, the exit and
entrance of vessels are effected by lowering their water-level on a
rising tide, and opening the gates, which are then closed at high
water to prevent the lowering of the water-level in the dock, and to
avoid closing the gates against a strong issuing current.

The tidal basins outside the locks at Tilbury and Barry are quite open
to the tide, and have been carried down to 24 ft. and 16 ft.
respectively below low water of spring tides, in order to afford
vessels a deep sheltered approach to the lock in each case, available
at or near low water (figs. 7 and 8). Such basins, however, open to a
considerable tidal range where the water is densely charged with silt,
are exposed to a large deposit in the fairly still water, and their
depth has to be constantly maintained by sluicing or dredging.

River quays.

Where the range of tide is moderate, or on large inland rivers, docks
or basins are usefully supplemented by river quays, which though
subject to changes in the water-level, and exposed to currents in the
river, are very convenient for access, and are sometimes very
advantageously employed in regulating a river and keeping up its banks
when deepened by dredging. Generally 10 to 12 ft. is the limit of the
tidal range convenient for the adoption of open basins and river
quays; but the banks of the Tyne have been utilized for quays, jetties
and coal-staiths, with a somewhat larger maximum tidal range; and a
long line of quays stretching along the right bank of the Scheldt in
front of Antwerp, constructed so as to regulate this reach of the
river, accommodates a large sea-going traffic, with a rise at spring
tides of 15 ft.

Excavations for docks.

When a dock has to be formed on land, the excavation is effected by
men with barrows and powerful steam navvies, loading into wagons drawn
in trains by locomotives to the place of deposit, usually to raise the
land at the sides for forming quays. Directly the underground
water-level is reached, the water has to be removed from the
excavations by pumps raising the inflowing water from sumps, lined
with timber, sunk down below the lowest foundations at suitable
positions, so that the lower portions of the dock walls and sills of
the lock or entrance may be built out of water. A cofferdam has to be
constructed extending out from the bank of the river or approach
channel in front