iron arches of cast voussoirs, somewhat like the
voussoirs of masonry bridges. Such a bridge was the Wearmouth bridge,
designed by Rowland Burdon and erected in 1793-1796, with a span of 235 ft.
Southwark bridge over the Thames, designed by John Rennie with cast iron
ribs and erected in 1814-1819, has a centre span of 240 ft. and a rise of
24 ft. In Paris the Austerlitz (1800-1806) and Carrousel (1834-1836)
bridges had cast iron arches. In 1858 an aqueduct bridge was erected at
Washington by M.C. Meigs (1816-1892). This had two arched ribs formed by
the cast iron pipes through which the water passed. The pipes were 4 ft. in
diameter inside, 11/2 in. thick, and were lined with staves of pine 3 in.
thick to prevent freezing. The span was 200 ft.

[Illustration: FIG. 28.--Arch of Bridge at Coblenz]

Fig. 28 shows one of the wrought iron arches of a bridge over the Rhine at
Coblenz. The bridge consists of three spans of about 315 ft. each.

[Illustration: FIG. 29.--St Louis Bridge.]

Of large-span bridges with steel arches, one of the most important is the
St Louis bridge over the Mississippi, completed in 1874 (fig. 29). The
river at St Louis is confined to a single channel, 1600 ft. wide, and in a
freshet in 1870 the scour reached a depth of 51 ft. Captain J.B. Eads, the
engineer, determined to establish the piers and abutments on rock at a
depth for the east pier and east abutment of 136 ft. below high water. This
was effected by caissons with air chambers and air locks, a feat
unprecedented in the annals of engineering. The bridge has three spans,
each formed of arches of cast steel. The centre span is 520 ft. and the
side spans 502 ft. in the clear. The rise of the centre arch is 471/2 ft.,
and that of the side arches 46 ft. Each span has four steel double ribs of
steel tubes butted and clasped by wrought iron couplings. The vertical
bracing between the upper and lower members of each rib, which are 12 ft.
apart, centre to centre, consolidates them into a single arch. The arches
carry a double railway track and above this a roadway 54 ft. wide.

The St Louis bridge is not hinged, but later bridges have been constructed
with hinges at the springings and sometimes with hinges at the crown also.

The Alexander III. bridge over the Seine has fifteen steel ribs hinged at
crown and springings with a span of 353 ft. between centres of hinges and
358 ft. between abutments. The rise from side to centre hinges is 20 ft. 7
i