THE COUNTERPOISE. 437
than 89 tons would have to be applied at its axis in order to affect the
transverse equilibrium.
The great majority of swing bridges, however, have their weights distri-
buted between the pivot and the wheels or rollers in varying proportions,
capable of adjustment by mechanical contrivances. The revolving members
must have conical surfaces with axes radiating to the centre of rotation.
Their diameters in existing examples vary from about 8 inches to 5 feet; but
such extremes are injudicious owing, in the first case, to the difficulty of
obtaining a satisfactory adjustment and, in the second, to the great depth of
the roller path. Between 18 inches and 3 feet will be found a suitable range
for practical purposes. Large rollers, on account of the correspondingly
obtuse angles which they subtend, have a tendency to work out of position
under pressure. They are restrained by their inner flanges or by axial rods
to the pivot, but in either case the friction is augmented.
Fig. 422.—Balancing Lever.
Sometimes a double wheel track is provided, or there is an intermediate
row of friction rollers near the centre. In order to secure a proportionate
pressure upon these intermediate supports, the bearing is communicated
through a volute or other spring or by means of counter-weighting. This
latter method is achieved by placing the wheel journals in a loose cast-iron
frame connected with a balancing lever as shown in fig. 422.
Some bridges move entirely upon a turntable of rollers, leaving scarcely
any appreciable weight to be borne by the pivot. A footbridge has been
constructed which revolved upon a row of cannon balls between two grooved
cast-iron plates.
The Counterpoise.—^sonry, gravel or rubble ballast, and cast-iron
kentledge have all been utilised for the purpose of counterweightmg
movable bridges. The last-named material, being heavier and easy to
mould in blocks of suitable shape and size, is most generally used, a very
inferior quality of iron being employed.