[Illustration:  The Lyman Viaduct.]

Doubtless in its day this structure was considered an engineering feat worthy of such posthumous immortality as is gained by an epitaph, and afforded such convenience for transportation as was needed by the commercial activity of that era.  From that time, however, to this, the changes which have occurred in our commercial and industrial methods are so fully indicated by the changes of our manner and method of bridge-building that it will not be a loss of time to investigate the present condition of our abilities in this most useful branch of engineering skill.

In the usual archaeological classification of eras the Stone Age precedes that of Iron, and in the history of bridge-building the same sequence has been preserved.  Though the knowledge of working iron was acquired by many nations at a pre-historic period, yet in quite modern times—­within this century, even—­the invention of new processes and the experience gained of new methods have so completely revolutionized this branch of industry, and given us such a mastery over this material, enabling us to apply it to such new uses, that for the future the real Age of Iron will date from the present century.

The knowledge of the arch as a method of construction with stone or brick—­both of them materials aptly fitted for resistance under pressure, but of comparatively no tensile strength—­enabled the Romans to surpass all nations that had preceded them in the course of history in building bridges.  The bridge across the Danube, erected by Apollodorus, the architect of Trajan’s Column, was the largest bridge built by the Romans.  It was more than three hundred feet in height, composed of twenty-one arches resting upon twenty piers, and was about eight hundred feet in length.  It was after a few years destroyed by the emperor Adrian, lest it should afford a means of passage to the barbarians, and its ruins are still to be seen in Lower Hungary.

With the advent of railroads bridge-building became even a greater necessity than it had ever been before, and the use of iron has enabled engineers to grapple with and overcome difficulties which only fifty years ago would have been considered hopelessly insurmountable.  In this modern use of iron advantage is taken of its great tensile strength, and many iron bridges, over which enormous trains of heavily-loaded cars pass hourly, look as though they were spun from gossamer threads, and yet are stronger than any structure of wood or stone would be.

[Illustration:  Blast-furnaces.]

Another great advantage of an iron bridge over one constructed of wood or stone is the greater ease with which it can, in every part of it, be constantly observed, and every failing part replaced.  Whatever material may be used, every edifice is always subject to the slow disintegrating influence of time and the elements.  In every such edifice as a bridge, use is a process of constant weakening, which, if not as constantly guarded against, must inevitably, in time, lead to its destruction.