392	NICHOLAS'S  SEAMANSHIP  AND   NAUTICAL KNOWLEDGE
additional 2 or 3 inches of dunnage should be laid on the ceiling and at
the turn of the bilges.
The Dunnage Wood is of various lengths and thicknesses, and it
should be kept clean and dry, as many cargoes, especially foodstuffs
in bags, generate heat and absorb moisture, dirt and oil stains from
dirty dunnage wood.
Regulations regarding the dunnaging, stowing and ventilating of
particular cargoes are enforced at some ports, particularly for rice
and grain, and the conditions of loading them must be complied with.
HOMOGENEOUS CARGOES
Coal as a homogeneous cargo ranks first in importance as it is the
only mineral product the United Kingdom possesses in excess of home
requirements, the normal export being about 60 million tons per annum,
and, in addition, some 16 million tons are delivered at coaling stations
abroad for bunkering ships. It is still the engineer's chief source of
energy.
Coal is shipped from ports in the Bristol Channel, North East
Coast of England, Firths of Forth and Clyde, and abroad from New-
castle (N.S.W.), Pennsylvania, (U S.A.), Natal, Calcutta and Shanghai.
This cargo is loaded usually alongside specially constructed "tips,"
the coal being tipped out of the railway waggon into a shute leading
into tlie ship's hold At some ports the waggon is lifted bodily with a
crane, swung over the hatchway and emptied into the hold. The
discharge is usually by tubs tilled in the hold, hoisted up and swung
outboard on to the quay or into lighters alongside, although elevator
conveyancers working on the principle of a bucket dredger are available
at some ports. At other places mechanical grabs are used. The
grab is lowered from the end of a crane into the hatchway, closes its
"jaws" on a few tons of coal and is then hoisted up and emptied into
waggons or lighters Specially constructed colliers making short
voyages have bees built for this particular trade, the features being
large hatchways and self-triniinirig holds.
Surface Ventilation is essential with ..a coal cargo as the gas is lighter
than air and must be given an opportunity of escaping upwards through
the ventilators and a hatch should be left of£ in fine weather* Through
ventilation is to be avoided as a current of air passing through the
mass of coal might stimulate into activity any dormant gases into
spontaneous combustion.