4-76 NTCHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

sides of the mast to carry clear of the deck the vapour given off by
benzine, naphtha, gas-oil or any low flash spirit, and are fitted with auto-
matic valves which are operated by the difference of pressure between
the vapour in the tank and the atmosphere outside. When the pressure
in the tank is the greater the gas presses the valve open and escapes
into the atmosphere; and, conversely, when the pressure within the tank
falls sufficiently below the pressure of the atmosphere, especially when
pumping out oil, the valve opens inwards and the air passes into the
tank and maintains an equality of pressure. Needless to say, these
automatic valves require attention and must be opened out and examined
periodically to ensure that they are working properly.

VENTILATION
The ventilation of large passenger ships with several heights of
decks presents a difficulty which is satisfactorily overcome by either
the pressure system or the exhaust system. In the pressure system
fresh air is drawn down the ventilator by fans and forced through sheet-
iron ducts to the various compartments; in the exhaust system, fans
draw the foul air from the compartment and exhaust it up the cowls,
the fresh air entering the ventilating ducts.
The Thermotank System is a combination of ventilating, heating
and cooling. The air is drawn by fans into a casing, comes into contact
with the surface of pipes and is i>hen forced through ducts to the various
parts of the ship. The temperature of the air may be left as it is, or
heated by circulating steam through the pipes, or cooled by circulating
brine through them.
Engine-room and Stokehold ventilators extend as far down into the
compartment as practical, with branches leading to both sides of the
ship, but fans are usually fitted in the bottom of the ventilators which
extend up to the weather deck. The air is drawn down the ventilator
and distributed as low down as possible, thus displacing the heated air
and expelling it upwards through skylights or other outlets.
Cargo Spaces.—With most cargoes, and especially with those of a
perishable nature, such as fruit, etc., ventilation is an important matter,
and is necessary for the proper care of the cargo and to prevent deteriora-
tion. The neglect of this may result in the ship being held responsible
for damage if it is proved that such damage might have been prevented
by proper ventilation. When, however, the cargo contains anything
of an inflammable nature, or wnich is likely to give off inflammable,