Now, in its simplest form, a suspension bridge has been known for ages. It is merely a pathway, or even a small movable car, suspended from a rope or ropes across a chasm. Ulloa describes suspension bridges built by the Peruvians in South America. Four stout cables span a river, and on these four is placed the platform of sticks and branches, while two other ropes connected with the platform are useful as hand rails. Such bridges sway with the wind and move with the passenger, but for light loads they appear to be perfectly safe.
In Telford’s Menai Bridge the carriage-way is hung from four huge chains or cables, each chain made up of four others, and passing over high piers. The chains are anchored on the landward side, sixty feet in pits, and grafted by iron frames to the rocks. The chains are so complex and so strong, that parts may be removed for repair without imperilling the safety of the structure. The length of the span thus gained is 560 feet, and it is 150 feet above high-water. The remainder of the bridge is composed of arches of stone, of 52½ feet span.
The piers from which the great span is suspended rise above the carriage-way fifty-two feet, and are topped by blocks of cast-iron, which can move on rollers to permit the chains passing over them to expand and contract freely with the temperature. There are two carriage-roads, and also a footpath. The roads are separated by iron lattice work, which also gives them stability and decreases vibration.
THE CLIFTON BRIDGE.
In its day, this stupendous bridge was as great a wonder as its later companion over the same Straits—the Britannia Tubular. Six years were occupied in building, and it was opened in 1825. Why, then, did not Stephenson construct a similar bridge when, twenty years or so later, he had to solve a similar problem?
The answer is, that suspension bridges are not—or were not—considered sufficiently strong and rigid for railway work. In America, however, they have been used for this purpose; witness the famous Niagara Suspension Bridge, 2⅓ miles below the Falls, and with a superb span of 822 feet; but American engineers appear to stiffen the roadway considerably, so as to distribute the stress of the rushing train over a large portion of the cable. The Niagara Bridge is not supported by plate-link chains, but by four immense wire cables, stretching from cliff to cliff over the roaring rapids. Four thousand distinct wires make up each cable, which pass over lofty piers, and from them hangs the railway by numerous rods.
THE BROOKLYN BRIDGE.
Probably the famous Brooklyn Bridge is the largest suspension bridge in the world, even as the Clifton Suspension Bridge, in England, is one of the most interesting. The Brooklyn Bridge has a magnificent central span of 1595½ feet over the East River between Brooklyn and New York; further, there are two land spans of 930 feet, which, together with the approaches, make up a total of about a mile and a furlong. The cables, four in number, are each composed of 5000 steel wires, and measure 15¾ inches in diameter. They are anchored to solid stone structures at either end, measuring 119 feet by 132 feet, and weighing 60,000 tons; while the towers from which the main span is suspended rise to the height of 276 feet, and are embedded in the ground 80 feet below high-water. It has been estimated that the weight hung between these towers is nearly 7000 tons.