432
DOCK ENGINEERING.
pivot illustrated is that it is provided with keys or wedges, whereby the
bridge can be more accurately balanced.
All the foregoing examples are instances of what may be termed the
solid pivot, in contradistinc-
tion to the hydraulic pivot,
exemplified in the two follow-
ing cases.
(e) A cylindrical pivot of
medium height (figs. 416 and
417), with a perfectly plane
top so disposed as to receive
only the vertical pressure due
to the weight of the bridge,
the axis of the pivot passing
through the centre of gravity
of the bridge. Any lateral
action due to surging or vibra-
tion is taken by a horizontal
ring of small rollers encircling
the lower part of the bridge
seating. The pivot is essenti-
ally a ram or piston raised
into position by hydraulic
pressure against its under
surface, and allowed to fall
after the completion of the
rotative work. This system
is practised at Marseilles.
With a slight modification it
has been also practised at
Liverpool. The modification
consists in a concavity in the
upper surface of the ram, to
receive the hemispherical
seating of the bridge, so
that the latter may revolve
about the pivot instead of the
pivot turning in the cylinder
with a tendency to wear the
sides. As a matter of faet,
it is difficult to ensure the
immobility of the pivot, so
that the object aimed at cannot be said to be achieved. It is important
to note that there is a grave risk attaching to the apparently simple and
effective contrivance just described. Should the hydraulic pressure not be