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. [Illustration: ROBERT STEPHENSON.] “Often at night,” he declared, “I would lie tossing about, seeking sleep in vain. The tubes filled my head. I went to bed with them, and got up with them. In the gray of the morning, when I looked across Gloucester Square, it seemed an immense distance across to the houses on the opposite side. It was nearly the same length as the span of my tubular bridge.” The principle adopted was to construct the shorter tubes on scaffolds in the places which they were to occupy. This could be done, for such scaffolding would not impede navigation. But scaffolding could not be built for the large tubes across the great spans of water. What then was to be done? It was decided to build them on platforms on the shore quite close to the water, and float them when ready on pontoons to their places between the piers, raising them to their position by hydraulic power. Such a task would be hazardous enough. It was first tried at Conway, where a similar bridge was being built by Robert Stephenson, being indeed part of the same railway. The Britannia was, however, a much greater enterprise, though the span of the Conway is 400 feet. The Conway bridge, indeed, is but of one span, and contains two tubes.