ar iron of commerce. From the iron reduced into this form the
various parts used in the construction of iron bridges are made by being
rolled into shape, the rolls through which the various parts pass having
grooves of the form it is desired to give to the pieces.

[Illustration: HOT SAW.]

[Illustration: RIVETING A COLUMN.]

These rolls, when they are driven by steam, obtain this generally from a
boiler placed over the heating-or puddling-furnace, and heated by the
waste gases from the furnace. This arrangement was first made by John
Griffin, the superintendent of the Phoenix Iron-works, under whose
direction the first rolled iron beams over nine inches thick that were
ever made were produced at these works. The process of rolling toughens
the iron, seeming to draw out its fibres; and iron that has been twice
rolled is considered fit for ordinary uses. For the various parts of a
bridge, however, where great toughness and tensile strength are
necessary, as well as uniformity of texture, the iron is rolled a third
time. The bars are therefore cut again into pieces, piled, re-heated and
rolled again. A bar of iron which has been rolled twice is formed from
a pile of fourteen separate pieces of iron that have been rolled only
once, or "muck bar," as it is called; while the thrice-rolled bar is
made from a pile of eight separate pieces of double-rolled iron. If,
therefore, one of the original pieces of iron has any flaw or defect, it
will form only a hundred and twelfth part of the thrice-rolled bar. The
uniformity of texture and the toughness of the bars which have been
thrice rolled are so great that they may be twisted, cold, into a knot
without showing any signs of fracture. The bars of iron, whether hot or
cold, are sawn to the various required lengths by the hot or cold saws
shown in the illustrations, which revolve with great rapidity.

[Illustration: FURNACE AND HYDRAULIC DIE.]

For the columns intended to sustain the compressive thrust of heavy
weights a form is used in this establishment of their own design, and to
which the name of the "Phoenix column" has been given. They are tubes
made from four or from eight sections rolled in the usual way and
riveted together at their flanges. When necessary, such columns are
joined together by cast-iron joint-blocks, with circular tenons which
fit into the hollows of each tube.

To join two bars to resist a strain of tension, links or eye-bars are
used from three to s