REDUCTION OF CURRENT RECORDS 51 
Form 270. For the long series of observations at lightships, which are recorded on 
Form 270a, it is usually more convenient to take the magnetic azimuth for the individual 
observations, the computation of the true azimuth being deferred until final means have 
been obtained and then any necessary correction for compass variation applied. For 
a discussion of compass variation and deviation see paragraphs 34-39. 
130. When the pelorus has been used to obtain the direction of the current pole, 
the true or magnetic direction of the current may be obtained by one of the following 
formulas. 
TRUE DIRECTION OF THE CURRENT 
=pelorus+ship’s head+deviation+ variation+ 180° (pole aft of beam) 
=pelorus+ship’s head+deviation+ variation (pole forward of beam) 
MAGNETIC DIRECTION OF CURRENT 
=pelorus+ship’s head+deviation+180° (pole aft of beam) 
=pelorus+ship’s head-+deviation (pole forward of beam) 
In order that the above formulas may be applicable, it is important that the pelorus 
shall have been mounted as described in paragraph 41 and that it has been read on the 
forward half of the disk in accordance with the instructions in paragraph 42. The 
pelorus reading must therefore be between 270° and 360° or between 0° and 90°. The 
ship’s head must be expressed in degrees of azimuth as read from the ship’s compass. 
If the ship’s head has been recorded in points of the compass as is sometimes done, 
the equivalent azimuth readings may be taken from Table 1 on page 15. The compass 
deviation and variation are considered as positive when east and negative when west 
and should have the corresponding sign when used in the above formulas. If the ship’s 
compass has been completely compensated for deviation, this term becomes zero. 
131. In the above formulas it will be noted that the angle +180° must be applied 
if the current pole is aft of the vessel’s beam when taking the observation, but is not 
included when the current pole is forward of the beam. It is therefore essential that 
the record kept by the observer should indicate clearly the position of the pole. If 
observations are being taken from a vessel anchored so that it is free to swing with the 
current as is usually the case with lightships, the current pole will, in general, move 
astern unless an opposing wind prevents the vessel from tailing with the current. 
With the vessel so anchored, it may usually be assumed that the pole is aft of the 
beam unless the observer specifically records otherwise. When the vessel is moored 
fore and aft so that it cannot swing with the current and is provided with two peloruses, 
one forward and the other aft, it is expected that the observer will alternate in the use 
of these instruments according to the direction of the current. In general, the pole 
will be ahead of the beam when the forward pelorus is in use and astern of the beam 
when the aft pelorus is in use, but there will be a number of exceptions. Special care 
is therefore required on the part of the observer to indicate not only the particular 
pelorus in use, but also to have the record clear as to whether the pole was forward or 
aft of the beam at the time of any observation. If the observer reports the changes 
between flood and ebb, it will aid in the later interpretation of the record in the office. 
132. If the bearing of the current pole has been taken directly by compass, cor- 
rections for deviation and variation only are required to obtain the true direction of 
the current; or, if the magnetic direction only is needed, the correction for deviation 
alone is sufficient. If the direction of the current pole is measured by a sextant angle 
from a charted reference object, the corresponding true direction of the current is 
obtained by applyimg this angle to the true bearing of the reference object from the 
