2

E.—2

The general structure of the bridge may be described as follows : —Piers were erected at either side of the river, at a distance of 300 feet apart, over which two chains were stretched, and carried by tunnels into the ground at either side. These tunnels were to be 35 feet in length; but on the south side were extended to upwards of 70 feet, in order to reach the solid rock. Shafts were sunk from the surface, meeting the ends of the tunnels, in order to provide access to the apparatus for anchoring the chains. Each chain was to be composed of seven twisted wire ropes, 4f-inches in circumference, laid side by side, and above them six other ropes, each made of thirty telegraph wires spliced together but not twisted, placed side by side, the whole united every 10 feet by clips, forming a flat chain of nearly 12 inches in width by 2 inches in depth. Of the fourteen twisted wire ropes used, however, ten were 5-|-inch and four were 4^-inch. The chains were not parallel, but curved outwards at the ends, being 8 feet apart—the breadth of the roadway—in the middle; 30 feet apart at the piers ; and about 65 feet apart at the bottom of the tunnels, where fastened to the ground. The bridge itself consisted of two trusses of 300 feet in length and 10 feet in height, with a wooden roadway 8 feet wide between them, carrying a railway in middle. The whole suspended from the ropes by iron rods bolted through cross beams underlying the trusses. The chains were fixed to the ground as follows :—A solid mass of masonry, of about 7 feet by 6 feet, was built into each wall of the tunnel, the inner face being brought to a sloping plane perpendicular to the strain of the chain. Against the inner edge of each wall was placed a cast-iron girder the whole height of the masonry; and across these girders, and crossing the opening of the tunnel, was a cast-iron beam, the anchor, to which the ropes were fastened. Each anchor was 4 feet 6 inches in length, 12 inches in width, and 5 inches in depth, with a web at the back of 3 inches in width, rising by a curve from the ends to 5 inches in depth in the middle ; so that the middle section of the anchor showed a flange of 5 by 12, with a web sby 3 inches. The unsupported space between the bearings on the girders was 3 feet 6 inches. At either side of the central web the anchor was perforated by holes —on the lower side seven in number, on the upper side six, each 2 inches in diameter. The several ropes, of which the whole chain was formed, were fastened to screw bolts 2 inches in diameter, passing through the holes in the anchor, and screwed up by nuts at the back; the seven lower bolts taking the twisted wire ropes, and the six upper ones the telegraph wire ropes. The strain of each chain was thus distributed over a space of about 2 feet in the middle of the 3^ feet between the bearings. It is obvious that the perforations of the anchor by so many holes greatly reduced its strength; but for this allowance has been made in the calculations. It was intended to build up the tunnels, in front of the anchors and surrounding the chains, with solid concrete, at least to the depth of the masonry, 6 feet; and there is no question that, had this been done, the whole apparatus for securing the ropes would have been more than sufficiently strong to support any weight which could have been brought upon the bridge. The Engineer-in-Chief stated to us that his intention was that this should be done as soon as the ropes were in position; and that he gave verbal instructions to that effect. Such instructions, however, were not in writing, and we notice that neither in the specifications nor in the plans is there any allusion to this filling up with concrete. On the other hand, we ascertained from the District Engineer, that as the contractor was required to bring the chains and bridge into a specific form, it was desirable, if not necessary, to retain the power of regulating the strain on the chains, by means of the screws, after the roadway was completed, which could not have been done had the chains been buried in concrete. Mr. Carruthers, however, is of opinion that no adjustment was required after the chains were once brought to a proper strain; and thought that the District Engineer understood that the concrete was to be built in as soon as the chains were in position.