frames or boxes filled with concrete which hardens and forms a solid basis, these are called “cassons.” Wooden bridges are now seldom made of any great size, but are very numerous in country places for crossing narrow streams, on account of their cheapness and the facility with which they can be constructed. Bridges of brick are not much used, but “viaducts” of this material often occur in the lines of railways, and very handsome arches of brick-work are used to span streets in towns where railways pass. Iron girder bridges also, are frequently employed for this purpose; they are made with girders of wrought or cast iron which generally pass in a straight line over the roadway from pier to pier. Some of them are made of iron-plate rivetted together, while others are strong girders of cast iron having small arches of brick-work built on them, running the length of the girders and resting upon ridges in them, so as to fill up the space between each girder. Illustration: THE BRITANNIA TUBULAR BRIDGE. Iron bridges are of almost every form and construction conceivable; some are tubes of iron-plate, as the Britannia Bridge, rivetted together; some of solid cast iron work as Southwark Bridge, others called suspension bridges are hung by rods of wrought iron, from large chains suspended from pier to pier, as Hungerford Bridge. In many places bridges of boats are used; the boats are moored to the river’s-bed and have timber-work connecting them. Another kind, which can hardly lay claim to the name of bridge, called the “floating-bridge,” is used for the conveyance of carts, horses, passengers, or merchandise, from one side of a river to the other. It consists of a sort of platform guided by chains laid down for the purpose, which pass over wheels or drums on the side of the bridge. The motive power necessary for turning these wheels, is supplied by a steam-engine within the structure itself. TUNNELS. Illustration: (‡ RAILROAD TUNNEL.)