VENTILATION 47'
explosive, or poisonous vapour, proper and efficient ventilation becomes
of vital importance, and neglect of this precaution may lead to disastrous
consequences, possibly involving loss of life.
No system of ventilation will be efficient unless arrangements arc
made for the free exit of possible foul, damp, or vapour-laden air, as
well as the introduction of fresh air, the object being to keep a circula-
tion of air through the entire hold or compartment. Movable ventilators
should be attended to so as to take advantage of the wind. If a hold
or compartment is ventilated entirely by cowl ventilators, they should
not all be turned to the wind. The one furthest to leeward should be
open to the wind; the one furthest to windward should be turned away
from the wind to allow free outdraft. See also page 606«
In fine weather, and when ike cargo is not of an explosive or inflammable
nature, such as petroleum spirit, etc., the ordinary means of ventilation
may be supplemented by taking off one or two of the hatches forward
and aft. This will give surface ventilation. In special cases where
additional ventilation is necessary, it can be obtained by rigging wind
sails into an open hatch.
The gas given off from coal cargoes is of a light nature, and rises.
It tends therefore to accumulate at the upper parts of the hold, and by
a system of surface ventilation it will be passed away from the interior
of the vessel. Petroleum vapour, on the other hand, is very much
heavier than air, and thus accumulates at the bottom of holds or other
spaces; and any system of ventilation, when carrying this spirit, to be
effective, should be such as will withdraw the vapour-laden air from the
bottom.
The holds of vessels, other than tankers carrying petroleum and other
similar spirits, should not be ventilated by removing the hatches. Proper
ventilators should be fitted extending to the bottom of the hold. These
should have large cowlheads, the openings being covered with fine
brass wire gauze.
All ventilation of crew spaces should be carefully attended to, and
kept open, when carrying any of these dangerous substances.
No one should be allowed to enter any hold or compartment where
there has been stowed any of these dangerous substances or liquids until
they have been thoroughly ventilated. There are gases which it is
dangerous to inhale, but which are odourless and give no warning of their
presence.
The presence of dangerous gases can be detected by means of a
safety lamp, specially devised for the purpose. If the light becomes