It was in April 1846 that the foundation stone of this great structure on the then Chester and Holyhead Railway was laid, on the Britannia Rock in the middle of the Straits. The rock itself obtained its name from the “Britannia,” a vessel wrecked some years before at this point, and in its turn suggested the very appropriate title of the bridge. The width of the Straits, here about 1,100 feet, presented an even graver problem to Robert Stephenson than had been faced by Telford, over twenty years before, for not merely a suspended road-bridge, but a rigid structure to sustain the burden of trains weighing 200 tons had to be designed. The Admiralty, jealous to preserve the navigation, forbade an arched bridge of any less height than a clear hundred feet above high water, and moreover refused to allow the interruption of the passage for a single day during the construction. Arches were out of the question, and eventually Stephenson resolved what was then the novelty of a bridge on the beam principle, consisting of rectangular iron tubes supported at intervals by giant masonry piers. There are three of these supports: the Britannia pier and tower, midway, and one on either shore. The two spans, or main tubes as they are called, across the water are each 472 feet in length, and that of the side spans 230 feet each; the whole length of the bridge 1841 feet, the weight of iron 10,000 tons, and the cost £602,000. The idea of this vast mass of riveted iron being elastic, seems grotesque, but when Stephenson designed the bridge he had sufficient forethought to allow for the expanding and contracting properties of iron under extremes of heat and cold. This precaution for securing free movement of the tubes, that would otherwise have broken down the supporting piers, took the form of allowing their ends to rest on cast-iron rollers and balls in a clear space left in the masonry. Experience has shown the expansion and contraction to be fully twelve inches. The first train ran through on March 5th, 1850, completing the present railway route from London to Holyhead; and although the weight of locomotives and trains has doubled since then, the bridge still serves its purpose.