nd
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