[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 establishment we can follow the iron from its original condition of ore to a finished bridge, and it is the only establishment in this country, and most probably in the world, where this can be seen.

[Illustration:  The engine-room.]

These works were established in 1790.  In 1827 they came into the possession of the late David Reeves, who by his energy and enterprise increased their capacity to meet the growing demands of the time, until they reached their present extent, employing constantly over fifteen hundred hands.

[Illustration:  Running metal into pigs.]