her side and worked from opposite side walls by hydraulic power, or
by a single hydraulic piston or bar hinged to the inner side of each
gate (figs. 19 and 20). The latter system has the advantages of being
simpler and occupying less space in the side walls, of avoiding the
slight loss of available depth over the sill due to the two closing
chains crossing on the sill when the gates are open, and especially of
keeping the gates closed against a swell in exposed sites.

Caissons for docks.

A sliding or rolling caisson is occasionally placed across each end of
a lock in place of a pair of dock gates, being Caissons drawn back
into a recess at the side for opening docks. the lock. As a caisson
chamber has to be covered for over to provide a continuous quay or
roadway on the top, a lowering platform is supplied to enable the
caisson to pass under the small girders spanning the top of the
chamber, or the caisson is sunk down sufficiently (fig. 24). The
caisson is furnished with an air chamber to give it flotation, which
is adjusted by ballast according to the depth of water. The advantages
of a caisson, as compared with a pair of gates, are that the gate
recesses, gate floor, hollow quoins and arrangements for working in
the side walls are dispensed with, so that the lock can be made
shorter, and the work at each head is rendered less complicated. The
caisson itself also serves as a very strong movable bridge, and
therefore is often preferred at dockyards to dock gates. By
improvements in the hauling machinery, a caisson can open or close a
lock as quickly as dock gates; the caissons at Zeebrugge lock, at the
entrance to the Bruges ship canal, are drawn across the lock or into
their chamber by electricity in two minutes. A caisson is specially
useful in cases where there may be a head of water on either side, as
then it takes the place of two pairs of gates pointing in opposite
directions, or for closing an entrance against a current. A caisson,
however, requires a much larger amount of material than a pair of dock
gates, and a considerable width on one side for its chamber, so that
under ordinary conditions gates are generally used at docks.

A ship caisson, so called from its presenting some resemblance in
section to the hull of a vessel, occupies too much time in being
towed, floated into position, and sunk into grooves at the bottom and
sides