rolls" are heavy cylinders of cast iron placed almost in contact, and
revolving rapidly by steam-power. The bloom is caught between these
rollers, and passed backward and forward until it is pressed into a flat
bar, averaging from four to six inches in width, and about an inch and a
half thick. These bars are then cut into short lengths, piled, heated
again in a furnace, and re-rolled. After going through this process they
form the bar 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,