Terence; _Hymns from the
Yattendon Hymnal_ (Oxford, 1899); and _Demeter, a Mask_ (Oxford, 1905).

[v.04 p.0533] BRIDGES. 1. _Definitions and General
Considerations._--Bridges (old forms, _brig_, _brygge_, _brudge_; Dutch,
_brug_; German, _Bruecke_; a common Teutonic word) are structures carrying
roadways, waterways or railways across streams, valleys or other roads or
railways, leaving a passage way below. Long bridges of several spans are
often termed "viaducts," and bridges carrying canals are termed
"aqueducts," though this term is sometimes used for waterways which have no
bridge structure. A "culvert" is a bridge of small span giving passage to
drainage. In railway work an "overbridge" is a bridge over the railway, and
an "underbridge" is a bridge carrying the railway. In all countries there
are legal regulations fixing the minimum span and height of such bridges
and the width of roadway to be provided. Ordinarily bridges are fixed
bridges, but there are also movable bridges with machinery for opening a
clear and unobstructed passage way for navigation. Most commonly these are
"swing" or "turning" bridges. "Floating" bridges are roadways carried on
pontoons moored in a stream.

In classical and medieval times bridges were constructed of timber or
masonry, and later of brick or concrete. Then late in the 18th century
wrought iron began to be used, at first in combination with timber or cast
iron. Cast iron was about the same time used for arches, and some of the
early railway bridges were built with cast iron girders. Cast iron is now
only used for arched bridges of moderate span. Wrought iron was used on a
large scale in the suspension road bridges of the early part of the 19th
century. The great girder bridges over the Menai Strait and at Saltash near
Plymouth, erected in the middle of the 19th century, were entirely of
wrought iron, and subsequently wrought iron girder bridges were extensively
used on railways. Since the introduction of mild steel of greater tenacity
and toughness than wrought iron (_i.e._ from 1880 onwards) it has wholly
superseded the latter except for girders of less than 100 ft. span. The
latest change in the material of bridges has been the introduction of
ferro-concrete, armoured concrete, or concrete strengthened with steel bars
for arched bridges. The present article relates chiefly to metallic
bridges. It is only since metal has been used that the great spans of 500
to 1800 ft. now accom