The Mariner’s Compass.—Every one is more or less familiar with the use of the mariner’s compass, the instrument which enables us to know the direction in which a vessel is sailing when out of sight of land.
Fig. 45.
The compass card ([Fig. 45]) is divided into thirty-two points, so, as a circle contains 360°, the distance from one point to the next is 11¼°.
Sometimes a point is expressed in its angular value, that is, the number of degrees the point is west or east of the meridional line joining north and south. Thus N. by E. would be called N. 11¼ E., and N.E., N. 45° E.
Compass bearings are calculated to quarter points, as N.E. ¼ E., which signifies N.E. and a quarter of a point towards E.
The compass card moves freely on a pivot in the compass bowl, which in its turn is balanced on gimbals, enabling it to preserve a horizontal position despite the vessel’s rolling and pitching.
The binnacle or case in which the compass bowl is swung is provided with a lamp, so that the man steering can read the compass card by night.
Inside the bowl is a small mark, called the “lubber’s line,” which is exactly in a line with the head of the ship. That portion of the card which touches the lubber’s line therefore indicates the direction in which the vessel is pointing.