ers are 120 ft. in length
and attached to the cantilevers by links. The main suspension chains are
carried across the centre span in the form of horizontal ties resting on
the high-level footway girders. These ties are jointed to the hanging
chains by pins 20 in. in diameter with a ring in halves surrounding it 5
in. thick. One half ring is rigidly attached to the tie and one to the
hanging chain, so that the wear due to any movement is distributed over the
length of the pin. A rocker bearing under these pins transmits the load at
the joint to the steel columns of the towers. The abutment towers are
similar to the river towers. On the abutment towers the chains are
connected by horizontal links, carried on rockers, to anchor ties. The
suspension chains are constructed in the form of braced girders, so that
they are stiff against unsymmetrical loading. Each chain over a shore span
consists of two segments, the longer attached to the tie at the top of the
river tower, the shorter to the link at the top of the abutment tower, and
the two jointed together at the lowest point. Transverse girders are hung
from the chains at distances of 18 ft. There are fifteen main transverse
girders to each shore span, with nine longitudinal girders between each
pair. The trough flooring, 3/8 in. thick and 6 in. deep, is riveted to the
longitudinals. The anchor ties are connected to girders embedded in large
concrete blocks in the foundations of the approach viaducts.

The two bascules are each constructed with four main girders. Over the
river these are lattice girders, with transverse girders 12 ft. apart, and
longitudinal and subsidiary transverse girders dividing the floor into
rectangles 3 ft. by 31/2 ft. covered with buckled plates. The roadway is of
pine blocks dowelled. The bascules rotate through an angle of 82 deg., and
their rear ends in the bascule chambers of the piers carry 365 tons of
counterweight, the total weight of each being 1070 tons. They rotate on
steel shafts 21 in. in diameter and 48 ft. long, and the bascules can be
lifted or lowered in one minute, but usually the time taken is one and a
half minutes. They are worked by hydraulic machinery.

9. (d) _Iron and Steel Girder Bridges._--The main supporting members are
two or more horizontal beams, girders or trusses. The girders carry a floor
or platform either on top (_deck_ bridges) or near the bottom (_through_
bridges). The platform is variously constructed. For rail