s, two footways
and two [v.04 p.0538] bicycle paths. There are four cables, one on each
side of the two main trusses or stiffening girders. These girders are
supported by the cables over the centre span but not in the side spans.
Intermediate piers support the trusses in the side spans. The cables are
183/4 in. in diameter; each weighs about 1116 tons, and has a nominal
breaking strength of 22,320 tons, the actual breaking strength being
probably greater. The saddles are 332 ft. above the water. The four cables
support a dead load of 7140 tons and a live load of 4017 tons. Each cable
is composed of 37 strands of 208 wires, or 7696 parallel steel wires, No. 8
B.W.G., or about 3/16 in. in diameter. The wire was required to have a
tensile strength of 89 tons per sq. in., and 21/2% elongation in 5 ft. and 5%
in 8 in. Cast steel clamps hold the cable together, and to these the
suspending rods are attached. The cables are wrapped in cotton duck soaked
in oxidized oil and varnish, and are sheathed in sheet iron. A later
bridge, the Manhattan, is designed to carry four railway tracks and four
tramway lines, with a wide roadway and footpaths, supported by cables 211/4
in. in diameter, each composed of 9472 galvanized steel wires 3/16 in. in
diameter.

[Illustration: FIG. 13.--Tower Bridge, London.]

The Tower Bridge, London (fig. 13), is a suspension bridge with a secondary
bascule bridge in the centre span to permit the passage of ships. Two main
towers in the river and two towers on the shore abutments carry the
suspension chains. The opening bridge between the river towers consists of
two leaves or bascules, pivoted near the faces of the piers and rotating in
a vertical plane. When raised, the width of 200 ft. between the main river
piers is unobstructed up to the high-level foot-bridge, which is 141 ft.
above Trinity H.W. The clear width of the two shore spans is 270 ft. The
total length of the bridge is 940 ft., and that of the approaches 1260 ft.
on the north and 780 ft. on the south. The width of the bridge between
parapets is 60 ft., except across the centre span, where it is 49 ft. The
main towers consist of a skeleton of steel, enclosed in a facing of granite
and Portland stone, backed with brickwork. There are two high-level
footways for use when the bascules are raised, the main girders of which
are of the cantilever and suspended girder type. The cantilevers are fixed
to the shore side of the towers. The middle gird