It has been stated by Mr. Stephenson, that the idea of forming a tubular bridge was suggested by experience gained in constructing the railway bridge at Ware, which consisted of a wrought-iron cellular platform; but a more exact representation of the principle on which the Britannia Bridge is constructed had been long previously seen across the Rhine, at Schauffhausen, where a rectangular tube, or hollow girder, made of wood, was erected in 1757. That bridge, though of different material, was in its principle of construction similar to the iron tubular bridges at Conway and at the Menai Straits. Another similar bridge, carried over the river Limmat, at Wettingen, constructed in 1778, had a span of 390 feet; and that, as well as the former, was raised to its position in one piece, by means of powerful screw-jaws. These curious and interesting structures, which may be considered the forerunners of the gigantic iron Tubular Bridges of the present day, were burnt by the French in 1799. In constructing the Britannia Bridge, Mr. Stephenson took advantage of a rock midway from shore to shore, whereon to erect the central pier. Two other piers, at a distance, on each side, of 460 feet, were built without much difficulty in shallower water, and between these and the masonry on each side was a distance of 230 feet. There are eight rectangular tubes resting on those piers, to form two lines of railway, each tube being 28 feet high and 14 feet wide, exclusive of the cellular cavities at the top and bottom. These cavities are rectangular, and extend from one end of the bridge to the other, and may be regarded as long tubes. There are eight of them at the top, each 1 foot 9 inches square, and there are six at the bottom, the latter being 2 feet 4 inches wide, and the same depth as those at the top. Sound is conveyed through these cavities as readily as through speaking tubes, and conversation can be thus easily carried on across the Straits. The height of the central pier of the Britannia Bridge, from the foundation to the top, is 230 feet; and the height of the roadway above high water mark is 104 feet. The length of the large tubes, through which the railway carriages pass, on each side of the central pier, is 460 feet: and the total length from shore to shore, 1,531 feet. The tubes are connected together at the piers to give the bridge additional strength, and they are composed altogether of 186,000 separate pieces of iron, which were pierced with seven millions of holes, and united together by upwards of two millions of rivets. The whole mass of iron employed weighed 10,540 tons.