SUSPENSION BRIDGES. In these bridges the roadway is suspended from chains passing over piers or towers, and firmly fixed at their extremities. When the roadway is equally loaded over its length, the curve of the chain is a parabola. The weight of the roadway being known, the strain upon the chain, and its requisite strength, are readily determined. For example, in fig. 1, if A be the center of the bridge, and it be required to find the strain upon the chain at the point B, it is evident that the weight of the roadway between A and B is supported by the chain at B; we have then to find what strain in the direc tion of the length of the chain will support this vertical load. By the principles of mechanics, if we draw a right-angled triangle BCD, of which the side BC is a tangent to the curve at B, CD is vertical, and RD horizontal; and if the length of CD represent numerically the load on AB, then BC will represent numerically tho 'tin on the chain produced by that load, and BD will be what is called the horizon1/..kimponent of this strain. This horizontal part of the strain is the same for every pa ,L of the curve; it is the total strain on the chain at the center A, and the strain carried over the towers and balanced by the backstays, which are firmly anchored to the ground behind them. In this manner 1116 conditions of strength and stability of a bridge uniformly loaded are easily determined, but when we have a rolling load which is heavy in proportion to the weight of the bridge, as for example a railway train, the case is very different, for when the train only occupies one half of the bridge, the chain will be depressed toward that side, and raised at the center; thus an undulation will be produced in the bridge, which, if the train be moving rapidly, would endanger its stability. Various combinations
have been devised to overcome this difficulty. The most simple, and practically the best, is to stiffen the roadway so that the strain of the passing load is distributed over a considerable length of the chain. In this manner large railway bridges have been con structed in America; among them is that over the Niagara above the falls, with three lines of rails on it, of which the span is 822 ft., and the height of the platform above the river, 250 feet; it is supported by four wire cables, each containing 3,640 wires. Trains pass over it at the rate of 10 m. per hour. An ordinary suspension bridge is liable to both vertical and horizontal oscillations, the former taking place when a train or other load is passing over it, and the latter being due to the action of the wind. These oscil lations cannot be altogether prevented, but can be se reduced as to be harmless by the use of stays, stretching both from the towers and from points on shore to various parts of the bridge. Suspension bridges are generally used in positions where the span is great, and the rolling loads neither great in proportion to the weight of the bridge itself, nor very rapid in their motion. Many beautiful examples are to be seen in this country; among others, we may instance the ?ulenai bridge, 580 ft. span, and the Clifton bridge, near Bristol, 703 ft. span