[AS.] A roadway over a stream, valley, or low ground. Viaduct is applied to bridges over which a road or railway passes; and aqueduct is applied to those for carrying a canal or water. They are made of wood, stone, iron, or steel. Bridges are built in various ways. In shallow water they are supported from the bottom, or stone piers are built, and arches thrown from pier to pier. Suspension bridges are held up by strong strands of wire stretched from shore to shore. A common way of building bridges now is by truss or girder work, the bridge being sustained by iron girders firmly bolted together. There are many remarkable bridges in existence, some of them of great length and width and able to support immense weights.