The construction of hinged arches of reinforced concrete is very rare, but is not unknown, and will probably come into greater use when their advantages are more fully realized. "We may consider that structurally they consist of curved ribs which have hinges at each abutment, and which may or may not have a hinge at the center of the arch. The advantage of the three-hinged arch lies in the fact that it is not subject to temperature stresses. The two-hinged arch is partially subject to temperature stresses, but not to so great an extent as the fixed arch, since the arch rib is not held rigid at the abutments as in the case of the fixed arch. Prac-tically the hinges are made by having at each hinge a pair of large cast-iron plates which are a little larger than the size of the rib, and which have at their centers a bearing for a pin of due proportionate size. The bearings are so made that one may turn, with respect to the other, about the axis of the pin through an angle of a very few degrees.