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1. Instruments

observed)

Introduction. The navigator of a ship uses distances, bearings, courses, time, and rates at every turn. How does he measure these quantities? When far from land how does he know the direction north? How does he know how fast the ship is steam­ing? How does he find the bearing of an object and its distance? How does he find the azimuth of a star and its altitude? With instru­ments. This chapter describes briefly some of the most important instruments used on shipboard and indicates the reason why they are effective. Before these instruments are described, we shall define precisely some of the quantities measured by them.

Half line

Fig. 2

(Ob server'i position)

Fig- 1

Definitions. The direction angle of a line segment (or ray) emanating from a point on the earth's surface is the inclination of the line to some reference line. It is measured clockwise from 0° at the reference line to 360° (See Fig. 1).

Three reference lines used in this connection are discussed in appropriate parts of this chapter. From a point (observer's posi­tion) on the earth's surface the bearing of another (object observed) is the direction angle of the line (line of sight) connect­ing the first point with the second (See Fig. 2).

The course С is the direction prescribed for the ship's movement or progress. It is defined also as the direction angle of the

line along which the ship sails from one place to another (See Fig. 3).

The ship's heading (S. H) is the direction angle of the line (or ray) containing the keel (assumed horizontal) at any particular mo­ment (See Fig. 4).

Ship's keel Fig. 4

True (geographic) meridians are semi-great circles of the earth passing through the north and south geographic poles.

True north is toward the geographic north pole along the tangent to the true meridian of the observer.

True North

True course

True direction angle

line Observer

True North

i

True North

Fig. 5

Direction angle, bearing, course, and heading when referred to the true (geographic) meridian are called respectively, true direc­tion angle, true bearing, true course and true" heading (See Fig. 5).

The compass. The compass is one of the most important instruments used by a navigator because it indicates direction. For centuries it has been the principal source of that knowledge of direction upon which the navigation and the safety of ships depend.

There are two general types of compasses, the gyro compass and the magnetic compass. Attached to each is a compass card.

Radio direction finder. This instrument consists essentially of a large coil A (See Fig. 6) connected to a dial, which registers the direction of an incoming radio wave. In operating a radio direction finder the coil is revolved until the reception is minimum and the direction of the beam is then read from the dial. Radio beams are sent out by shore stations located at inter­vals along the coasts.

Magnetic compass. The magnetic compass is an instru­ment consisting usually of a group of magnets, attached to a compass card pivoted at the center and allowed to swing freely in a horizontal plane. The needles point in the di­rection of the earth's lines of force when not affected by magnetic disturbances within the ship. Modern compasses are set in a bowl filled with liquid to steady the card. On the inner surface of this bowl is marked a vertical line called the lubber's line. The compass is so mount­ed that a plane containing the pivot and the lubber's line is parallel to the keel (considered horizontal) of the ship. Hence the reading on the card opposite the lubber's line is the ship's heading with reference to the axis of the compass card. The bowl is mounted in a binnacle. Com­pensating magnets contained in the binnacle and hollow iron spheres mounted on either side of the compass reduce certain compass errors.

Azimuth circle. An azimuth circle is a graduated ring made to fit over the compass bowl, and free to turn about a vertical axis coin­ciding with the vertical axis of the bowl. It is used to mea­sure the bearings of objects and the azimuth (bearing) of the sun. On the azimuth circle the rays of the sun are received upon a convex mirror, and reflected through a right-angled prism on the opposite side of the ring to the lens below, appearing on the card as a bright bar of light upon the graduations. This can readily be seen when the sun is much obscured, enabling accurate readings to be made. At right angles to this are placed the usual sight vanes and the reflector for dim sun observations, or for taking bearings. One of these vanes attached to the other circumference of the ring has a peep sight, the other diametrically opposite it has a vertical sighting wire. The double arrangement enables the circle to be tested for accuracy by com­paring one reading with the other. The inner rim of the circle is grad­uated from 0° to 360°.

P e 1 о r u s. A pelorus is essentially a compass without magnets and equipped with an azimuth circle. It facilitates the observing of bearings from convenient positions on the ship. A portable type mounted in a square wooden box may be moved to any position. More elaborate instruments are permanently mounted at points on the ship that afford a clear vision, as on the winds of the bridge.

Definitions. At any place on the earth's surface the line having the direction assumed by a compass needle when under the influence of the earth's magnetic force only is called the magnetic

meridian of that place. For example, if at sea a compass were placed on a wooden ship containing no iron or other magnetic- substance, the compass needle would point in the direction of the magnetic meridian. It is to be regreted that the word meridian is used in this sense as one generally associates meridian with a semigreat circle. Perhaps a better name for magnetic meridian would be magnetic line.

Magnetic north is in the general direction of north along the magnetic meridian of the observer.