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.

[Illustration: DUMPING ORE AND COAL INTO BLAST-FURNACES.]

In a wooden or stone bridge a beam affected by dry rot or a stone
weakened by the effects of frost may lie hidden from the inspection of
even the most vigilant observer until, when the process has gone far
enough, the bridge suddenly gives way under a not unusual strain, and
death and disaster shock the community into a sense of the inherent
defects of these materials for such structures.

The introduction of the railroad has brought about also another change
in the bridge-building of modern times, compared with that of all the
ages which have preceded this nineteenth century. The chief bridges of
ancient times were built as great public conveniences upon thoroughways
over which there was a large amount of travel, and consequently were
near the cities or commercial centres which attracted such travel, and