For bridge-building the value of this metal has lately been much disputed, though we have several notable examples of its use in the earlier days for such structures. In fact, the use of cast iron for structural purposes is not older than the time of Smeaton, who in 1755 employed it for mill construction, and about the same time the great Coalbrookdale Viaduct was erected across the Severn near Broseley, which gave an impetus to the use of cast iron for bridge construction. The viaduct had a span of 100 feet, and was composed of ribs cast in two pieces; it was erected from castings designed by Mr. Pritchard, of Shrewsbury, an architect, and this circumstance is worthy of note as showing that an architect really was the first to employ this material for important structural work, and that the same profession was the first to reject it upon traditional grounds. It is quite certain, however, the bridge-builder lost no time in trying his hand upon so tractable a material; for not long after Telford erected a bridge at Buildwas of even a greater span, and the famous cast-iron bridge over the river Wear at Sunderland was erected from the designs of Thomas Paine, the author of the "Age of Reason." Iron bridges quickly followed upon these early experiments, for we hear of several being built on the arched system, and large cotton-mills being erected upon fireproof principles at the commencement of the present century, the iron girders and columns of one mill being designed by Boulton and Watt. A little later, Eaton Hodgkinson proved by experiments the uncertainty of cast iron with regard to tensile strength, which he showed to be much less than had been stated by Tredgold. Cast iron was afterwards largely adopted by engineers. The experiments of Hodgkinson supplied a safe foundation of facts to work upon, and cast iron has ever since retained its hold. Thomas Paine's celebrated bridge at Sunderland had a span of 236 feet and a rise of 34 feet, and was constructed of six ribs, and is remarkable from the fact that the arched girder principle used in the Coalbrookdale and Buildwas bridges was rejected, that the ribs were composed of segments or voussoirs, each made up of 125 parts, thus treating the material in the manner of stone. Each voussoir was a cast-iron framed piece two feet long and five feet in depth, and these were bolted together. The Southwark bridge over the Thames, by Sir John Rennie, followed, in which a similar principle of construction is adopted. There is much to be said in favor of a system which puts each rib under compression in the manner of a stone arch, and which builds up a rib from a number of small pieces. At least, it is a system based on the legitimate use of cast iron for constructive purposes. The large segmental castings used in the Pimlico bridge, and the new bridge over the Trent at Nottingham, from Mr. M. O. Tarbotton's design, are excellent examples of the arched girder system. The Nottingham bridge