plished have been made possible.

2. In a bridge there may be distinguished the _superstructure_ and the
_substructure_. In the former the main supporting member or members may be
an arch ring or arched ribs, suspension chains or ropes, or a pair of
girders, beams or trusses. The bridge flooring rests on the supporting
members, and is of very various types according to the purpose of the
bridge. There is also in large bridges wind-bracing to stiffen the
structure against horizontal forces. The _substructure_ consists of (a) the
piers and end piers or abutments, the former sustaining a vertical load,
and the latter having to resist, in addition, the oblique thrust of an
arch, the pull of a suspension chain, or the thrust of an embankment; and
(b) the foundations below the ground level, which are often difficult and
costly parts of the structure, because the position of a bridge may be
fixed by considerations which preclude the selection of a site naturally
adapted for carrying a heavy structure.

3. _Types of Bridges_.--Bridges may be classed as _arched bridges_, in
which the principal members are in compression; _suspension bridges_, in
which the principal members are in tension; and _girder bridges_, in which
half the components of the principal members are in compression and half in
tension. But there are cases of bridges of mixed type. The choice of the
type to be adopted depends on many and complex considerations:--(1) The
cost, having regard to the materials available. For moderate spans brick,
masonry or concrete can be used without excessive cost, but for longer
spans steel is more economical, and for very long spans its use is
imperative. (2) The importance of securing permanence and small cost of
maintenance and repairs has to be considered. Masonry and concrete are more
durable than metal, and metal than timber. (3) Aesthetic considerations
sometimes have great weight, especially in towns. Masonry bridges are
preferable in appearance to any others, and metal arch bridges are less
objectionable than most forms of girder.

Most commonly the engineer has to attach great importance to the question
of cost, and to design his structure to secure the greatest economy
consistent with the provision of adequate strength. So long as bridge
building was an empirical art, great waste of material was unavoidable. The
development of the theory of structures has been largely directed to
determining the arrangements of materi