EXAMPLES FOR PRACTICE.

Required the difference of longitude between a place A and a place B in each of the following examples :—

89. To find the longitude in, having given the longitude from (or left) and the difference of longitude.

RULE XXIX.

1o. When the longitude from and the difference of longitude have like names-To the longitude from add difference of longitude (turned into degrees, if necessary): the sum, if not more than 180°, will be the longitude in, of the same name as the longitude from; but if the sum exceed 180°, subtract it from 360°, and the remainder is the long. in and of a contrary name to long. from.

2°. When the longitude left and difference of longitude have unlike names-Under longitude from put difference of longitude (in degrees and minutes, if necessary); take the less from the greater; the remainder, marked with the name of the greater, is the longitude in.

NOTE.-If the ship is receding from the first meridian, her longitude is evidently getting greater; if approaching it, less.

In this example the ship from a place A in W. long. has made 113 miles diff. long. West, and, consequently, increased her West long. by 1° 53′ (see Fig. 22).

In this example the ship from a place A in long. 2° 30′ E. has made 5° 30′ diff. long. West, and thereby crossed the meridian of Greenwich, having run from East into West longitude (see Fig. 23).

In this example the ship has gone from Eist into West longitude, and by crossing the meridian of 180° has changed the name of her longitude from East to West (see Fig. 24).

Required the longitude in, or arrived at, in each of the following examples:

13. Define meridian of the earth, equator, parallel of latitude. great circles, and why?

THE COMPASS.

90. The Compass is simply an instrument which utilises the directive power of the magnet. A magnetised bar of steel, apart from disturbing forces and free to move, points in a definite direction, and to this direction all others may be referred, and a ship guided on any desired course.

There are various adaptations of the instrument, according to tho use it is specially intended for. The compass intended for use on board ship is called the "Mariner's Compass," and according to the purpose it is intended for it is named the Steering Compass, the Standard Compass, and the Azimuth Compass.

91. The Mariner's Compass consists of a circular card, which represents the horizon of the observer; the circumference or edge of the card being divided according to two systems of notation into points and degrees.

The origin of the compass is very obscure. The ancients were aware that the loadstone attracted iron, but were ignorant of its directing property. The instrument came into use in Europe sometime in the course of the thirteenth century.

(1.) By Points.-There are 32 points; and each of those divisions is again sub-divided into four parts called quarter points. A point of the compass being therefore the 32nd part of the circumference of a circle is equal to 11° 15'. The four principal points, or, as they are called, the cardinal points, are the North (represented by N.), South (S.), East (E.), West (W.), the East being to the right, and West to the left, when facing the North.

All the points of the Compass are called by names composed of these four terms.

Thus, the points half way between the cardinal points are called after the two adjacent cardinal points; hence the point midway between the North and East is called North-east, and represented by N.E.; so midway between South and East is called South-east (written S.E.); in like manner we get South-west (written S.W.), and North-west (written N. W.)*

A point half-way between one of these last and a cardinal point is called, in like manner, by a name composed of the nearest cardinal point and the adjacent points, N.E., N.W., S.E., and S.W. Thus, the point half-way between N. and N.E. is called North-north-east (written N.N.E.); the point between E. and N.E. is called East-north-east (written E.N.E.); and so we have E.S.E., S.S.E., S.S.W., W.S.W., W.N.W., and N.N.W. The points next the eight principal points, namely, N., N.E., E., S.E., S., S.W., and N.W., are named by placing by between the letter representing the point to which it is adjacent and the next cardinal point in the same direction. Thus, the point next to N., on the east side, is called North by East, i.e., North in the direction towards East (written N. by E.); that next N.E., towards the North, is called North-east by North (N.E. by N.), i.e., North-east in the direction towards North; and so we have N.E. by E., E. by N., E. by S., S.E. by E., S.E. by S., S. by E., S. by W., S.W. by S., S.W. by W., W. by S., W. by N., N.W. by W., N.W. by N., N. by W.; in this manner we get other sixteen points. We have thus got names to all the thirty-two points of the compass.

Each point is again sub-divided into half points and quarter points.

A half point, which is the middle division between two points, is called after that one of its adjacent points which is either a cardinal point or is the nearest to a cardinal point. Thus, the middle division between N. and N. by E. is called North-half-east (written N. E.) Half points near N.E., N.W., S.E., and S.W., take their name from these points. Thus we say N.E. N., N.E. by E. E.f

The same holds for a quarter and for three-quarters as for a half point, all of which are named upon the same principle as the subordinate points.

These new directions also give names to the four quarters of the compass, as, when we say that "the wind is in the S. W. quarter," meaning thereby not exactly S. W., but somewhere between S. and W.

+ In naming the half and quarter points it is advisable in some cases to sacrifice system to simplicity. Thus, for example, seamen commonly say N.N.E. E. instead of N.E. by N. N.; we do not, however, say E.N.E. E., though this is simpler than E. by N. † N., since it is at once seen to be 6 points. It would of course be more systematic, as a matter of geometry, to reckon the half points always from N. or S., because the ship's course is reckoned from the meridian; but on the other hand, as a matter of names, regard will be had to the whole points between which it falls, and to the order in which these are taken.