154 NICHOLAS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
pointer on the scale of degrees the two points limiting the arc of silence
and the mid-point between which gives the required direction but, in
the first instance, the direction is relative to the ship's head only. The
actual direction of the ship's head by compass should be taken at the
same time as the wireless observation to enable the true bearing of the
distant station to be obtained.
Exampk.—Ths radio direction finder* records a bearing 120°, ship's
head 030°. Required the true bearing of the station.
Direction of ship's head.......030°
Eadio angle between ship's head and station - * 120
True bearing of station .... S. 30° E or 150
The magnetic field of the ship interferes with the normal direction
of the incoming wireless wave, consequently the apparent direction of
the distant wireless station is altered. The errors caused are quadrantal,
that is to say, they reach a maximum value every 90°, or four maxima
and four minima in a complete swing. In a ship having perfect electrical
symmetry the induced currents in the structure produce a magnetic
field in the athwartship line and the quadrantal error is zero in directions
0°, 90°, 180°, and 270° from the bow, the maximum error appearing
at 45° on port and starboard bows and on port and starboard quarters.
In most cases, however, the ship's magnetic field is unsymmetrical so
that the quadrantal error is zero at four other angles from the bow, but
the maximum, errors are still separated by 90°. The apparatus is
provided with permanent correction adjustment so that the correct
angle between the ship's head and the station may be read direct from
the scale. There are other types of wireless directional indicators known
as radio beacons for transmitting wireless fog signals as described under
Notices to Mariners, page "248.
SLUICES AND WATERTIGHT DOORS.
A sluice valve is a watertight vertically sliding shutter over a hole
cut in the lower edge of a bulkhead to allow water to flow from one
compartment to the next. The valve is raised and lowered by means
of a rod secured to it and operated by hand from an upper deck above
the; waterline. The fewer sluices fitted in a ship the better. None
is fitted in the collision bulkhead nor at any other watertight bulkhead
unless arranged so as to be at all times accessible. They should be