p at the hinge,
diminishing to 1 ft. 7 in. at the centre of the span. The counterweight is
a depressed cantilever arm 12 ft. long, overlapped by the fixed platform
which sinks into a recess in the masonry when the bridge opens. In closed
position the main girders rest on a bed plate on the face of the pier 4 ft.
3 in. beyond the shaft bearings. The bridge is worked by hydraulic power,
an accumulator with a load of 34 tons supplying pressure water at 630 lb
per sq. in. The bridge opens in 15 seconds and closes in 25 seconds.

At the opening span of the Tower bridge (fig. 13) there are four main
girders in each bascule. They project 100 ft. beyond and 62 ft. 6 in.
within the face of the piers. Transverse girders and bracings are inserted
between the main girders at 12 ft. intervals. The floor is of buckled
plates paved with wood blocks. The arc of rotation is 82 deg., and the axis of
rotation is 13 ft. 3 in. inside the face of the piers, and 5 ft. 7 in.
below the roadway. The weight of ballast in the short arms of the bascules
is 365 tons. The weight of each leaf including ballast is about 1070 tons.
The axis is of forged steel 21 in. in diameter and 48 ft. long. The axis
has eight bearings, consisting of rings of live rollers 4-7/16 in. in
diameter and 22 in. long. The bascules are rotated by pinions driven by
hydraulic engines working in steel sectors 42 ft. radius (_Proc. Inst.
C.E._ cxxvii. p. 35).

As an example of a swing bridge, that between Duluth and Superior at the
head of Lake Superior over the St Louis river may be described. The centre
opening is 500 ft., spanned by a turning bridge, 58 ft. wide. The girders
weighing 2000 tons carry a double track for trains between the girders and
on each side on cantilevers a trolley track, roadway and footway. The
bridge can be opened in 2 minutes, and is operated by two large electric
motors. These have a speed reduction from armature shaft to bridge column
of 1500 to 1, through four intermediate spur gears and a worm gear. The end
lifts which transfer the weight of the bridge to the piers when the span is
closed consist of massive eccentrics having a throw of 4 in. The clearance
is 2 in., so that the ends are lifted 2 in. This gives a load of 50 tons
per eccentric. One motor is placed at each end of the span to operate the
eccentrics and also to release the latches and raise the rails of the steam
track.

At Riga there is a floating pontoon bridge over the Duna. It cons