of an entrance for closing it, and then refloated and towed away
for opening the entrance again, to be used at entrances and locks to
docks (fig. 25). Being, however, simple in construction, taking up
little space, and requiring no chamber or machinery for moving it,
this form of caisson is generally used for closing the entrance to a
graving dock, where it remains for several days in place during the
execution of repairs to a vessel in the dock. A ship caisson only
requires the admission of sufficient water to sink it when in position
across the entrance to a graving dock; and this water has to be pumped
out before it can be floated, and removed to some vacant position in
the neighbouring dock till it is again required. Like a sliding or
rolling caisson, it provides a bridge for crossing over the entrance
of the graving dock when in position.

_Graving Docks._ - Provision has to be made at ports for the repairs of
vessels frequenting them. The simplest arrangement is a timber gridiron,
on which a vessel settles with a falling tide, and can then be inspected
and slightly cleaned and repaired till the tide floats it again.
Inclined slipways are sometimes provided, up which a vessel resting in a
cradle on wheels can be drawn out of the water; and they are also used
for shipbuilding, the vessel when ready for launching being allowed to
slide down them into the water. Graving or dry docks, however, opening
out of a dock, are the usual means provided for enabling the cleaning
and repairs of vessels to be carried out.

[Illustration: FIG. 26.--Plan of Southampton Graving Dock.]

[Illustration: FIG. 27.--Cross Section of Southampton Graving Dock.]

A graving dock consists of an enclosure, surrounded by side walls
stepped on the face, and paved at the bottom with a thick floor
sloping slightly down from the centre to drains along the sides, long
enough to receive the longest vessel likely to come to the port. Its
entrance, at the end adjoining the dock, is just wide enough to admit
the vessel of greatest beam, and deep enough over the sill to receive
the vessel of greatest draught, when light, at the lowest water-level
of the dock (figs. 26 and 27). Graving docks are constructed of
masonry, brickwork or concrete, or formerly in America of timber; they
should be founded on a solid impervious stratum, or, where that is
impracticable, they should be built upon bearing piles and e