256 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
In Uhe case of motor boats plying on rivers or inland lakes, a horn
or an electric bell audible for at least half a mile may be allowed instead
of a whistle, if the owners so desire.
Fog Horns.—Article 15 of the Collision Regulations requires that
the necessary signals for vessels under way shall be given (1) by steam
vessels on the whistle or siren; (2) by sailing vessels and vessels towed,
on the fog horn. Whistles required for steam vessels are described
above, but all steam vessels must, m addition, be provided with an
efficient fog horn to be sounded by mechanical means.
Fog horns of the "rotary" and ''crank bellows" type are the most
efficient at present in use. "Plunger" type fog horns are rarely found
to be efficient, and should only be accepted if entirety satisfactory.
Horns blown by mouth cannot be accepted as efficient on vessels
plying on the high seas, or in waters connected therewith navigable
by sea-going vessels.
SCREENING OF LIGHTS.
Side-Lights: Screening Abaft the Beam.—The wick or wicks of a
side-light must be placed at an angle of 112^° with the fore-and-aft
line of the ship; in other words they must be parallel to the direction
two points abaft the beam. The burner must be so placed that a
line drawn in this direction from the after edge of the wick in the case
of a single burner, and of the forward wick in the case of a duplex
burner, shall cut the edge of the housing of the lens.
Side-Lights: Screening Forward.—The screens of side-lights, the
length of which should never be less than 36 inches from the flame
to the chock or its equivalent, must always be placed parallel to the line
of the keel. The chocking must be so arranged to show a "thwart-
ship value" of at least 1 inch of wick in a forward direction; that is to
say, a person looking past the edge of the chock in a line parallel to
the keel must be able to see at least 1 inch of wick.
Masthead Lights: Screening.—In a masthead light the wick or
wicks must be at right angles to the line of the keel, and their setting"
must be such that lines drawn from the centre of the after edge of the
wick in the case of a single burner, and of the forward wick in the case
of a duplex burner, in directions two points abaft the beam on each
side, shall cut the edges of the housing of the lens.
Stern Lights: Screening,—If a fixed stern light is fitted, the wick,