248 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
sharp descending note termed the "grunt," the Siren a medium powered
note, either high or low or a combination of the two, and the Reed a
high note of less power. Reeds may be hand-operated, in which case
the signals from them are of small power.
The Nautophone (4) is an electrically-operated instrument also
fitted with a horn, and emits a high note signal similar in power and tone
to that of the Reed
Gun (5) and Explosive (6) signals are produced by the firing of
explosive charges, the former being discharged from a gun, and the
latter being exploded in mid air.
Bells (7) may be operated either mechanically or by wave action,
in which latter case the sound is irregular. The notes may be high,
medium or low according to the weight of the bell. Gongs are also
sometimes employed.
A Whistle (8) is a signal of low power and tone sometime** fitted on
a floating body, when this is the case the sound is produced by air
drawn in and compressed during the upward and downward movement
of the body due to wave action, and is consequently irregular.
II. Submarine Sound Signals.
The Oscillator (9) is an electrically-operated instrument sounding
a high note signal.
Bells (10) may be operated either mechanically or by wave motion,
in which latter case the sound is irregular.
The effective range of submarine sound signals far exceeds that of
air sound signals having been known to exceed 50 miles in the case of
an oscillator and 15 miles in that of a bell. Their bearings can be
determined with sufficient accuracy for safe navigation in a fog if a
vessel is equipped with receivers, and even should a vessel be not so
-equipped submarine signals may be heard from below the waterline
for distances which are well outside the range of air fog signals, though
their bearings cannot then be so well determined.
III. Wireless Fog Signals. *
These are provided for the purpose of position finding.
There are three types employed (11) Beacon, (12) Revoking Beam,
and (13) Rotating Beacon or Loop Stations.
The Beacon Station (11) consists of an all-round wireless trans-
mitter sending out a code signal in every direction, the bearing of
wMch is obtained by means of a wireless direction finder or wireless
compass fitted on board ship. When the wireless signal is combined