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be applied), much greater steadiness, no swing to and fro, more sensitiveness, and as a consequence the ability to steer a very straight course.
The Disadvantages are : Expensiveness, the need of electric installation, some knowledge required of electricity to be able to work the attachments in ordinary working or a breakdown.
A breakdown in the electric current would put the compass out of use.
THE MARINER'S COMPASS
When out of sight of land, the MARINER'S COMPASS is the only instrument that shows the DIRECTION in which the ship is moving; it is therefore necessary to understand its construction and use, together with the corrections which must be applied to its indications.
In the first place you must learn the divisions of the compass card, and know how to box the compass, that is, be able to repeat the points in their order, commencing at any given point, and going to right or left.
DIVISIONS OF THE COMPASS CARD.—The Compass Card is a circle divided into 32 equal parts, called points of the compass; and standing on the deck of a vessel at sea you must always suppose yourself to be in the centre of such a circle, the circumference of which is the visible horizon, and towards some point of which the vessel's head is directed. The standard of direction is the meridian passing through the place of the vessel, and a small part of this meridian is represented on the compass by a line drawn from one part of the circumference to another,-passing through the centre of the circle: One end of the meridional line is named North, because the line trends in direction towards the pole of that name, and the other end, directed towards the south pole, is named South. A second line, passing through the centre of the circle, but at right angles to the meridian, has one end named East, and the other West. When facing the North, the East (in which direction the sun and other heavenly bodies appear to rise) is on your right hand, and the West (towards which the same bodies set) is on your left. Hence the letters N., S., E., W., represent the four cardinal or chief points of the compass—corresponding to similar points of the horizon; and the two principal lines of which we have spoken (and which give these four points), by cutting each other at right angles, divide all the horizontal space around you into quarters, called quarters of the compass.
All the other points are named after the cardinal points as follow:
By halving (bisecting) each of the four quarters of the compass, and drawing four lines in a new direction from the centre of the circle to the circumference, we get 4 more points; the N.E. (north-east) point, midway between N. and E., and so on with S.E. (south-east), S.W. (south-west), and N.W. (north-west), all midway between cardinal points; these new directions also give names to the four quarters of the compass, as when we say “the wind is in the N.E. quarter,”—meaning thereby not exactly N.E., but somewhere between N. and E.
Thus far, by halving the four quarters, or right angles, we have 8 halfright angles, thus making 8 points of the compass; and proceeding in a similar manner to halve these, we get 8 additional lines, which derive their names from the points to which they are contiguous, always placing first the nearest cardinal point, as N.N.E. (north-north-east), midway between N. and N.E.; E.N.E. (east-north-east), midway between E. and N.E.; and so on with E.S.E. (east-south-east), S.S.E. (south-south-east), S.S.W. (south-south-west), W.S.W. (west-south-west), W.N.W. (westnorth-west), and N.N.W. (north-north-west).
We have now 16 points of the compass, and the remaining 16 (to make up 32) are also derived by a similar process to that already explained, viz., halving the 16 angles last obtained, and the new points are named after the 8 principal ones by writing the word by before the next nearest cardinal point, as when we say N. by E. (north by east, i.e., north in the direction towards east) for the point midway between N. and N.N.E.; N.E. by N. (north-east by north, i.e., north-east in the direction towards north) for the point midway between N.N.E. and N.E.; and so forth, as best seen by looking at the compass points in their order on p. 73, “ Mariner's Compass.'
The space from point to point is usually divided into four equal parts, so that we say N. 1 E. (i.e., N. a quarter point towards E.), N. 1 E., N. E., N.E. I N., N.E. I N., and so forth with any other point of the compass.
Note on expressing Half and Quarter Points.—There is no fixed system for expressing the divisions between two points, but the simplest (learnt by practice and attention) is the best ; thus N.N.W. } W. is the same as N.W. by N. } N., but the first is preferable ; you also hear E.N.E. | E, as commonly as E. by N. IN.; but you must beware of such absurdities as using E. by N. 1 E. instead of E. I N., or S. by E. 1S. instead of S. } E.
The angular value of each point of the compass, or, more properly speaking, of the space from point to point, is 11° (obtained by dividing 360° by 32). The outer edge of the card is often also divided into degrees, and it is well to be able to convert points into degrees at sight, without direct reference to the compass card.
The points are frequently, in calculations, expressed in their numerical value, counting from the meridional line (N. or S.) in the direction of E. or W.; thus W.N.W.is N. 6 points W., and its angular value, in degrees, in the same direction is N. 671° W. (i.e., IIļo multiplied by 6); similarly, N.E. 1 N. is N. 3} points E., and its angular value N. 39° 22}' E. (see Table of Angles, Plate III).
Finally, it is to be observed that the term point is commonly used as expressive of two different things; thus, when the question is put "How is her head ? ” and the reply given is S.E., the meaning is that the ship is sailing towards the S.E. point, i.e., direction; but when the helmsman is told to “bring her up a point,” in this case the meaning of point is a change of direction through an angular space of 11°; the mode of expression always removes any doubt as to what is intended.
The latest pattern compass is now graduated in degrees, and reckoned continuously from oo at north through E., S., and W. to 360° at N.
Such is the principle on which the compass card is divided, and it is of the first importance that you make yourself master of its indications, both as to the points, and the rendering of these into degrees, as they are of constant application in Navigation and Nautical Astronomy.
The COMPASS CARD—HOW MADE.- For practical use the card, on which the points and degrees are figured, is made of stiff cardboard, or mica covered with paper, so as to be as light as possible ; and before mounting it is not unfrequently termed the fly.
But to turn this card into the mariner's compass it is necessary to resort to magnetism ; a small well-tempered steel bar is fully magnetised by drawing its opposite ends (from the middle in the direction of the ends) across the two opposite ends of a powerful magnet; the bar thus acquires what is called polarity, and when suspended, but free to move only in a horizontal direction, its tendency is to rest, one end towards north, the other towards south; and the same end will invariably turn (at any given place) towards the same point of the horizon, not indifferently, sometimes to one point, sometimes to its opposite. Two or more such bars, called magnetic needles, are fixed below the circular compass card, but parallel with its meridional (N. and S.) line, and in such manner that the North-seeking ends of the needles shall coincide (in direction) with the N. end of that line, and the South-seeking 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, when accurately balanced on a pivot, will rest horizontally.
COMPASS BOWL.—The compass bowl is of brass or copper, and sufficiently large to admit of the card moving freely within it when placed on a hard metal pivot rising from the middle of the bottom of the bowl; the point of the pivot on which the cap of the card rests should always be sharp and smooth; 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 ; finally, the bowl, weighted at the bottom, is hung in gimbals, so that it shall always rest horizontally whether the ship rolls or pitches.
When long and powerful needles were used it was customary, and, indeed, necessary, to make the bowl of pure copper, and to leave as little clearance as possible for the needles; then as the compass swung a magnetic current in the opposite direction was induced in the copper bowl, thereby bringing the compass rapidly to rest without any loss of directive force.
In a Thomson compass the bowl is made of brass and weighted underneath so as to keep it steady by means of an expansion chamber partly filled with castor oil.
The BINNACLE.—To the deck, in front of the helmsman's position, there is firmly fixed a stand called a binnacle, which may be of any shape-square, octagonal, or pillar-like—sometimes of wood, sometimes of brass (like the annexed figure); within it are supports (bearings), on which rest the gimbals of the compass bowl; and its movable top or cover is furnished with a glass front, and a lamp, or perhaps two lamps, to cast a light on the compass card by night.
Note.—The compass will be perfect in its indications (or readings) conditionally ; (1) that the divisions of the points are equidistant; (2) that