e will take place when the load is applied. The speaker does
not claim that arches are maintained by the cement or mortar joining the
voussoirs together, but that the lines of pressure will be materially
changed, and the same calculations are not applicable to both the
unloaded and the loaded arch.

It is quite true, as the author states, that a few cubic yards of
concrete placed in the ring will strengthen the arch more than a like
amount added to the abutments, provided, however, that this material be
placed properly. No good can result from an attempt to strengthen a
structure by placing the reinforcing material promiscuously. This has
been tried by amateurs in bridge construction, and, in such cases, the
material either increased the distance from the neutral axis to the
extreme fibers, thereby reducing the original section modulus, or caused
a shifting of the neutral axis followed by a large bending moment;
either method weakening the members it had tried to reinforce. In other
words, the mere addition of material does not always strengthen a
structure, unless it is placed in the proper position, and, if so
placed, it should be placed all over commensurately with the stresses,
that is, the unit stresses should be reduced.

The author has criticized reinforced concrete construction on the ground
that the formulas and theories concerning it are not as yet fully
developed. This is quite true, for the simple reason that there are so
many uncertain elements which form their basis: First, the variable
quantity of the modulus of elasticity, which, in the concrete, varies
inversely as the stress; and, second, the fact that the neutral axis in
a reinforced concrete beam under changing stress is migratory. There are
also many other elements of evaluation, which, though of importance, are
uncertain.

Because the formulas are established on certain assumptions is no reason
for condemning them. There are, the speaker might add, few formulas in
the subject of theoretical mechanics which are not based on some
assumption, and as long as the variations are such that their range is
known, perfectly reliable formulas can be deduced and perfectly safe
structures can be built from them.

There are a great many theorists who have recently complained about the
design of reinforced concrete. It seems to the speaker that such
complaints can serve no useful purpose. Reinforced concrete structures
are being built in steadily increasing n