BRITANNIA TUBULAR BRIDGE, a railway bridge over the Menai strait, remark able alike for its gigantic dimensions, and as being the first construction of the kind ever undertaken. With a view to facilitate communication with Ireland ria Holyhead, the directors of the Chester and Holyhead railway in 1845 sought the aid of Mr. Hobert Stephenson, the great engineer, to bridge the strait with such a structure as should admit of the safe passage of heavily laden trains without in any way interfering with the navigation of the channel. About a mile above the suspension-bridge, and nearer Carnarvon, a rock in the middle of the strait rose 10 ft. above the water at low tide; and on this site, provided by nature, it was resolved to erect a bridge in the form of a rectangular tube, composed of wrought-iron plates riveted together in a manner to com bine the greatest strength with the greatest lightness. See STRENTII OF MATERIALS and TT:mm.1km BRIDGES. In the spring of 1846, the undertaking was commenced; by the 22d of June, 1849, the Britannia tower on the rock in the center of the strait was com pleted (height, 191 ft. 6 in. above high-water mark). Other two towers, some 18 ft. lower, were erected on each side of the Britannia tower; thus dividing the space into four spans, of which the two center ones are 960 ft. each, the other two being compara tively narrow. The short tubes between the abutments and the short towers were con structed, by means of strong scaffolding and stages, in the places they were to occupy when finished; the long central tubes were built at the water-edge, from whence they were floated off on pontoons to the base of the towers, which had grooves or recesses made to receive them, and then elevated gradually (supports being built under their ends as they ascended) by powerful hydraulic presses to the requisite height, 102 ft. above high
-water mark. On the 13th of Oct. 1849, the first long tube, 472 ft. in length (12 ft. being allowed for the rest at both ends), and about 1800 tons in weight, was safely fixed 2t its proper height above the sea. The other center tube was got up by Dec.; and on the .5th of Mar., 1850. a train swept through, and the bridge was open for traffic. In Aug. the parallel line of tubes was completed, and the up and down trains could now over the Menai with as little delay and danger as over any other part of the line. The total length of the bridge is 1841 ft.. of the tubes, 1513 feet. The extreme height of the tube at the Britannia tower is 30 ft., diminishing to 22 ft. 9 in. at the abutments, "the difference being made to give a true parabolic curve to the top while the bottom is straight." Inside, the width is 13 ft. 8 in. throughout, and the height 26 ft. at the middle, and 18 ft. 9 in. at the ends. To provide for the expansion and contraction of the metal, the bed-plates in the shore towers and in the abutments, on which the tubes rest, are made to move freely on cast-iron rollers and balls. This precaution, for securing free movement to the tubes, was not unnecessary, as it has been found that between the expansion of summer and contraction of winter there is a difference of fully 12 inches. The total weight of iron used was nearly 12,000 tons, of which the tubes contain 9360 tons of malleable iron. 1015 tons of cast iron, and 175 of permanent rail way. In their fabrication 186,000 different pieces of iron, fastened together by more than 2,000,000 rivets, were used; and in the towers, abutments, etc., there is 1,492,151 cubic feet of masonry. The total cost was about £602,000. The whole structure was completed in less than five years. See TUBULAR BRIDGE.
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