ea,
as exemplified by the Sunderland docks adjacent to the mouth of the
river Wear, and the Havre docks at the outlet of the Seine estuary (fig.
2). Some old ports, originally established on sandy coasts where a
creek, maintained by the influx and efflux of the tide from low-lying
spaces near the shore, afforded some shelter and an outlet to the sea
across the beach, have had their access improved by parallel jetties and
dredging; and docks have been readily formed in the low-lying land only
separated by sand dunes from the sea, as at Calais, Dunkirk (fig. 6) and
Ostend (see HARBOUR). In sheltered places on the sea-coast, docks have
sometimes been constructed on low-lying land bordering the shore, with
direct access to the sea, as at Barrow and Hartlepool; whilst at
Mediterranean ports open basins have been formed in the sea, by
establishing quays along the foreshore, from which wide, solid jetties,
lined with quay walls, are carried into the sea at intervals at right
angles to the shore, being sheltered by an outlying breakwater parallel
to the coast, and reached at each end through the openings left between
the projecting jetties and the breakwater, as at Marseilles (fig. 5) and
Trieste, and at the extensions at Genoa (see HARBOUR) and Naples. Where,
however, the basins are formed within the partial protection of a bay,
as in the old ports of Genoa and Naples, the requisite additional
shelter has been provided by converging breakwaters across the opening
of the bay; and an entrance to the port is left between the breakwaters.
The two deep arms of the sea at New York, known as the Hudson and East
rivers, are so protected by Staten Island and Long Island that it has
been only necessary to form open basins by projecting wide jetties or
quays into them from the west and east shores of Manhattan Island, and
from the New Jersey and Brooklyn shores, at intervals, to provide
adequate accommodation for Atlantic liners and the sea-going trade of
New York.

[Illustration: FIG. 5.--Port of Marseilles. Basins and Extensions.]

Approach channels.

The accessibility of a port depends upon the depth of its approach
channel, which also determines the depth of the docks or basins to which
it leads; for it is useless to give a depth to a dock much in excess of
the depth down to which there is a prospect of carrying the channel by
which it is reached. The great augmentation, however, in the power and
capacity for work of modern dr