INSTRUMENTS FOR OBSERVING 17 
40. Direction of current by compass.—The compass may be used either inde- 
pendently or in connection with the pelorus to obtain the direction of the current. When 
used independently, the compass equipped with an azimuth circle is mounted in the 
stern of the vessel. Immediately after a velocity observation has been taken with a 
current pole and log line, the sights of the azimuth circle are directed toward the pole and 
the bearing read from the compass and recorded. Necessary corrections are afterwards 
applied for deviation and variation. When the pelorus is used, its reading must be 
combined with a compass reading of the ship’s head as explained in paragraph 130. 
Pelorus 
41. While the most modern type of pelorus is a, somewhat elaborate instrument, 
the pelorus used in taking current observations is a simple disk about 8 inches in 
diameter and graduated clockwise for every 5 or 10 degrees. In conformity with the 
usual practice it should be mounted rigidly on the vessel with the 0° mark forward 
and the diameter through this mark parallel to the keel of the boat. Sometimes only 
the forward half of the disk is graduated, beginning with 270° and extending through 
0° to 90°. If the vessel is anchored so that it is free to swing with the current, a single 
pelorus in the stern is sufficient. This should be installed in a position to allow the 
current line to swing through as large an unobstructed sector as possible. If the 
vessel is moored fore and aft so that it cannot swing, a second pelorus mounted for- 
ward is also necessary. 
42. Observing with pelorus.—The pelorus is used in connection with the ship’s 
compass and with the current pole and log line to determine the direction of the current. 
Immediately after the velocity observation the log line is stretched across the center 
of the pelorus and a reading taken where the line intersects the forward arc of the 
instrument. This reading, usually taken to the nearest 5°, is then entered in the 
record book (Form 270 or 270a). The compass reading of the ship’s head for the 
same time must also be recorded. In general, for observations taken from the stern 
of the vessel the current pole will be aft of the beam and for observations taken from 
the forward pelorus the pole will be forward of the beam. There may be occasional 
exceptions, and these should be carefully noted in the record. Also, when there is 
more than one pelorus on the vessel, the record should be clear as to which pelorus is 
‘in use at any time. The computations for obtaining the actual current direction from 
the pelorus reading are explained in paragraph 130. 
Sextant 
43. Aside from its use as an instrument of navigation, the sextant is used in 
current observations to determine the location of the station occupied and also in de- 
termining the direction of the current. The sextants generally used in connection with 
current surveys are of lighter construction than those used for navigation and are read 
only to minutes. The sextant is designed to measure the angle between two objects 
by bringing into coincidence rays of light received directly from one and by reflection 
from the other, the measure depending upon the angle between two reflecting surfaces. 
One of these surfaces is fixed and is known as the “horizon mirror,’”’ and the other, 
