Poughkeepie the Missouri River Bridge came the great steel bridge over the Hudson at Poughkeepsie (fii. 46,fig-. 4), fin ished in ISM This fine structure has a total length of 6667 feet. It has three spans of 54o feet each and two connection spans of 525 feet each. At the present time this bridge has the longest span of any structure of its type. This structure is a most important one in its relation to the trans portation system of an extensive region, forming as it does the connecting link required to establish direct communication between the railway SVS tems of the New England States and those tapping the coal-fields of Penn sylvania.
Cantilever late years bridges constructed on the principle of the cantilever—that is, in which the superstructure is formed of project ing beams or levers fixed at one end to the abutments or piers and free at the other end—have come into high favor among engineers. This type of bridge structure, like the suspension type to be later considered, is an ancient one, and its revival is due principally to American bridge-builders who have adopted the system successfully in the case of a number of im portant structures.
Ancient Jaioanese Cantilever interesting example of a bridge on the cantilever principle, which is of undoubtedly high though of unknown antiquity, is shown on Plate 38 (fig. 7). The existence of this structure was made known to the engineering world through a corre spondent of Van Nostrancl's Engineering alagaline,wlio furnished a plio tog-raph of the bridge, on which the Figure is based, and also a brief descrip tion, which is here substantially reproduced. The picture is a view of a verv fine and costly bridge of red lacquered work, of cantilever construc tion, built by native engineers at the sacred city of Nikko, in Japan. " The abutments aye of hewn stone, the shore piers of hewn gjanite, octagonal, monolithic, mortised for stone girders; monolithic plate beams receive the wooden superstructure. The stringers, which are fastened into the abutments, balance over the stone beams, but do not reach, by a con siderable distance, the gap being fitted by middle stringers let into the shore stringers. The Niagara Bridge is a mere amplification of this one, which was built as a religious duty, and, like a bridge of angels, its planks are never profaned by the feet of the laity." Pape's a now somel.vhat scarce work by Thomas Pope, published in 1811, with the title A Treatise on Bria'ge Arekitceture, whick the Superior Advantages of the Flying Bridge are fully Proz,rd, etc., the author describes a method of construct ing a timber or iron bridge " with a single arc, on the principle of the lever." This plan is identical in principle with what is now designated
the cantilever. Pope prepared detailed plans and estimates for the erec tion of a timber bridg-e on this system across the East River at New York, and constructed a model to illustrate the practicability of his ideas. In his treatise there occurs, in a foot-note, the following reference to this model. It " was built to illustrate a bridge suitable to span the East River at New York with a single arc the chord of which would be iSoo feet; the altitude, or versed-sine, 223 feet. The abutments were built in the form of so many warehouses, and the whole was erected on a scale of of an inch to one foot; the length of model of half bridge, in real measure, is nearly 5o feet. The weight borne at one time by the unsupported arm of this diminutive model, since finished, has been ten tons, and has astonished the mind of every beholder." This ingenious man, like others who think in advance of their time, failed to see his plans realized. Figures 5 and 6 GM 3S) are illustrations of this unique project, which is worthy of record because of its istoric in terest.
Xiagara Falls Cantilever of the most interesting speci mens of the cantilever bridge is exhibited by the beautiful structure span ning the Niagara River a short distance below the Falls (see Frontisfiica). It has a total length of 910 feet and crosses the river with a single span of 47o feet; the roadway is 239 feet above the water-level. On account of its great height, as well as the rapid current of the river at this point, the erection of staging in the stream was impossible, and the mode of building by " overhang " (described in the reference to the Forth Bridge) was adopted. The materials employed are steel and iron.
Firth of Forth Brieige.—The Poughkeepsie Bridge will be far exceeded in length by the great steel bridge now in course of erection over the Firth of Forth at Oueensferry, in Scotland. This colossal structure, the design of which is shown on Plate 46 (Ars. 2, 3), with viaducts, will be S3oo feet long-, with two main spans of r700 feet each and two side spans of 675 feet each, with a clear way between water-level and the central 5oo feet of 15o feet. Each span consists of two cantilevers joined by a central girder. The method of building is that known as " erection by overhang "—that is to say, each of the great cantilevers will be built out like wings from the piers, section by section, until they shall have been advanced far enough to be united to their connecting girders.