in principle to locks on canals and canalized
rivers, but are on a much larger scale. The lock-chamber has its water
raised or lowered in proportion to the difference in level between the
water-level in the dock and the water in the entrance channel, by
passing water, when the gates are closed at both ends, from the dock
into the lock-chamber or from the lock-chamber into the entrance
channel, through large sluiceways in the side walls, controlled, as at
entrances, by vertical sluice-gates. In this way the vessel is raised
or lowered in the chamber, till, when a level has been reached, the
intervening pair of gates is opened and the vessel is passed into the
dock or out to the channel. Generally the upper and lower sills of a
lock are at the same level, a foot or two higher than dock-bottom; and
the depth at which they are laid is governed by the same
considerations as the sill of an entrance. Vessels longer than the
available length between the two pairs of gates can be admitted close
to high water, when the water in the dock and outside is at the same
level, and both pairs of gates can be opened. When the range of tide
at a port is large, and the depth in the approach channel is
sufficient to allow vessels to come up or go out some time before and
after high water, and also where the water in the dock is kept up to a
high level from an inland source to exclude very silty tidal water, it
is expedient to reduce the cost of construction by limiting the depth
of the excavations for the dock, and consequently also the height of
the dock walls, to what is necessary to provide a sufficient depth of
water below high water of the lowest neap tides, or below the
water-level to which the water in the dock is always maintained, for
the vessels of largest draught frequenting the port, or those which
may be reasonably expected in the near future. The upper sill of the
lock is then determined by the level of dock-bottom; but the lower
sill is taken down approximately to the depth of the bottom of the
approach channel, or to the depth to which it can be carried by
dredging, so as to enable the lock to admit or let out at any time all
vessels which can navigate the approach channel. Thus, for instance,
the outer and intermediate sills of the lock at the Barry docks are 9
ft. lower then the upper sill.

The foundations for the sill and side walls at each end of a lock