. Le chemin de fer américain, sa construction, son développement, sa gestion et ses appareils . Aduct à Portage, N. Y. cette construction a été de plus de 800 pieds de long, et de 234 pieds de haut du lit de la rivière au rail. Les fondations de maçonnerie étaient de 30 pieds de haut, les tréteaux de 190 pieds, et la structure de 14 pieds; il contenait plus que de l'amillion et un demi-pied, mesure de la planche, de bois. Les jetées de bois, qui étaient à 50 pieds l'un de l'autre, sont formées par trois tréteaux, regroupés à la géthère. Il a été encadré de sorte que les pièces défectueuses puissent être prises et remplacées à tout moment. Ce pont a été terminé en 1852 et était complètement le plus âgé

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. The American railway; its construction, development, management, and appliances . aduct at Portage, N. Y. This con-struction was over 800 feet long, and 234 feet high from the bed ofthe river to the rail. The masonry foundations were 30 feet high, the trestles 190 feet, and the truss 14 feet; it contained more than amillion and a half feet, board measure, of timber. The timber piers, which were 50 feet apart, are formed by three trestles, grouped to-gether. It was framed so that defective pieces could be taken outand replaced at any time. This bridge was finished in 1852 andwas completely destroyed by fire in 1875. The new metal struct-ure which took its place is shown on the opposite page, and is an THE AMERICAN METAL VIADUCT 79 interesting example of the American method of metal viaduct con-struction, an essential feature of that construction being the concen-tration of the material into the least possible number of parts. Thisbridge has ten spans of 50 feet, two of 100 feet, and one of 118 feet.The trusses are of what is called the Pratt pattern, and are supported. The New Portage Viaduct. by wrought-iron columns, two pairs of columns forming a skeletontower 20 feet wide and 50 feet long on the top. There are six ofthese towers, one of which has a total height from the masonry tothe rail of 203 feet 8 inches. There are over 1, 300, 000 pounds ofiron in this structure. The fundamental idea of a bridge is a simple beam of wood.If metal is substituted it is still a beam with all superfluous partscut away. This results in what is called an I beam. Whengreater loads have to be carried, the I beam is enlarged andbuilt up of metal plates riveted together and thus becomes a plategirder. These are used for all short railway spans. For greaterspans the truss must be employed. Before referring, however, to examples of truss bridges, a de-scription should be given of the Britannia Bridge, built by Robert 8o FEATS OF RAILWAY ENGINEERING.

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