rry piers each 145 ft.; two
central girders between cantilevers each 350 ft.; and six cantilevers each
680 ft. The two main spans are each 1710 ft. The clear headway is 157 ft.,
and the extreme height of the towers above high water 361 ft. The outer
ends of the shore cantilevers are loaded to balance half the weight of the
central girder, the rolling load, and 200 tons in addition. An internal
viaduct of lattice girders carries a double line of rails. Provision is
made for longitudinal expansion due to change of temperature, for
distortion due to the sun acting on one side of the structure, and for the
wind acting on one side of the bridge. The amount of steel used was 38,000
tons exclusive of approach viaducts. (See _The Forth Bridge_, by W.
Westhofen; _Reports of the British Association_ (1884 and 1885); _Die Forth
Bruecke_, von G. Barkhausen (Berlin, 1889); _The Forth Bridge_, by Philip
Phillips (1890); Vernon Harcourt, _Proc. Inst. C.E._ cxxi. p. 309.)

(2) The Niagara bridge of a total length of 910 ft., for two lines of
railway. Clear span between towers 495 ft. Completed in 1883, and more
recently strengthened (_Proc. Inst. C.E._ cvii. p. 18, and cxliv. p. 331).

[Illustration: FIG. 24.--Lansdowne Bridge.]

(3) The Lansdowne bridge (completed 1889) at Sukkur, over the Indus. The
clear span is 790 ft., and the suspended girder 200 ft. in length. The span
to the centres of the end uprights is 820 ft.; width between centres of
main uprights at bed-plate 100 ft., and between centres of main members at
end of cantilevers 20 ft. The bridge is for a single line of railway of 5
ft. 6 in. gauge. The back guys are the most heavily strained part of the
structure, the stress provided for being 1200 tons. This is due to the half
weight of centre girder, the weight of the cantilever itself, the rolling
load on half the bridge, and the wind pressure. The anchors are built up of
steel plates and angle, bars, and are buried in a large mass of concrete.
The area of each anchor plate, normal to the line of stress, is 32 ft. by
12 ft. The bridge was designed by Sir A. Rendel, the consulting engineer to
the Indian government (_Proc. Inst. C.E._ ciii. p. 123).

(4) The Red Rock cantilever bridge over the Colorado river, with a centre
span of 660 ft.

(5) The Poughkeepsie bridge over the Hudson, built 1886-1887. There are
five river and two shore spans. The girders over the second and fourth
spans are extended as cantilevers over th