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

1. Long. from 5° 48′ W. D. long. 110' W. 7. Long. from 41° 29′ W. D. long. 139′ E.

2.

"" • 59 W.

39 137 E.

8.

220 W. 242 E.

94 4 E. 98 54 E.

115 W. 302 E.

12 E.

177 6 W.

" 237 W.

237 W.

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

Which of these are

THE COMPASS.

176. 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 a'l others may be referred, and a ship guided on any desired course.

There are various adaptations of the instrument, according to the 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.

177. 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.t

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.

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.

In chosing the name to use we must be guided by circumstances. In some problems it is convenient always to reckon uniformly from North or South, but generally the simpler name will be the preferable one; and similarly for quarters and three-quarters of a point.

(2). By Degrees.-The whole circumference is divided into three hundred and sixty degrees (360°), each degree into sixty minutes (60'). This furnishes a notation for the compass more minute than points, half points, and quarter points. We still reckon from the cardinal points: thus, to indicate a division which has 72° 48' to the East of North we write N. 72° 48′ E.

178. The name of the opposite point to any proposed point is known at once without referring to the compass, by simply reversing the name or the letters which compose it-thus, the opposite of N. being S. and of E. being W., the opposite point of N.E. by N. is at once known to be S.W. by S., the opposite of W. S. is E. † N., and so on.

179. Repeating the points in any order is called boxing the compass; to do this is, of course, one of the first things a seamen learns.

180. As the ship's course, which is sometimes expressed in points and sometimes in degrees, is always reckoned from the North or South point, the seaman has to refer at once, in using the Tables, to the number of points or degrees in any course given by name. The following table, which exhibits the degrees, minutes, and seconds in each quarter point of the compass, will be convenient for reference.

A TABLE OF THE ANGLES,

which every Point and Quarter Point of the Compass makes with the Meridian.

181. The card for practical use is generally made of mica covered with paper, so as to be as light as possible. Two or more magnetic needles,* which are small steel bars magnetised, are fixed below the circular compass card, but parallel with its meridional line, so that the N. ends of the needles shall coincide (in direction) with the N. end of that line, and the S. ends of the needles with the S. end of the same line. An inverted conical brass socket, called a cap, with a hard stone in its centre, is passed through a hole in the centre of the card, and the whole is then accurately balanced on a sharp centre or pivot rising from the middle of a brass or copper bowl, and sufficiently large to admit of the card moving freely within it: the cover of the bowl is glass, which, while protecting the card from wind and weather, admits of its indications being distinctly seen. There is also a vertical line

drawn inside the bowl which is called the lubber's line. The bowl, having a weight fixed to it below, is placed in gimbals, which are brass hoops or rings so arranged as to admit of motion about two horizontal axis at right-angles to one another, i.e., each turning upon two pivots at opposite points of the hoop next greater in size; by this means the loaded bowl remains nearly horizontal during the confused and irregular motion of the ship.

To the deck, in front of the helmsman's position, a stand called a Binnacle is firmly fixed, which may be of any shape-octagonal, square, or pillar-like -sometimes of wood, sometimes of brass: within it are supports or bearings into which the pivots or outer rings of the compass bowl fit, and its movable top or cover is fitted with a glass front and a lamp or lamps to cast a light on the compass card by night. This constitutes the Steering Compass.

182. The helmsman steers the ship so that a line parallel to the keel passes over the centre of the card, and the point prescribed as the course. Care is taken to place the box so that the lubber's point in the bowl and the centre of the card are in a line fore-and-aft, or parallel to the keel; but as the lubber's point deviates a little from its proper position when the ship is heeled over, seamen do not implicitly depend upon it, as, indeed, the name implies.

183. The Azimuth Compass is a compass of superior construction, particularly adapted to observe bearings. It is mounted on a stand, and is fitted with two small frames carrying vertical wires, called sight-vanes, for the purpose of observing objects elevated above the horizon. In one of these vanes there is a long and very narrow slit, and in the other is an opening of the same kind, but wider, and having a wire up and down the middle of it, exactly opposite the slit.

184. In the best modern instruments, a horizontal ring is expressly provided to carry the vertical wire frame, and instead of having a wire next the eye, a glass prism, acting by internal reflection, is placed there, so arranged that one-half of the pupil of the eye can observe the wire on the further side of the horizontal ring and the distant object, and the other half of the pupil can see the graduations of the compass card by internal reflection in the The object of using several magnets is to increase the magnetic moment of a given weight of steel.