in the most recent extensions. Thus at the port of London the
sills of the first lock of the Albert dock were 26-1/2 ft. below high water
of neap tides, and of the second lock adjoining, 32-1/2 ft. deep; whilst
the sills of the lock of the Tilbury docks are 40-1/2 ft. below high water
of neap tides. Moreover, in spite of the great range of tide at the
South Wales ports on the Severn estuary, the available depth at high
water of neap tides of 25 ft. at the Roath lock, Cardiff, was increased
in the lock of the new dock to 31-1/2 ft.; the depth at the entrance to
the Barry docks, opened in 1889, was 29-1/2 ft., but at the lock opened in
1896 was made 41-1/3 ft.; whilst a depth of 34 ft. has been proposed for
the new lock of the Alexandra dock extension at Newport, nearly 10 ft.
deeper than the existing lock sills there. Similar improvements in depth
have also been made or designed at other ports to provide for the
increasing draught of vessels.

The length of locks has also been increased, from 550 ft. at the Albert
dock, to 700 ft. at Tilbury in the port of London, from 300 ft. to 550
ft. at Hull, and from 350 ft. to 660 ft. at Cardiff. The lock at the
Barry docks is 647 ft. long, though only 65 ft. wide. A lock constructed
in connexion with the improvement works at Havre, carried out in
1896-1907, was given an available length of 805 ft. and a width of
98-1/2 ft., with a depth over the sills of 34-3/4 ft. at high water of
neap tides.

[Illustration: FIG. 19.--Barry Docks, Entrance.]

Entrances to docks.

Entrances with a single pair of gates, closing against a raised sill
at the bottom and meeting in the centre, have to be made long enough
to provide a recess in each side wall at the back to receive the gates
when they are opened, and to form a buttress in front on each side to
bear the thrust of the gates when closed against a head of water
inside. A masonry floor is laid on the bottom in continuation of the
sill, serving as an apron against erosion by water leaking between or
under the gates, and by the current through the sluiceways in the
gates, when opened for scouring the entrance channel or to assist in
lowering the water in a half-tide dock for opening the gates (fig.
19). A sluiceway in each side wall, closed by a vertical sluice-gate,
generally provided in duplicate in case of accidents and worked by a
machine actuated by hydraulic pressure, enables the half-tide basin to