NAVIGATION LIGHTS 259
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Further, in lights fitted with dioptric lenses, the luminous centre
of the filament should always be the same distance above the cap, and
to ensure that this condition is fulfilled a sufficient supply of spare
lamps of identical dimensions should be carried on board. This reserve
stock should be inspected.
25. Side-Lights: Screening Abaft the Beam,—Lamp sockets should
be so placed m the lantern cases that a line drawn in a direction two
points abaft the beam, touching the forward edge of a circle, f-mch
diameter, concentric with the socket, will cut the edge of the housing.
The centre of the lamp socket should be placed iV inch abaft the
centre from which the curvature of the lens is struck.
26. Side-Lights: Screening Forward.—The chocking must be so
arranged so as to show a "thwartship value" of at least 1 inch of
filament in a forward direction.
27. Masthead Lights Screening —Lamp sockets should be so placed
in the lanterns that lines drawn in directions two points abaft the beam
on each side touching, tangentially, the forward side of a circle f-mch
diameter concentric with the lamp socket will cut the edges of the
housing of the lens. The centre of the lamp socket should be placed
i^r inch abaft the centre from which the curvature of the lens is struck.
28. Fittings,—The usual type of bayonet fitting should be used.
The inside diameter of the socket should be £-inch, or J-inch larger than
the diameter of the circle to which the light is screened abaft the beam,
as described in paragraphs 25 and 27.
Special care should be taken to see that the lamps fit closely into
the sockets, so that when in position they are upright and secure.
NOTICES TO MARINERS
Questions
1. Who issues Notices to Mariners and where are they obtained?
2. What signals are shown when a port is closed and what would
you do on entering a closed port and no instructions were issued?
3. What lights are shown by an Examination vessel?
£. Give the day and night signals for a vessel sweeping for mines.
5. What precautions should be taken when approaching a squadron
of warships?
6. What flag is displayed by a submarine escort and what action
would you take on approaching her?