PORT ECONOMICS

the open end. After entrance, the second pair of gates
is closed, and by means of sluices, the level of the water
in the lock chamber is either raised or lowered as the
circumstances may require, in the former case by admitting
water from the dock, and in the latter case by discharging
water into the outer waterway. When the level of the
water in the lock chamber has been adjusted to the new
level, the first pair of gates can be opened and the ship is
free to leave the chamber by the other end from that at
which it entered.

Not only does the system involve an important out-
lay in construction and equipment and also in working
cost, but each locking operation causes the withdrawal
of a volume of water from the dock which cannot in the
majority of cases lightly be spared, and this consideration
reduces the periods within which locking is economically
practicable.

The disadvantage of a Basin in comparison with a Dock
in places where the tidal range is at all considerable, is
that the rise and fall of the water level necessitates constant
attention to the mooring ropes, that is, they have to be
slackened or tightened at frequent intervals. Also, the
change in the relative levels of a ship’s deck and the quay
makes cargo handling difficult, and, in some cases, im-
practicable. These objections apply equally to riverside
quays. Moreover, unless there is sufficient depth of water
in a basin, ships will take the ground at low water, and
this is inadvisable, as strains may be induced in the ships’
frames, owing to the inequality of support. Modern
mercantile vessels, constructed of slender steel framing
and thin plating, are not designed to withstand stresses of
this nature. Generally, a dock, by the very nature of its
complete enclosure, affords a greater degree of tranquillity
and shelter for shipping, and more effective security for
goods from pilferage and theft.

Formerly, a wet dock was commonly known as a floating
dock, because it kept vessels afloat, but this designation

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