cides with a meridian when sailing true north or south, and with a parallel when sailing true east or west. The angle between the rhumb line and the meridian is the course.

THE COMPASS.

Before taking up the actual problem of navigating the ship we must consider the compass and its behavior, together with its corrections. A short description of this most important navigating instrument will be sufficient; its action and errors, however,

will be explained at greater length. The mariner's compass consists of a circular brass card C (Fig. 3), covered with white enamel, on which are marked the points and degrees as shown. The figure shows marks at 10 degrees, but a regular compass has the one degree marks as well. The smaller compasses often do not have degree marks, but simply the points. B represents the edge of the compass bowl, which swings in gimbals and is carried in a binnacle. A is a spheroidal air vessel, N are the bundles of magnetic

90. Cases III and IV are usually solved by first obtaining formulas connecting the given parts and a required part, but as we have not deduced these formulas we will solve these cases by breaking up the oblique triangles into right triangles.

91. Case III. b = 35.31; c = 42.18; A = 31° 26'.

From the right triangles we have the following:

Factoring (b+a) (b− a) = (AD+D B) (A D — D B)

= C(AD-DB)

Hence

AD-DB=

(b+a) (b− a)

e

(2)

- DB

D B we

By taking half the sum of AD+DB and A D find a value for AD. By taking half the difference of these same

needles. The card, needles and air vessel are fastened rigidly together and rest on a pivot at the center of A, the bowl being filled with alcohol and covered by glass.

Fig. 4.

00

BINNACLE.

Fig. 4 shows a compass in a box, and Fig. 5 an improved form of compass card.

The circular card is divided into degrees and points to represent every different point of the horizon. Every circle being divided into 360 degrees, and there being 32 points to the compass card, the angle between the single points will be 2 of 360°, or 11° 15'. The points of the compass are as follows, commencing at the north point and going around right handed:

Each point is further divided into quarter points; for instance,

a point E of N is called NE, a quarterway between SW and SW by S is SWS, and so on. These points, or "boxing the compass," as it is called, should be well learned.

A course is spoken of commonly as a 1, 2, 3, or 4 point course, as the case may be; for instance, N by W is a 1 point course, NNE, SSE, SSW, NNW are 2 point courses, NE, SE, SW and

NW are 4 point courses. It will be seen at a glance that the notation is not difficult.

Naming the Course. As already stated, the course is the angle reckoned from the N or S towards E or W; for instance, if a ship is steering by compass N by E, the course is a one-point course, and is one-point to the right of N. E by S is not a ninepoint course, but a seven-point course, seven points to the left of S.

MS

SE

જેકટ રનેટ કેટલી છ સાત બા

Fig. 5.

E and W are eight-point courses, in reality, although never spoken of as such.

On the inside of the compass bowl is a vertical black line in the direction of the ship's bow; this line, called the lubber line, shows the helmsman the direction of the ship's head. If, for example, he is told to steer SSE, he puts the helm over so as to bring the lubber line opposite SSE on the compass card.