The two general types of compasses are: magnetic compass, which depends on the earth's magnetic field for its directive force; and gyrocompass, which depends upon the inherent space stability of a rapidly spinning wheel plus mechanical or electrical torquing to keep its axis aligned with that of the earth.
A gyrocompass can transmit constant indications of true headings electrically to gyrocompass repeaters located at various positions throughout the ship. It looks like a magnetic compass.
The term azimuth, means the bearing of a celestial body. The terms azimuth and bearing are used to mean the direction of an object from the observer. Bearings and azimuths are expressed in degrees, using three digits, from 000° at north clockwise through 360°. True azimuth or bearing refers to the direction with respect to true north, magnetic azimuth with respect to magnetic north. A relative bearing or azimuth is from the ship's head measuring clockwise with 000 being dead ahead.
An azimuth circle is an instrument for determining both bearing of terrestrial objects and azimuths of celestial objects. It consists of a non-magnetic ring formed to fit over the top of a compass or onto the top of a gyrocompass repeater, about which it can be turned to any desired direction. Its inner lip is graduated from 00 to 360 counterclockwise, for measuring relative bearings. A telescopic alidade is similar to a bearing circle except that the azimuth circle mounts a telescope instead of the sighting vanes. The telescope contains a reticle for greater precision in taking bearings. The image is magnified, making distant objects appear larger to the observer. A prism arrangement which reflects the bearing of the object from the compass card enables the observer to sight the object and its bearing simultaneously.
Since a clear view in all directions may be unobtainable from the compass, peloruses or dumb compasses may be mounted at convenient points, such as the wings of a bridge.
A pelorus consists of a flat, nonmagnetic, metallic ring mounted in gimbals on a vertical stand. The inner edge of the ring is graduated in degrees from 0° at the ship's head clockwise through 360°. This ring snugly encloses a compass card called a pelorus card. The card, flush with the ring and the top of the bowl, is rotatable, so that any chosen degree of its graduation may be set to the lubber's line. A small set screw is provided for temporarily securing the card to the ring. On the card is mounted a pair of sighting vanes similar to those of a bearing circle. They may be revolved about the center of the card, by itself, and held in any desired position by a central clamp screw.
True bearings are obtained as follows: set the pelorus to the ships course, by turning the card until its true course with the lubber's line. Secure the card. Line up the sighting vanes approximately on the object to be observed. Tell the steerman say "Mark! Mark! Mark)" when he is steady on his steering course, and when he does take the bearing and the degree on the card.
Another method of obtaining a true bearing, the navigator gives the steersman a warning "Stand by - followed by a Mark, the instant of the observation. If the steersman was on his course the bearing was true. If not, it may be corrected by applying the number of degrees the steersman was off, but be careful to apply the correction in the right direction.
Magnetic or compass bearings are taken in exactly the same as true bearings, the pelorus card being set to the magnectic or compass course. The pelorus is used for taking relative bearings by setting the card to the lubber's line and observing the object. Relitive bearings are converted to true bearings for plotting by adding to the bearings observed, the true heading of the ship. In most cases, a gyro repeater is mounted in the pelorus stand of the pelorus card so that gyro bearings can be obtained. It used to be we would take soundings with a lead line,but this is a thing of the past,it is a lead weighing 7 - 14 pounds and marked to 25 fathoms.
Lines are generally marked as follows:
2 fathoms from the lead, with 2 strips of leather
3 fathoms from the lead, with 3 strips of leather
5 fathoms from the lead, with a white rag
7 fathoms from the lead, with a reg rag
10 fathoms from the lead, with leather having a hole in it
13 fathoms from the lead, same as at 3 fathoms
15 fathoms from the lead, same as at 5 fathoms
17 fathoms from the lead, same as at 7 fathoms
20 fathoms from the lead, a line with 2 knots
25 fathoms from the lead, a line with 1 knot
Fathoms which correspond with the depths marked are called marks and intermediate fathoms are called deeps. The only fractions of a fathom used are a half and a quarter.
A practice sometimes followed is to mark the hand lead line in feet at the critical depths of the vessel using it.
Lead lines should be measured while wet and the correctness of the marking checked.
A lead may be armed by filling its hollow lower end with tallow, by which means a sample of the bottom sediment is brought up.
