elevation of half the bridge; Fig. 15, a transverse section through the bow tube above, and stringer or chord tube below; and Fig. 16, a partial section through two of the cross-beams which carry the longitudinal timbers and rails. Several bridges of similar design to the one last described have been since constructed. Cap

line of the Blackwall Railway from Stepney to Bow: "These two bridges are of a peculiar form, and the first of their class erected for railway purposes. The roadway upon them is supported on wrought-iron girders, placed transversely between two arches, or ribs, formed entirely of wrought iron. The clear span of one is 120 feet, of the other 116 feet 8 inches. Each arch or rib of the latter bridge, which carries the railway over the Regent's Canal, is formed of a box built with iron boiler plates inch in thickness, and angle iron, firmly riveted together, its breadth being 2 feet 10 inches, its depth about 2 feet, and sectional area 81 square inches, and is connected at the base by a wrought-iron tie-bar, which receives the horizontal thrust of the arch, and is formed of links having a total sectional area of 69 square inches, bolted together with bolts 2 inches in diameter, aided by eight others at each joint inch in diameter. Between the tie-bars and the arch a system of vertical and diagonal bracing has been introduced, so as in a manner to distribute the weight of passing loads equally over the whole arch. These ribs, so formed, are laid in cast-iron plates, fixed at one end, and free to move at the other over rollers, so as to allow scope for the expansion and contraction of the metal. The clear interval between the bearings is 116 feet 8 inches, and the rise of the arch is 8 feet to the under side of the box of which it is formed, the roadway being beneath the arch, and about 2 feet above the bottom

of the tie-bar. The structure is exceedingly light, but appears, nevertheless, sufficiently strong to carry the weights which may come upon it in practice, so far as the areas of the arch and bow-string, or tie, are concerned, and has stood the test of a dead weight of 240 tons, in addition to its own weight of 59 tons, distributed in weights of 34 tons at equal distances over its length, with a deflection of 31 inches, and recovered entirely its original position upon the removal of the load. As this proof exceeds considerably any weight that can be brought upon it in practice, I am of opinion that it may be used with safety for the passage of trains; but as it is of so novel and light a construction, and the action of the cross-bracing and connection of the tie-bars has not been ascertained by continued experiments of moving weights, I should recommend that it be examined from time to time, so that any defect, if it should exist, might be ascertained, more particularly as the weight of the whole bridge, including the double line of roadway and covering, only amounts to 194 tons, and is very easily set in vibratory motion by any moving power."

Upon a limited scale, tubular girders of wrought iron appear to have been applied to the purpose of bridge building nine years ago, although in a very different manner from their improved construction, as invented by Mr. W. Fairbairn. The instance here alluded to is a bridge which carries the Carmunnock road over the Polloc and Govan railway, near Glasgow. This bridge, which crosses the railway obliquely, was erected by Mr. A. Thompson, and is 31 feet 6 inches in span on the face, or 30 feet square with the railway. The width of the bridge, from outside to outside of parapet, is 25 feet 6 inches, and the roadway is supported upon six girders, each 35 feet 3 inches long, resting upon stone abutments, and at a distance of 5 feet 1 inch apart between their centres. Each girder stands upon a wrought-iron plate at each end, and is constructed of the best boiler-plate inch thick, in the manner hown in Figs. 17 and 18, of which Fig. 17 is a sectional view

clear at the top and 6 inches at bottom. The upper and lower plates are 6 inches wider than the beam, the projection. of 3 inches on each side being provided to receive the angleirons, 3 x 3 x inches, which are riveted to the side plates and upper and lower plates respectively with 4-inch rivets placed 1 inch apart from centre to centre. These girders are filled with concrete, with the view of increasing their resistance against a pressure from the outside, and they are tied together with transverse bars of Low Moor iron, 3 inches by

inch, attached by bolts to T-irons riveted to the side plates of the girders. The spaces between the beams were filled in with two courses of 9-inch arched brick-work, the rise of the arches being 1 inch. The crown of these arches was payed over with hot tar, and a layer of clay puddle well rammed down over the tar. Over the puddle a metalling of whinstone was laid to form the roadway, covered with a binding course, 2 inches thick, of engine ashes. The foot pavement

side of the bridge is 4 feet wide, with a gutter laid between it and the roadway. This bridge was built for W. Dixon, Esq., of the Govan Iron Works, at Glasgow. The communication between the furnaces of these works is by platforms carried upon tubular beams 33 feet in length. The transverse sectional form of these beams is rectangular, instead of having the sides inclined, as described of the bridge girders, and their dimensions are as follows: depth in the clear, 19 inches; width in the clear, 7 inches; plates, inch thick. The side and bottom plates are connected by inner angle-irons, with -inch rivets, placed 2 inches apart between their centres. The side plates rise 2 inches above the top plate, and are connected with it by external angle-irons placed upon the top plate and between the side plates, riveted as the bottom plate is to the sides.

It is scarcely necessary to point out here the many differences between the tubular beams used in this bridge and the patented tubular girders; but the former are probably the earliest application of a tubular plate-iron girder in any form to bridge building, and are therefore historically interesting.

SECTION V.

Chester and Holyhead Railway-General Sketch of the Line-Telford's Holyhead Road-The Menai and Conway Suspension Bridges-Railway Tunnel, Sea-wall, and Viaduct, at Penmaen Mawr-Parliamentary Proceedings, and Engineers' Reports upon the Communication between London and Dublin-Iron Bridges proposed by Mr. Rennie in 1802— Mr. Robert Stephenson's Design for Cast-Iron Arched Bridges, and selection of Site over the Britannia Rock—Admiralty Opposition, and Mr. Stephenson's consequent Design of the Tube.

THE railway from Chester to Holyhead, forming an important part of the shortest line of communication between London and Dublin, is highly interesting in its general features, as it is peculiarly so in comprising the two fine structures known as

the Conway and Britannia bridges. The length of this railway is 84 miles; and its several stations, starting from Chester, and their respective distances from that ancient city, are as follows:

The journey between Chester and Holyhead is now usually performed by the mail trains in 3 hours and 5 minutes, of which 35 minutes are occupied between Bangor and Llanfair, a distance of 4 miles, by road carriages. About 25 minutes of this time will be saved when the Britannia Bridge, situated between these two stations, is completed; and the total time of the journey will be thus reduced to about 2 hours and 40 minutes. The royal assent was given to the Bill for this railway on July 4, 1844, and the works have been conducted with considerable vigour since their commencement. The general direction of the line is nearly east and west, but its course is in several parts extremely tortuous, which is rendered necessary by the mountainous character of the country traversed. On this account nearly the whole of the line is constructed on or near to the coast, the first 25 miles from Chester pursuing a direction nearly N.w. At this distance the course is turned towards the south, in the direction w.s.w., and