Whether a bridge is
to cross a brook, a river, a strait, an inlet, an arm of the sea, a
canal, or a valley, are so many diverse contingencies which modify the
calculations and plans of the engineer. Here liability to sudden
freshets, there to overwhelming tides, now to the enormous weight of
railway-trains, and again to the corrosive influence of the elements,
must be taken into consideration; the navigation of waters, the
exigencies of war, the needs of a population, the respective uses of
viaduct, aqueduct, and roadway, have often to be included in the
problem. These considerations influence not only the method of
construction, but the form adopted and the material, and have given
birth to bridges of wood, brick, stone, iron, wire, and chain,--to
bridges supported by piers, to floating, suspension, and tubular
structures, many of which are among the remarkable trophies of modern
science and the noblest fruits of the arts of peace. Railways have
created an entirely new species of bridge, to enable a train to
intersect a road, to cross canals in slanting directions, to turn amid
jagged precipices, and to cross arms of the sea at a sufficient
elevation not to interfere with the passage of ships,--objects not to be
accomplished by suspension-bridges because of their oscillation, nor
girder for lack of support, the desiderata being extensive span with
rigid strength, so triumphantly realized in the tubular bridge.
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