Bridges form one of the noteworthy varieties of the modern bridge which are made to do valuable service for railway purposes. Of these, several remarkable examples were noticed under the head of Wooden Bridges (p. 25o). Some of these have been replaced by iron structures of the same type, and a number of others of exceeding boldness of design have been erected within a recent period.
Trestle [7(707110ES. —Of some interest are the cast-iron trestle viaducts erected over the Cheat River for the tracks of the Baltimore and Ohio Railroad. These were built in 1853 by Albert Fink. They cross the river at an elevation of 25o feet, the trestles being- 6o feet high. They are still in service. The substitution of wrought iron for this form of structure is believed to have been first made by C. Shaler Smith, an eminent American engineer. He employed Plicenix columns (pi. 42, fig. 7) for posts, stiff ening them with cross-struts united with diagonal tie-rods. This struc ture, which supported a trussed girder carrying- the roadway, has been the model after which a number of similar structures have been copied. A notable example is the Portage Bridge (fi/. 45,.fis-. 1), an iron viaduct over the Genesee River, in New York, on the line of the Erie Railway. It has ten spans of 5o feet, two of TOO feet, and one of I'S feet, and a 50-foot span placed between each of the last-named long spans.
The l'errugas 'lad net is on the line of the Oroyo Railroad, in Peru; it crosses the Agua de Verrugas at a height of 5478 feet above sea-level. As shown in Figure 2, it is composed of three iron piers respectively 145, 252, and 177 feet high, each being- 5o feet long by 15 feet wide at the top. The spans are formed of trusses of the Fink pattern, three of which are too feet wide and the fourth 125 feet. The length of the structure over all is 575 feet.
Kinszta L'iaa'uct. —An even more extensive structure than this is the Kinzua Viaduct, on the line of the Erie Railway, over the Kinzna gorge (pi. 42, fig. 15; 46, Ar. t). It is 2o52 feet in length and at its highest point 3or feet above water-level. The foundations on which the trestles are supported are piers of sandstone one hundred and twelve in number. The posts are built of Plicenix columns stiffened substantially as above described. A notable structure of the same character, 302.5 feet high and
1662 feet long, has been thrown across the Rio Pecos, in Texas, on thc line of the Galveston, Harrisburg- and San Antonio Railroad.
European T i'aduas. —A number of notable viaducts have been built in Europe in imitation of the high wooden-trestle viaducts in this country. (See Wooden Bridges, p. 25o.) Among these may be mentioned the Crum lin Viaduct (185o) at Newport, in South Wales, with trestles 173.87 feet high, and the SaOne Viaduct at Freiburg- (1862), with trestles 141.7 feet hig-h. In these structures the posts and cross-struts are formed of cast iron and the diagonal tie-rods of wrought iron. This system has been followed more recently in quite a number of trestle-constructions in F,tirope—the Creuse Viaduct at Bousseau d'Ahun, the Cere Viaduct of the Orleans Railway, four viaducts on the Commentry-Gannat line—reaching a height of 196.8 feet—the Iglau 'Viaduct at Eibenschfiss, etc.
In America, where this type of bridge is represented by the most remarkable structures in existence, the use of cast iron has been entirely abandoned for wrought iron, and in the United States, where iron-trestle bridges are comparatively numerous, they are made to do valuable service for railway purposes.
Steel late, steel has come into use, and a number of nota ble bridg-es have been erected of this material. The first extensive experi ment with steel was in the construction of the great arch bridge over the Mississippi at St. Louis, Missouri (fii. 47, fig. 2), built from the plans of the late Captain James B. Eads between the years 1868-1874. This will be referred to under the bead of Arched Briclgt's. A steel-girder bridge over the Missouri River between Bismarck, Missouri, and Mandan, Dakota, built for the Northern Pacific Railroad, was opened in the year IS82. It has three main spans over the river of 400 feet each, and two shore spans of 125 feet. The Lachine Bridge over the St. Lawrence at Montreal, also a notable example of a steel bridge, has a leng-th of 3514 feet, with two main channel spans of 4o8 feet each, and a number of smaller ones.