This surprising and unique example of engineering skill has been constructed across the river Conway, and forms a portion of the Chester and Holyhead line of railway. The enormous iron tube, or more properly trunk or chest, is made of wrought iron plates, varying in thickness from a quarter of an inch to one inch, riveted together and strengthened by T irons, and, to give additional strength to the whole, a series of cells is formed, both at the top and at the bottom of the tube, between an inner ceiling and floor, and the exterior plates. The upper cells, eight in number, are nearly square, being 1ft. 9in. high, and 1ft. 81/4in. wide. The lower cells, six in number, are 2ft. 31/2 in. wide, by lft. 9in. high. The space between wall and wall of the tube, if we may so speak, is 14ft.; and the height of the whole, inclusive of the cells, is 22ft. 31/2 in. at the ends, and 25ft. 6in. at the centre; the total length of the tube is 412ft.

One end of this gigantic tube is fixed to the masonry of one of the stone piers on which it is supported, and the other end is so arranged as to allow for the expansion of the metal, owing to the variations of atmospheric temperature, and rests on eleven iron rollers, lying on a bed plate; but, in order that the weight of the whole tube may not be carried by these rollers, six girders are carried over the tube, and riveted to the upper part of its sides; the extremities projecting from the sides, and resting on twelve balls of gun metal, running in grooves, which are fixed to iron beams let into the masonry. The weight of the Conway tube is about 1300 tons. For the purpose of constructing this triumph of engineering skill, an enormous platform was raised on piles, on a piece of land projecting into the Conway, and about 100 yards from the site of the piers of the bridge, and on this the work of erecting the tube was carried on; its construction occupied twelve months, and when complete it was floated to the piers on which it was to be raised, on six huge pontoons, three at either end, constructed near the spot where the tube was erected; each of these pontoons was 100 feet in length, 25 feet in width, 10 feet high; they were floated at low water under the platform on which the tube rested.

The piles supporting the platform being taken away, the whole mass of the tube rested upon two stone piers, temporarily erected at either end for that purpose, and as the tide rose, the pontoons lifted the tube from these piers, and transported it to the shelf prepared for it, on the permanent piers, and below two enormous hydraulic presses, by means of which it was afterwards raised into the place it was intended permanently to occupy.

At each end of the tube two strong lifting beams were fixed transversely, to which the lifting apparatus of the hydraulic presses was attached; the chains by which this was effected were of wrought iron, each link being 6 feet in length from pin hole to pin hole; at each lift the tube was raised 6 feet; the cylinder in which the ram of the press worked was 3 feet 1 1/2 inches outside measure, and the hollow for the ram lft. 8in. in diameter. The space allowed for water all round the ram was very narrow, and at the top of the cylinder was a deep collar, at which part the space for water was stopped, and a water-tight joint formed by means of a leather packing in it. The aperture for the entrance of water into the cylinder was bored through the collar, and only three-eighths of an inch in diameter, and the quantity of water each cylinder held, was 66 gallons; each press, being worked at the pressure of 3 tons to the inch, was capable of lifting 972 tons; but as it was calculated they would bear 4 tons to the inch, each of them, in that case, would have been able to support the enormous weight of 1296 tons.

On Monday, the 6th of March, 1848, the operation of moving this enormous mass, and placing it in its proper position, began, and on the first of May it was firmly seated on the permanent piers, and formed part of the railway. The Engineer by whom this great work was effected is Robert Stephenson, Esq. and Messrs. Easton and Amos were the engineers on whom devolved the onerous duty of lifting this tube, by means of the hydraulic apparatus.

See other side.