ing the stiffening girders at intermediate
points of the span. A curved chain supported the oblique chains and kept
them straight. In 1860 a bridge was erected over the Danube canal at
Vienna, of 264 ft. span which had two parallel chains one above the other
and 4 ft. apart on each side of the bridge. The chains of each pair were
connected by bracing so that they formed a stiff inverted arch resisting
deformation under unequal loading. The bridge carried a railway, but it
proved weak owing to errors of calculation, and it was taken down in 1884.
The principle was sound and has been proposed at various times. About 1850
it was perceived that a bridge stiff enough to carry railway trains could
be constructed by combining supporting chains with stiffening girders
suspended from them. W. J. M. Rankine proved (_Applied Mechanics_, p. 370)
that the necessary strength of a stiffening girder would be only
one-seventh part of that of an independent girder of the same span as the
bridge, suited to carry the same moving load (not including the dead weight
of the girder which is supported by the chain). (See "Suspension Bridge
with Stiffened Roadway," by Sir G. Airy, and the discussion, _Proc. Inst,
C.E._, 1867, xxvi. p. 258; also "Suspension Bridges with Stiffening
Girders," by Max am Ende, _Proc. Inst. C.E._ cxxxvii. p. 306.)

[Illustration: FIG. 11.--Niagara Suspension Bridge.]

The most remarkable bridge constructed on this system was the Niagara
bridge built by J. A. Roebling in 1852-1855 (fig. 11). The span was 821
ft., much the largest of any railway bridge at that time, and the height
above the river 245 ft. There were four suspension cables, each 10 in. in
diameter; each was composed of seven strands, containing 520 parallel
wires, or 3640 wires in each cable. Each cable was carried on a separate
saddle on rollers on each pier. The stiffening girder, constructed chiefly
of timber, was a box-shaped braced girder 18 ft. deep and 25 ft. wide,
carrying the railway on top and a roadway within. After various repairs and
strengthenings, including the replacement of the timber girder by an iron
one in 1880, this bridge in 1896-1897 was taken down and a steel arch built
in its place. It was not strong enough to deal with the increasing weight
of railway traffic. In 1836 I. K. Brunei constructed the towers and
abutments for a suspension bridge of 702 ft. span at Clifton over the Avon,
but the project was not then carried further; in