both European and American engineers. A
bar of the best wrought-iron an inch square will not break under a
tensile strain of less than sixty thousand pounds. Only a small part
of this, however, is to be used in practice. A bar or beam may be
loaded with a greater weight applied as a permanent or dead-load than
would be safe as a rolling or moving weight. A load may be brought
upon any material in an easy and gradual manner, so as not to damage
it; while the same load could not be suddenly and violently applied
without injury. The margin for safety should be greater with a
material liable to contain hidden defects, than with one which is not
so; and it should be greater with any member of a bridge which is
subjected to several different kinds of strain, than for one which
has to resist only a single form of strain. Respect, also, should be
had to the frequency with which any part is subjected to strain from
a moving load, as this will influence its power of endurance. The
rule in structures having so important an office to perform as
railroad or highway bridges, should be, in all cases, absolute safety
under all conditions.

The British Board of Trade fixes the greatest strain that shall come
upon the material in a wrought-iron bridge, from the combined weight
of the bridge and load, at 5 tons per square inch of the net section
of the metal. The French practice allows 3-8/10 tons per square inch
of the cross section of the metal, which, considering the amount
taken out by rivet-holes, is substantially the same as the English
allowance. The report of the American Society of Civil Engineers,
above referred to, recommends 10,000 pounds per inch as the maximum
for wrought-iron in tension in railroad bridges. For highway bridges
a unit strain of 15,000 pounds per square inch is often allowed. A
very common clause in a specification is that the _factor of safety_
shall be four, five, or six, as the case may be, meaning by this that
the actual load shall not exceed one-fourth, one-fifth, or one-sixth
part of the breaking-load. It is a little unfortunate that this term,
factor of safety, has found its way into use just as it has; for it
by no means indicates what is intended, or what it is supposed to.
The true margin for safety is not the difference between the
working-strain and the breaking-strain, but between the
working-strain and that strain which will in any way unfit the
material for use. Now, any material is unfitted