There is probably no single problem, wherein the laws of science and mechanical skill combine, which has so won the attention and challenged the powers of inventive minds as the construction of bridges. The various exigencies to be met, the possible triumphs to be achieved, the experiments as to form, material, security, and grace, have been prolific causes of inspiration and disappointment. In this branch of economy, the mechanic and the mathematician fairly meet; and it requires a rare union of ability in both vocations to arrive at original results in this sphere. To invent a bridge, through the application of a scientific principle by a novel method, is one of those projects which seem to fascinate philosophical minds; in few have theory and practice been more completely tested; and the history of bridges, scientifically written, would exhibit as remarkable conflicts of opinion, trials of inventive skill, decision of character, genius, folly, and fame, as any other chapter in the annals of progress. How to unite security with the least inconvenience, permanence with availability, strength with beauty,--how to adapt the structure to the location, climate, use, and risks,--are questions which often invoke all the science and skill of the architect, and which have increased in difficulty with the advance of other resources and requisitions of civilization. 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. The day when the great Holyrood train passed over the Strait of Menai by this grand expedient established the superiority of this principle of