d by an arch of 541 ft. span, and 213 ft. rise. The bridge
is similar to that at Oporto, also designed by Seyrig. It is formed by a
crescent-shaped arch, continued on one side by four, on the other side by
two lattice girder spans, on iron piers. The arch is formed by two lattice
ribs hinged at the abutments. Its depth at the crown is 33 ft., and its
centre line follows nearly the parabolic line of pressures. The two arch
ribs are 651/2 ft. apart at the springings and 201/2 ft. at the crown. The
roadway girders are lattice, 17 ft. deep, supported from the arch ribs at
four points. The total length of the viaduct is 1715 ft. The lattice
girders of the side spans were first rolled into place, so as to project
some distance beyond the piers, and then the arch ribs were built out,
being partly supported by wire-rope cables from the lattice girders above.
The total weight of ironwork was 3200 tons and the cost L124,000 (_Annales
des travaux publiques_, 1884).

The Victoria Falls bridge over the Zambezi, designed by Sir Douglas Fox,
and completed in 1905, is a combination of girder and arch having a total
length of 650 ft. The centre arch is 500 ft. span, the rise of the crown 90
ft., and depth at crown 15 ft. The width between centres of ribs of main
arch is 271/2 ft. at crown and 53 ft. 9 in at springings. The curve of the
main arch is a parabola. The bridge has a roadway of 30 ft. for two lines
of rails. Each half arch was supported by cables till joined at the centre.
An electric cableway of 900 ft. span capable of carrying 10 tons was used
in erection.

12. (g) _Movable Bridges_ can be closed to carry a road or railway or in
some cases an aqueduct, but can be opened to give free passage to
navigation. They are of several types:--

[Illustration: FIG. 33.]

(1) _Lifting Bridges._--The bridge with its platform is suspended from
girders above by chains and counterweights at the four corners (fig. 33 a).
It is lifted vertically to the required height when opened. Bridges of this
type are not very numerous or important.

(2) _Rolling Bridges._--The girders are longer than the span and the part
overhanging the abutment is counter-weighted so that the centre of gravity
is over the abutment when the bridge is rolled forward (fig. 33 b). To fill
the gap in the approaches when the bridge is rolled forward a frame
carrying that part of the road is moved into place sideways. At Sunderland,
the bridge is first lifted by a hydraul