About 1868 iron bridges began to take the place of wooden bridges. Die-forged eyebars and pin connections allowed of longer panels and longer spans. One of the first long-span bridges was a single-track railway bridge of 400-foot span over the Ohio at Cincinnati, which was considered to be a great achievement in 1870. The Kinzua viaduct, 310 feet high and over half a mile long, belongs to this era. It is the type of the numerous high viaducts now so common. About 1885 a new material was given to engineers, having greater strength and tenacity than iron, and commercially available from its low cost. This is basic steel. After many experiments, the proper proportions of carbon, phosphorus, sulphur, and manganese were ascertained, and uniformity resulted. The open-hearth process is now generally used. This new chemical metal, for such it is, is fifty per cent. stronger than iron, and can be tied in a knot when cold. The effect of improved devices and the use of steel is shown by the weights of the 400-foot Ohio River iron bridge, built in 1870, and a bridge at the same place, built in 1886. The bridge of 1870 was of iron, had panels twelve feet long, and its height was forty-five feet, and span 400 feet. The bridge of 1886 was of steel, had panels thirty feet long, and its height was eighty feet. Its span was 550 feet. The weights of the two were nearly alike. The cantilever design, which is a revival of a very ancient type, came into use. The great Forth Bridge, in Scotland, 1600-foot span, is of this style, as are the 500-foot spans at Poughkeepsie, and now a new one is being designed to cross the St. Lawrence near Quebec, of 1800-foot span. This is probably near the economic limit of cantilever construction, but the suspension bridge can be extended much farther, as it carries no dead weight of compression members. The Niagara Suspension Bridge, of 810-foot span, built by Roebling, in 1852, and the Brooklyn Bridge, of 1600 feet, built by Roebling and his son, twenty years after, marked a wonderful advance in bridge design. Thirty years later, when a new bridge of 1600 feet was wanted to cross another part of the East River at New York, the same lines of construction were followed, and they will be followed in the 2700-foot span, designed to cross the North River some time in the present century. The only radical advance is the use of a better steel than could be had in earlier days. Steel-arched bridges are now scientifically designed. Such are the new Niagara Bridge, of 840-foot span, and the Alexandra Bridge at Paris.