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
were therefore placed where they were seen by great numbers. Now,
however, the connection between the chief commercial centres is made by
the railroads, and these penetrate immense distances, through
comparatively unsettled districts, in order to bring about the needed
distribution; and in consequence many of the great railroad bridges are
built in the most unfrequented spots, and are unseen by the numerous
passengers who traverse them, unconscious that they are thus easily
passing over specimens of engineering skill which surpass, as objects of
intelligent interest, many of the sights they may be traveling to see.

[Illustration: ELEVATOR.]

The various processes by which the iron is prepared to be used in
bridge-building are many of them as new as is the use of this material
for this purpose, and it will not be amiss to spend a few moments in
examining them before presenting to our readers illustrations of some of
the most remarkable structures of this kind. Taking a train by the
Reading Railroad from Philadelphia, we arrive, in about an hour, at
Phoenixville, in the Schuylkill Valley, where the Phoenix Iron-and
Bridge-works are situated. In this est