Probably the most important branch of engineering work is railroad construction, already considered under steam railways. Closely related to the railroad, however, is bridge building, and many of these noble structures hang between heaven and earth, conspicuous monuments of the engineer’s skill. [Illustration: FIG. 228.--THE FORTH BRIDGE. LARGEST VIADUCT IN THE WORLD. FROM A PHOTOGRAPH WHEN IN PROCESS OF CONSTRUCTION. LENGTH, 8,290 FEET; HEIGHT ABOVE WATER, 361 FEET; MAIN SPANS, 1,710 FEET LONG, 150 FEET HIGH.] _The Forth Bridge._--This massive structure, of the cantilever type, is shown in Fig. 228. It was begun in 1882 and finished in 1890, and is the largest and most costly viaduct in the world. It is built across the Firth of Forth, and is the most important link in the direct railway communication of the North British Railway, and associated roads, between Edinburgh on the one side, and Perth and Dundee on the other. The total length of the viaduct is 8,296 feet, or nearly 1⅝ miles. The extreme height of the structure is 361 feet above the water level, and the foundations extend 91 feet below the water level. The two main spans are 1,710 feet, and these both give a clear headway for navigation of 150 feet height. There are over 50,000 tons of steel in the superstructure, and about 140,000 cubic yards of masonry and concrete in the foundation piers. The three main piers consist each of a group of four masonry columns faced with granite, 49 feet in diameter at the top, and 36 feet high, which rest on solid rock, or on concrete carried down in most cases by means of caissons of a maximum diameter of 70 feet to rock or boulder clay. No intelligent conception of the enormous size of this great structure can be obtained except by comparison. Estimating from the bottom of the masonry piers to the towering heights of the cantilevers, it reaches above the dome of St. Peter’s at Rome, and is only a little short of the height of the greatest of the pyramids of Egypt. The cost of the bridge is given as £3,250,000 or nearly $16,000,000. _The Brooklyn Bridge._--Having for its successful construction and maintenance the same foundation principle upon which the spider builds its web, this magnificent bridge of steel wires spans the East River between New York and Brooklyn, with a total length of 5,989 feet, and in length of span and cost is second only to the great Forth Bridge. It is shown in Fig. 229, and among suspension bridges it ranks first. It has a central span of 1,595½ feet between the two towers, over which the suspension cables are hung, and has a clear headway beneath of 135 feet. It has two side spans of 930 feet each between the towers and the shore. [Illustration: FIG. 229.--THE BROOKLYN BRIDGE. LONGEST SUSPENSION BRIDGE IN THE WORLD. TOTAL LENGTH, 5,989 FEET; SPAN BETWEEN TOWERS, 1,595 FEET 6 INCHES.]