656	NICHOLLS'S  SEAMANSHIP  AND   NAUTICAL   KNOWLEDGE
(a) Sound signalling in fog should be reduced to a minimum. Signals
other than single-letter signals should be used only in extreme emergency
and never in frequented navigational waters.
(6) For the reasons just stated the procedure indicated in the following
example should be observed,
Example.—You are instructed to signal  "What is the depth of
water" by means of the steam whistle, no other system of transmission
being convenient.    Describe the procedure.
I would make a succession of double separate  A A's,  until the
receiving ship or station answered with T's.
I would then morse the  message, "What is the depth of water,"
then give (AH) to indicate signal ended.
The receiving ship or station would then answer with (R) meaning
received.
Assuming the reply to be 19 feet then
She would make A A's     • —   • —       «~   • —••       • —•   • —
I would answer with T's       «« — «*n —• — —
She would make   •••«•••••••••••••••••••«•«    •   •    •*•
then (AR)       . —._ -
1 would answer with (R)    « ••» «   to indicate that I have received
the message, viz., 19 feet,
note.—The transmitting ship sends her message right through
without waiting for acknowledgment by the receiving ship, but if the
receiver misses a word or group he makes the repeat sign (UD), on
hearing which the transmitting ship will cease signalling and then go
back a few words or groups and continue the message,
QUARANTINE SIGNALS
The following signals are to be shown on arrival by vessels requiring
or required to show their state cff health:—
In the Daytime,
Q flag—signifying        -      -      -    "My ship is 'Healthy/ and I request
free pratique."
Q flag over first substitute (Q Q)—
signifying      -      .      -      .   "My ship is 'Suspect/ i.e.t 1 have had
cases of infectious diseases more
than five days ago, or there has
been unusual* mortality among
jfche rats on board my sbip."