The rivets, of which there are 2,000,000--each
tube containing 327,000--are more than an inch in diameter. They are
placed in rows, and were put in the holes red hot, and beaten with heavy
hammers. In cooling, they contracted strongly, and drew the plates
together so powerfully that it required a force of from 1 to 6 tons to
each rivet, to cause the plates to slide over each other. The weight of
wrought iron in the great tube is 1600 tons.
Each of these vast bridge tubes was constructed on the shore, then
floated to the base of the piers, or bridge towers, and raised to its
proper elevation by hydraulic machinery, the largest in the world, and
the most powerful ever constructed. For the Britannia Bridge, this
consisted of two vast presses, one of which has power equal to that of
30,000 men, and it lifted the largest tube six feet in half an hour.
The Britannia tubes being in two lines, are passages for the up and down
trains across the Straits. Each of the tubes has been compared to the
Burlington Arcade, in Piccadilly; and the labour of placing this tube
upon the piers has been assimilated to that of raising the Arcade upon
the summit of the spire of St. James's Church, if surrounded with water.
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