There are two tubes, to accommodate a double track (one would have done in this country, but in England they do nothing by halves), and each is 1513 feet long. The total length of the bridge is 1841 feet. These tubes are not round or oval, but nearly square at the termini; the bridge being constructed on the principle of the arch. A section of one of the tubes at the Britannia pier is in the form of a parallelogram, where it is 30 feet high, gradually diminishing towards each end to 20 feet. The tubes are riveted together into continuous hollow beams; they are of the uniform width of 14 feet 8 inches throughout; they are constructed entirely of iron, and weigh about 12,000 tons, each tube containing 5000 tons of wrought iron, and about 1000 tons of cast iron. The tubes were constructed each in four sections; the sections extending from the abutments to their corresponding piers, each 250 feet long, were built _in situ_, on immense scaffolding, made of heavy timbers for the purpose, even with the railway; but the middle sections, each 470 feet long, were built on piers on the Carnarvonshire shore, then floated into the stream, and elevated to their position; each of these sections weighed 1800 tons. CONSTRUCTION OF THE TUBES. The sides, bottom, and top of these gigantic tubes are formed of oblong wrought iron plates, varying in length, width, and thickness, according to circumstances, but of amazing size and weight. They are so arranged as to obtain the greatest possible strength, the whole being riveted together in the strongest manner. In addition to the 1600 tons of wrought iron in each of the four large pieces, an additional 200 tons was used to form lifting frames, and cast iron beams for the purpose of attaching the tube to those huge chains by which they were elevated. The construction of the tubes is thus described in the London Illustrated News, from which this account is derived: