weigh-bridge, and the other for the
return of the empty wagons to the siding where the empty train is made
up for returning to the colliery (fig. 8). Each full wagon is either run
at a low level upon a cradle at the tip, then raised on the cradle
within a wrought-iron lattice tower to a suitable height, and lastly,
tipped up at the back for discharging the coal; or it is brought along a
high-level road on to a cradle raised to this level on the tower, and
tipped up at this or some slightly modified level. The coal is
discharged down an adjustable iron shoot, gradually narrowed so as to
check the fall; and on first discharging into the hold of a vessel, an
anti-breakage box is suspended below the mouth of the shoot. When full,
this is lowered to the bottom of the hold and emptied, thereby gradually
forming a cone of coal upon which the coal can be discharged directly
from the shoot without danger of breakage. Other contrivances are also
adopted with the same object.

Dock extensions.

In designing dock works, it is expedient to make provision, as far as
possible, for future extensions as the trade of the port increases.
Generally this can be effected alongside tidal rivers and estuaries by
utilizing sites lower down the river, as carried out on the Thames for
the port of London, or reclaiming unoccupied foreshores of an estuary,
as adopted for extensions of the ports of Liverpool, Hull and Havre.
At ports on the sea-coast of tideless seas, it is only necessary to
extend the outlying breakwater parallel to the shore line, and form
additional basins under its shelter, as at Marseilles (fig. 5) and
Genoa (see HARBOUR). Quays also along rivers furnish very valuable
opportunities of readily extending the accommodation of ports. Ports,
however, established inland like Manchester, though extremely
serviceable in converting an inland city into a seaport, are at the
disadvantage of having to acquire very valuable land for any
extensions that may be required; but, nevertheless, some compensation
is afforded by the complete shelter in which the extensions can be
carried out, when compared with Liverpool, where the additions to the
docks can only be effected by troublesome reclamation works along the
foreshore to the north, in increasingly exposed situations.

_Dock Entrances and Locks._--The size of vessels which a port can admit
depends upon the depth and width of the entrance to the d