“What are the distances?”
“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.”