for use when it is
so far distorted by overstraining that it cannot recover, or,
technically speaking, when its elastic limit has been exceeded. The
elastic limit of the best grades of iron is just about half the
breaking-weight, or from 25,000 to 30,000 pounds per inch. A poor
iron will often show a very fair breaking-strength, but, at the same
time, will have a very low elastic limit, and be entirely unfit for
use in a bridge. A piece of iron of very inferior quality will often
sustain a greater load before breaking than a piece of the best and
toughest material, for the reason that a tough but ductile iron will
stretch before giving way, thus reducing the area of section, while a
hard but poor iron will keep nearly its full size until it breaks. A
tough and ductile iron should bend double, when cold, without
showing any signs of fracture, and should stretch fifteen per cent of
its length before breaking; but much of the iron used in bridges,
although it may hold 40,000 or 50,000 pounds per inch before failing,
will not bend over 90 degrees without cracking, and has an elastic
limit as low as 18,000 pounds. It is thus full as important to
specify that an iron should have a high elastic limit as that it
should have a high breaking-weight. A specification which allowed no
material to be strained by more than 10,000 pounds per inch, and no
iron to be used with a less elastic limit than 25,000 pounds, would,
at the same time, agree with the standard requirement, both in
England and in the United States, and would also secure a good
quality of iron.

Two documents published some time since illustrate the preceding
remarks. The first is the account of the tests of the iron taken from
the Tariffville bridge after its failure, and the second is the
specification for bridges on the Cincinnati Southern Railroad. The
Tariffville bridge, though nominally a wooden one, like most
structures of the kind relied entirely upon iron rods to keep the
wood-work together. Although the rods were too small, and seriously
defective in manufacture, the bridge ought not to have fallen from
that cause. The ultimate strength of the iron was not what it should
have been, but yet it was not low enough to explain the disaster; but
when we look at the _quality_ of the iron, we have the cause of the
fall. The rods taken from the bridge show an ultimate tensile
strength of 47,560 pounds per inch, but an elastic limit of only
19,000 pounds; while the