A cantilever bridge consists of two inverted trussed beams, each balanced on a pier, one part extending over the river and the other to the shore, where it is firmly anchored in solid, heavy masonry. The ends extending over the river toward each other from the opposite piers are joined by a short truss in such a manner as to permit expansion and contraction consequent on changes of temperature, and yet be proof against vertical or lateral pressure. Such a bridge, it is said, sustains scarcely any strain in the center of the span. Each half of the entire bridge is self-balanced on its pier; and when a long, heavy train is on it, the part of the train on one side of the pier is balanced as on a “teeter” by the part on the other side of the pier--in front or behind. The bridge across the Niagara River was the first of the cantilever kind ever constructed, and the one over the Hudson River was erected upon substantially the same principle, the cantilever being utilized as nearly as possible. In building the bridge it was important to obstruct the Hudson as little as possible, much opposition having been raised against it by those interested in the navigation of the river. Therefore a combination of anchorage trusses and cantilever spans was adopted. The river is crossed in five spans, with four piers in the channel. On each of the two piers nearest the shore, four sets of steel rollers carry the ends of the anchorage trusses and of the cantilevers of the east and west spans. The bridge is made of steel. The cantilever principle is again introduced in the famous Forth Bridge. At a distance of six hundred and eighty feet from the ends of either approach viaduct are the north and south cantilever piers, with their great arms stretching out to and joining with the girder approaches. In the opposite direction the cantilever arms extend for six hundred and eighty feet toward Inchgarvie, and come within three hundred and fifty feet each of meeting the arms of the cantilever built on that island. This cantilever pier is founded in the bottom of the shallow water close to the west of the islet. The gaps of three hundred and fifty feet between the extremities of the cantilever arms and of the ends of their neighbors to the north and south are filled in by connecting or central girders of the hogback lattice pattern. The total length of each of the north and south cantilevers is one thousand five hundred and five feet, while that of the central one, owing to its having a longer foundation base, is one thousand six hundred and twenty feet. The two main spans measure each one thousand seven hundred and ten feet, with a clear headway above high water, for five hundred feet in the center of the span, of one hundred and fifty feet, while the half cantilever spans to the approach viaducts north and south are each of six hundred and eighty feet. The measurement from the extremity of one approach viaduct to the