ed upon wrought-iron when under a tensile strain. Similar loads
have been determined for other materials, and for other kinds of
strain.

The preceding remarks in regard to the loads for which bridges should
be designed, and the safe weight to be put upon the material, are
given to show how far the safety of a bridge is a matter of fact, and
how far a matter of opinion. It will be seen that the limits within
which we are at liberty to vary, are quite narrow, so that
bridge-building may correctly be called a science; and there is no
excuse for the person who guesses, either at the load which a bridge
should be designed to bear, or at the size of the different members
of the structure. Still less can we excuse the man who not only
guesses, but who, in order to build cheaply, persistently guesses on
the wrong side.

It will, of course, be understood, when it is said that
bridge-building may be called a science, that it can only be so when
in the hands of an engineer whose judgment has been matured by wide
experience, and who understands that no mechanical philosophy can be
applied to practice which is not subject to the contingencies of
workmanship. There are many bridges which will stand the test of
figures very well, and which are nevertheless very poor structures.
The general plan of a bridge may be good, the computations all right,
and yet it may break down under the first train that passes over it.
There are many practical considerations that cannot be, at any rate
have not yet been, reduced to figures. It is not enough that the
strains upon each member of a bridge should be correctly estimated,
and fall within the safe limits: the different members of the bridge
must be so connected, and the mechanical details such, as to insure,
under all conditions, the assumed action of the several parts. In
fine, while we can say that a bridge that does not stand the test of
arithmetic is a bad bridge, we cannot always say that a structure
which does stand such a test is a good one.

We often hear it argued that a bridge must be safe, since it has been
submitted to a heavy load, and did not break down. Such a test means
absolutely nothing. It does not even show that the bridge will bear
the same load again, much less does it show that it has the proper
margin for safety. It simply shows that it did not break down at that
time. Every rotten, worn-out, and defective bridge that ever fell has
been submitted to exactly that test.