In 1830 as the first American railroad train sped on its way, a new era dawned for the bridge builder as well as for his neighbors. At once sprang up a demand for bridges longer and stronger than those which in the past had served well enough. A score of wagons laden with wheat or potatoes were a good deal lighter than a locomotive followed by a train of loaded freight cars. A market-wagon, too, could easily be taken aboard a ferry-boat, but for an engine and its cars a bridge was imperative, if the stream were not so wide as to forbid all opportunity to the bridge builder. His response to the demands of the railroad was two-fold. First in the use of metal instead of wood, beginning with iron rods to bind together frames of timber. As iron became cheaper and its value more and more evident, he employed it for additional parts of his structure until at last he built the whole bridge of iron. To-day good steel is so cheap that railroad bridges are seldom reared of anything else. Besides using stronger materials, the designer has gradually improved the form of his structure, not only in its parts but as a whole, so that to-day, strength for strength, a bridge may be only one tenth as heavy as a bridge of fifty years ago. Advances in form have been due to experience as one type has been compared with another; meanwhile the mathematicians have carried their analysis of strains as far as the extreme complexity of their problems will allow, greatly to the betterment of designs. In building a bridge, as in rearing many other structures, girders of various contours are used. In bridge building the I-beam is most employed. When the roadway proceeds on the top chord, as DH, in the queen-post figure, page 21, we have a deck bridge; when it is built on the bottom chord, as CM, we have a through bridge. [Illustration: HOWE TRUSS] [Illustration: PRATT TRUSS] The Burr Bridge Simplified by Howe and Pratt.