The deck span, for instance, weighed 12,000 pounds to each foot of bridge. The total weight of this very long bridge reaches the enormous figure of 32,000,000 pounds. And yet, even the untechnical observe the extreme simplicity of its lines of construction, and feel that the engineer, A. W. Thompson, has done his work well. The construction of the giant took two years and a half. During that time, the trains of the B. & O. were diverted to the closely adjacent Pennsylvania, so that the bridge-builders might continue with a minimum of delay. The truss span reaches its limitations at a little over 500 feet in length--we have just seen how the Susquehanna structure had its spans cut in halves in the non-navigable portions of the river. The spans of two great railroad bridges over the Ohio at Cincinnati reached 519 and 550 feet, but they were built in a day when the weights of locomotives and of train-loads had not yet begun to rise. Nowadays the shorter span is the safer and by far the best. The engineer builds plenty of midstream piers, looking out only for a decent width for any navigable channels. And when because of peculiarities of location he cannot place his pier midstream, then it is time for him to get out his pencils and begin his drawings all over again. He can perhaps build a suspension bridge--a clear span of 1,500 feet will be as nothing to it,--but suspension bridges take a long time to build and are fearfully expensive in the building. It is more than likely, then, that he will turn to the cantilever. In the cantilever, two giant trusses are cunningly balanced upon string supporting towers. They are constructed by being built out from the towers, evenly, so that the balance of weight may never be lost for a single hour. The two projecting arms are finally caught together in mid-air and over the very centre of the span--caught and made fast by the riveters. The result is a bridge of surpassing strength and fairly low cost, a real triumph for the bridge engineer. The first of these cantilever bridges built in the United States was of iron. It was designed and constructed by C. Shaler Smith across the deep gorge of the Kentucky River in 1876-77. Mr. Smith also built the second cantilever, the Minnehaha, across the Mississippi, at St. Paul, Minn., in 1879-80. The third and fourth were the Niagara and the Frazer River bridges built in the early eighties. In their trail came many others--one of the most notable among them being the great Poughkeepsie Bridge. * * * * *