14 MANUAL OF CURRENT OBSERVATIONS 
book. The direction of the current as indicated by the position of the current pole 
is determined by compass, pelorus, or sextant angles as will be explained later. 
30. For a very strong current, a 30-second run may be used instead of a full 
minute, and in this case the velocity is double that read directly from the log line. 
For a very weak current, the line may be allowed to run 2 minutes, in which case the 
velocity is one-half as great as read directly from the Ime. Care must be taken not 
to pay out the line faster than the pole can take it away. In a very weak current, 
trouble may be experienced because of the sinking bight in the log line. Corks fastened 
at each tenth of a knot graduation have sometimes been used to eliminate this trouble 
Compass 
31. There are two types of compasses now in use for the independent determina- 
tion of direction—the magnetic and the gyro. The magnetic compass, which has been 
in common use for a great many years, is actuated by a magnet or group of magnets 
and indicates direction relative to the earth’s magnetic poles but is subject to errors 
arising from local attractions which will be explained later. The magnetic compass is 
in general use on the smaller vessels that are usually employed in taking current ob- 
servations. The gyro compass, which is a more elaborate and modern invention, is 
actuated by a rapidly spinning rotor which tends to place its axis of rotation parallel 
to the earth’s axis of rotation. This compass, therefore, indicates direction relative 
to the true meridian and is subject only to small corrections depending upon latitude, 
course, and speed of the vessel. The larger and more modern vessels are usually 
equipped with gyro compasses. 
32. Hither type of compass has a compass card, circular in shape, with circumfer- 
ence graduated to indicate direction. These graduations may be either in degrees of 
azimuth or in points of the compass, or in both degrees and points. In modern times 
there is a tendency towards the use of degrees of azimuth reckoned clockwise through 
360° entirely around the circle, with north taken as 0° or the initial point of the reckon- 
ing. In the older magnetic compasses preference was given to the designation of direc- 
tion by points. There are 32 such points in the entire circumference, these points being 
spaced 114° apart; each point is usually subdivided into quarters. The naming of 
these points in order is called boxing the compass. The cardinal points are the four 
principal compass points—north, east, south and west. The intercardinal points are 
the points midway between the cardinal points and are northeast, southeast, southwest 
and northwest. The names of all of the points and quarter-points with corresponding 
azimuth to the nearest whole degree are shown in Table 1. 
33. The azimuth circle is a fitting for a compass employed in taking bearings and 
consists essentially of a pair of sight vanes at the extremities of the diameter of a ring 
that revolves concentrically with the compass bowl with the line of sight always passing 
through the vertical axis of the compass. A system of mirrors and prisms brings into 
the field of view of the observer the compass reading corresponding to the direction 
of the line of sight. 
34. Variation of compass.—The variation of the compass, also called magnetic 
declination, is the difference between true north as determined by earth’s axis of rota- 
tion and magnetic north as determined by the earth’s magnetism. The variation is 
designated as east or positive when the earth’s magnetism deflects the magnetic needle. 
