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 
20