“Why, it is like a huge extension of the primitive log-bridge of our ancestors.”
“If you like,” replied the engineer, laughing.
Robert Stephenson—for he it is whom we suppose to be speaking to his friend on this gigantic engineering enterprise—became satisfied by reflection that the principles involved in constructing an immense tubular beam were but a development of those commonly in use; and Sir William Fairbairn was entrusted with the duty of experimenting as to the strength of tubes, the directors of the Railway Company voting a sum of money for the purpose.
Sir William, then Mr., Fairbairn concluded that rectangular tubes were the strongest, and a model was made of the suggested bridge. It proved successful, and indicated that the tube would be able to stand the strain of a heavy train passing rapidly over it.
In September, 1846, Mr. Fairbairn read a paper on the subject at the meeting of the British Association at Southampton, as also did Professor Hodgkinson, a mathematician, who had verified Fairbairn’s experiments. Not long afterwards Stephenson became satisfied that chains were not needed to assist in supporting the bridge, and that his tubes would be strong enough to support themselves entirely between the piers.
Work therefore went forward. Some 1500 men were engaged on the Britannia Bridge, and the quiet shores of the Menai Straits resounded with the busy hum of hammers and machinery. Cottages of wood were built for the men, and workshops for the punching and rivetting of the plates for the gigantic tubes.
The design included two abutments of masonry on either side of the Strait, and three towers or huge piers, one of which, the centre pier, was to rise from the Britannia Rock, 230 feet high. There are four spans, two over the water of 460 feet each, and two of 230 feet each over the land. Two tubes, quite independent of each other, but lying side by side, form the bridge across. Each tube or beam is 1510 feet long, and weighs 4680 tons. Its weight at one of the long spans is 1587 tons.
Now how could these gigantic tubes be put together and raised to their positions? Here was a problem almost as great as the original one of the bridge itself, and it troubled the engineer sorely.
