d. Chalk was thrown in in
large quantities outside the piling, but without effect. Cement concrete
was at last put within the coffer-dam, until it set, and the bottom was
then found to be secure. A bed of concrete was laid up to the level of
the heads of the piles, the foundation course of stone blocks being
commenced about two feet below low water, and the building proceeded
without further difficulty. It may serve to give an idea of the
magnitude of the work, when we state that 400,000 cubic feet of ashlar,
rubble, and concrete were worked up in the piers, and 450,000 cubic feet
in the land-arches and approaches.

The most novel feature of the structure is the use of cast and wrought
iron in forming the double bridge, which admirably combines the two
principles of the arch and suspension; the railway being carried over the
back of the ribbed arches in the usual manner, while the carriage-road
and footpaths, forming a long gallery or aisle, are suspended from these
arches by wrought-iron vertical rods, with horizontal tie-bars to resist
the thrust. The suspension-bolts are enclosed within spandril pillars of
cast iron, which give great stiffness to the superstructure. This system
of longitudinal and vertical bracing has been much admired, for it not
only accomplishes the primary object of securing rigidity in the roadway,
but at the same time, by its graceful arrangement, heightens the beauty
of the structure. The arches consist of four main ribs, disposed in
pairs with a clear distance between the two inner arches of 20 feet 4
inches, forming the carriage-road, while between each of the inner and
outer ribs there is a space of 6 feet 2 inches, constituting the
footpaths. Each arch is cast in five separate lengths or segments,
strongly bolted together. The ribs spring from horizontal plates of cast
iron, bedded and secured on the stone piers. All the abutting joints
were carefully executed by machinery, the fitting being of the most
perfect kind. In order to provide for the expansion and contraction of
the iron arching, and to preserve the equilibrium of the piers without
disturbance or racking of the other parts of the bridge, it was arranged
that the ribs of every two adjoining arches resting on the same pier
should be secured to the springing-plates by keys and joggles; whilst on
the next piers on either side, the ribs remained free and were at liberty
to expand or contract according to temperature--a spac