certain distances where they are requisite. As many of these prin cipal trusses are used as the strength of the bridge demands, and they are connected by transverse braces, and distance pieces of cast iron. thus preventing undue outward pressure, sockets are cast upon the girders to receive the timber, joists, and the platform is covered with Dantzie deal spiked to the joists. The wrought-iron struts at the top, clasp the girders, to which they are also firmly' bolted, and their lower extremities pass through the truss, so that on tire nuts being screwed up, the truss is brought to its proper degree of tension, and being made sufficiently strong to bear the weight calculated fur the bridge, independent of the seg mental girders, the weight and strain are brought upon the abutments in the most favourable manner. Bridges thus constructed do not require any centering for their erection, as each side may be put together near the spot, and, by means of purchases, may be lifted entire on to the abutments, or the whole bridge may be put together before the earth is excavated from between the abutments, excepting only as much as is necessary for receiving the trussing.
The dimensions are given of occupation-bridges calculated to bear S tons, which is stated to be a greater weight than is required by the land-owners. The total weight of cast and wrought iron in an oblique bridge of a span of S6 feet 3 inches, and 11 feet wide, is 11 tons 7 ewt., and that I d' a square bridge of 106 feet 6 inches span and 11 feet wide, is I41- tons ; their total cost, including excavating the ground, the mase•y, stone penning on the sides of the excavations, the timber work, and the painting, was for the former £'280. and 1hr
the latter, £34'2; these sums are stated to be much less than the expense of similar bridges of stone, or even of timber.
A design is also given of a stronger kind of bridge of similar construction, for carrying, two lines of railway. The span is 90 feet, and the width 23 feet between the side rail ings. The weight is 43 tons, and the total cost, including the masonry, is estimated not to exceed X1.200. It is calculated to bear about 50 tons, which is as much as could be brought upon it by any passing train.
Numerous railway-bridges have been erected within the last few years, in a similar manner to that suggested by Mr. Storey, and also by other engineers. Many of these designs show much ingenuity, and great variety of arrange ment, and exhibit almost every form of combining wrought with cast iron. A bridge of a very novel construction, how ever, requires particular notice, not only from the boldness of its design, but from its being, we believe, the first applica tion of the suspension principle to a railway-bridge.
The iron tie-bridge over the Regent's Canal at Camden Town, on the line of the London and Birmingham railway, is one of the most ingenious struetures on the whole line, and was designed by Mr. Fox. ene of the resident engineers. for the purpose of conveying the railway at an elevation of only 13 feet above the stu face of the water, and with a span of not less than 50 feet. The structure consists of three main cast-iron ribs, each composed of two large castings, extend ing, the whole width of the opening, and having a bearing at each end of 4 feet. These minor ribs are connected together by transverse iron bracing. The pedestal; on which the ribs take their bearings, are 10 feet long, 6 feet wide, and 30 feet high. and are built of brick and stone, founded on a bed of concrete 2 feet in thickness. Eight cross girders run from either side main rib to the centre main rib, from Which they are severally suspended at each end by vertical rods and keys. The girders are offish-bellied frm, and are each 28 feet in length, and 2 feet deep in the middle. The thrust of each arch is sustained by wrought iron tie-bolts running from end to end of the ribs. Ti the front of the ribs is !Jolted open ornamental work, which gives to the whole a pleasing appearance. The extreme width of this bridge is GO feet. The rails over the briilge are set in chairs, and bolted thereto. The chairs are fixed un oak slabs running longitudinally, and resting on the tops I If the girders. The side and intermediate spaces, and also the space beneath each pair of rails, are covered with iron gratings, furnished with flanges underneath to strengthen them.