SOUNDING PIPES	475
length of the ship, with a separate pipe for bilge suctions, led from the
engine-room forward through the bilges and aft through the tunnel
Auxiliary suction pipes branch ofi from the main pipes to the different
water ballast tanks and bilges, the main pipes being led to a valve
chest in the engine room containing the several valves by which com-
partments and pumps may be so connected that any particular compart-
ment can be emptied. The valves are of the non-return type to prevent
the possibility of water passing in from the sea, or from water tanks
into cargo and machinery spaces, or from one compartment to another.

Fig   77—Valve.
Bilge Suction Pipes are fitted with strum boxes or strainers, placed
in accessible positions for inspection when the holds are empty and so
constructed that they can be easily cleared when choked. * Pipe lines
are fitted with expansion joints or bends so that they may not be
strained or fractured with the working of the ship. See page 604.
Sounding Pipes extending above the load waterline are fitted in each
compartment and ballast tank, with a thick doubling plate under the
bottom end of the pipe for the sounding rod to strike upon. Air pipes
are also fitted at each end of ballast tanks, the caps of which must be
taken ofi before the tank can be filled with water.
It is invariably the .daily duty of the carpenter to sound all com-
partments, tanks and bilges, and to note on a board in the engine-room
the depth of water in each as indicated by his sounding rod. It is also
the duty of an officer on joining a new ship to become acquainted as
soon as possible with the positions on deck of all sounding pipes, hand
pump and sluice valve connections.
In oil tankers the air pipes are led, a considerable distance up the