Engineering Wonders of the World
Volume I

THE BRIDGES OF NEW YORK CITY. 265 the towers one by one, looped round the anchorage pin at on© end, drawn up to exactly the right curve, and secured similarly at the other end, to ensure that each, wire of the 31,000 shall bear its exact proportion of the total strain when the load of the bridge is applied. Clearly this is a tedious process, but bridge engineers maintain it to be the only correct one. The reason for such painstaking methods is just this : that the whole idea of the wire-cable type of bridge is based on the high efficiency of drawn wire as a medium to resist pull, and, of course, this efficiency is in great part wasted unless all wires share the load equally. Steel wire, as used in these cables, is about four times as strong, and may be loaded about four times as heavily, as a piece of ordinary bridge steel of equal section, and it is this high efficiency that makes it possible to build such long, graceful suspension bridges. But if the wires were laid together on the ground, then hoisted up and bent over the saddles, many of the wires at the curves would spring out, showing that they carry no load at all ; and there would be no way of telling how many wires were actually carrying load, or of calculating the strength of the bridge. After all, this cable-spinning is not so tedious an undertaking as it may seem. Systematized Fig. 10.—BROOKLYN ANCHORAGE, MANHATTAN BRIDGE, PARTLY BUILT. Fig. 11.—LAYING THE ROADWAY OF THE WILLIAMSBURGH BRIDGE. methods do wonders. It is true that two years were required for forming the cables of the Brooklyn Bridge, and one year in the case of the Williamsburgh Bridge; but in the latest, the Manhattan Bridge, the work was com- pleted in the marvellously short space of four months. Because the spinning operation was done so much more perfectly on the Manhattan Bridge, we will consider it in detail when we come to that structure, bearing in mind that the description will in substance apply also to the two earlier bridges. When the cables of the Williamsburgh Bridge were in place, heavy cast-steel bands were clamped around them at the proper intervals —about 20 feet—and over these were hung the wire-rope sus- Wrapping- penders which hold up the floor-beams of the roadway. Between the bands, the cable was wrapped with cotton duck strip soaked in an oil varnish composition,