There are two kinds of docks--dry and wet. A dry-dock is usually
constructed with gates, to admit or shut out the tide. When a ship
arrives from a long voyage, and needs repair to the lower part of her
hull, she must be got out of the water somehow or other.
This object is frequently attained in regard to small vessels by simply
running them gently on the flat sand or mud beach of a bay or harbour,
so that, when the tide retires, they shall be left dry. But it would be
dangerous as well as inconvenient to do this with large ships, therefore
dry-docks have been constructed for this purpose. They are so built
that when the tide is full the dry-docks are also full. When thus full
of water, the gates of a dry-dock are opened, and the large ship is
dragged slowly in, after which the gates are shut. The tide then
retires, leaving it in this basin of water. The ship is then propped up
on all sides with timbers, in such a way that she stands upright, "upon
an even keel," and thus, the pressure on her hull being equally
distributed, she is not damaged. Then the water is let out by means of
sluices in the gates, or it is pumped out, and the ship left dry. When
the tide returns, the gates and sluices are all shut, and its entrance
into the dock prevented, until such time as the ship is repaired, when
water is let slowly in. As the vessel floats, the props and supports
fall away, the gates of her hospital are opened, and off she goes again,
in all the vigour of recruited health, to wing her way over the billows
of the great deep.
A wet-dock is somewhat similar to a dry-dock, the chief difference being
that ships while in it are kept floating in water.
Docks are not only used, however, for repairing and building ships.
They are also used for loading and unloading them; and as ships are
entering and departing from them almost constantly, the busy, bustling,
active scene they present is always agreeable.