be brought down to the level of the approach channel outside with a
rising tide, so that vessels may be brought into or passed out of the
basin towards high water. The advantages of these entrances are, that
they occupy comparatively little room where the space is limited, and
are much less costly than locks; whilst in conjunction with a
half-tide basin they serve the same purpose as a lock with a rising
tide. Vessels also pass more readily through the short entrances than
through locks; and as entrances are only used towards high water,
their sills need not be placed so low as the outer sills of locks to
accommodate vessels of large draught. On the other hand, they are
accessible for a more limited period at each tide than locks; and they
do not allow of the exclusion of silt-bearing tidal water, and
therefore necessitate a greater amount of dredging in the docks, and
especially in half-tide basins, for maintenance. Entrances, however,
at large ports are frequently supplemented by the addition of a lock
at some convenient site, rendering the ports accessible for the
smaller class of vessels for some time before and after high water, as
for instance at Liverpool, Barry, Havre and St Nazaire. A small basin
with an entrance at each end--an arrangement often adopted--is in
reality, for all practical purposes, a lock with a very large
lock-chamber. An entrance or passage with gates has also to be
provided at the inner end of a large half-tide basin like the basins
adopted at Liverpool, to shut off the half-tide basin from the docks
to which it gives access, and maintain their water-level when the
water is drawn down in the basin to admit vessels before high tide.

Reverse gates pointing outwards are sometimes added in passages to
docks and at entrances, to render the water-level in one set of docks
independent of adjacent docks, to exclude silty tidal water and very
high tides, and also to protect the gates of outer entrances in
exposed situations from swell, which might force them open slightly
and lead to a damaging shock on their closing again.

Locks at docks.

Locks differ from entrances in having a pair of gates with
arrangements similar to an entrance at each end, separated from one
another by a lock-chamber, which should be large enough to receive the
longest and broadest vessel coming regularly to the port. These dock
locks are similar