“From coast to coast the span of the Strait is some 1100 feet, with that rock in the centre. Now the problem is, to build a bridge across that gulf of surging water strong enough to bear heavy trains at high speeds, and sufficiently above the water to prevent any interference with navigation.” “And how will you manage it?” “First I thought of large cast-iron arches, but they will not do. I doubt if they would stand the strain; and moreover we should impede navigation by raising scaffolding during the building. At length I came to the idea of a tube bridge.” “What! a tube bridge! I’ve never heard of it!” “No, it is a new idea. By reconsidering a design I had made for a small bridge over the Lea at Ware in 1841, and thinking over the matter, I came to the idea that a bridge consisting of a hollow beam or tube might solve the difficulty.” “A huge hollow girder, so to speak!” exclaimed his friend. “Exactly so. Accordingly,” the engineer continued, “I had drawings prepared and calculations made, by which to ascertain the strength of such a bridge, and they were so satisfactory that I decided on attempting one.” “It is like constructing one huge hollow beam of iron by rivetting plates together. Can it be done?” remarked his friend. “The making of the high-level bridge over the Tyne, in which I had a part—the bridge between Newcastle and Gateshead, you know—was a transition between an arched bridge and a girder bridge. A girder of course is a beam, it may be of iron or wood, and the little bridge at Ware has been built of girders made of plates of wrought-iron rivetted together. Therefore, you see, I am not unused to wrought-iron girders, and what they will bear.” “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.