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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, 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 follows:- .
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 half-right angles, or 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, 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), E.S.E. (south-south-east), S.S.W. (south-southwest), W.S.W. (west-south-west), W.N.W. (west-north-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 thé 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, norih 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.. northeast 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. 14, “ Mariner's Compass."
You should know the compass-points, starting from any point, and going to right or left.
The space from point to point is usually divided into 4 equal parts, so that we say N. 1 E. (i.e. N. a quarter point towards E.), N. E., N. *. E., N.E. I N., N.E. N., and so forth with any point of the compass.
The angular value of cach point of the compass, or, more properly speaking, of the space from point to point, is 111° (obtained by dividing 360° by 32).
The points are frequently, in calculations, expressed in their numerical value, counting from the meridional line in the direction of E. or W.; thus W.N.W. is N. 6 points W., and its angular value in the same direction is N. 671° W. (i.e. 117° multiplied by 6); similarly, N.E. N. is N. 3 points E., and its angular value N. 39° 22E. (see p. 14.). The outer edge of the card is often also divided into degrees.
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 111°; the mode of expression always removes any doubt as to wha' is intended.
A TABLE OF THE ANGLES Which every Point and Quarter Point of the Compass makes with the Meridian,
Opposite or Back Bearings. Points. Degrees, &c. Points. Opposite or Back Bearings.
• We now know the principle on which the compass card is divided.
For practical use the card 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 produce the mariner's compass we require the aid of magnetism ; & small steel bar of proper temper 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 line, so that the N. ends of the needles shall coincide (in direction) with the N. end of that line, and the 8. 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.
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 rest horizontally whether the ship rolls or pitches.
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 moveable 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. THE STEERING COMPA88.- When the compass card, with needles attached; rests on the pivot in the bowl, and the bowl is placed in the binnacle with the lubber's line directed forward, i.e. towards the ship's head, and at the same time in the midship line, the instrument for steering purposes is complete. The point of the compass close by the lubber's line is said to be the direction of the ship’s head by compass; if the ship is under sail, it is her COURSE by compass.
An AZIMUTH Compass is an instrument similar to the steering compass, but of superior make; the card is more carefully divided, and the outside of the compass bowl is fitted with a moveable ring, to which are attached (exactly opposite to each other, and in a line with the centre of the compass card) two sight vanes that can be turned down when not in use. Sight vanes are merely oblong
pieces of brass with a vertical slit in each; the slit of one vane is very narrow, and the wider slit of the other vane is fitted with a single horse-hair for a vertical line; when you look through the sight .vanes and turn them in the direction of an object, you take the object's bearing; which is indicated by the compass card. In some cases a prism is substituted for one of the sight vanes. The azimuth compass is placed in a
binnacle like the steering one, or on a tripod removable at pleasure, and in the latter case the compass is kept in a box (for safety) when not in use.
The terms azimuth and bearing are synonymous.
When observing bearings, if the compass is vibrating, take the mean of two or more bearings read off as quickly as convenient.
To reverse a bearing or course, by which you get the opposite point, simply reverse the letters which compose it; thus, the opposite to N.W, N. is S.E. į S.
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 thë first is preferable; you also hear E.N.E. | E. as commonly as E. by N. & N.; but you must beware of such absurdities as using E. by N. & E. instead of E. I N., or S. by E. I S. instead of S. ] E.
THE MAGNETISED NEEDLE.—What I am now about to explain is not Navigation in its strictest sense, but it is, nevertheless, fitting that you should know something of the nature of Magnetism, without which we could have no Mariner's Compass; and unless you understand some, at least, of the properties of the magnetised needle, you had better not go out of sight of land.
A simple steel bar (like that used on a compass), if delicately suspended at its middle by a single fibre of silk so as to rest only in a horizontal direction, would turn as you turned it, and rest there: not so after you have rubbed its ends on a magnet, and made it a magnetised needle. Then—let us say in London-however you pushed it about, it would always come to rest with a particular end towards the north, but at the same time that particular end would dip downwards at an angle of 671° with a horizontal line passing through its centre. It does this in virtue of the magnetic force it has acquired, and which is at once horizontal and vertical. Elsewhere it might point in a different direction and at a different angle. Let us see: if this freely suspended needle be taken to the vicinity of the terrestrial equator, it will rest horizontally, pointing north and south nearly, but it will not dip; on leaving the equator, and going north, the particular end which pointed northward at the equator will continue to do so, but as you advance more and more north it will dip more and more; and if, on your voyage northward, you came to a spot where the needle had no horizontal direction, but stood vertically, in a line with the suspending silk fibre, you would then be at the magnetic pole of the northern hemisphere, which was reached by Capt. James Ross, the Arctic explorer, in
1831. In the southern hemisphere, the freely suspended needle acts in the same manner as I have just described, brut there it is the end pointing southward that dips; and as this end would be upwards in the northern hemisphere, so the north trending end is upwards in the southern hemisphere,
You will now understand that an irregular curved line around the earth, at or near the terrestrial equator, where the magnetised needle rests in a perfectly horizontal position, is appropriately called the magnetic equator, or line of no dip; that the direction in which the needle trends, which is more or less in the direction of a terrestrial meridian, is a magnetic meridiun; and that the two places, one in each hemisphere, where the needle stands : vertically, ate the magnetic poles, and the dip 90°—that being the angle the needle makes with the horizon. At any intermediate station between the magnetic equator and magnetic poles the dip will, of course, be at some angle between 0° and 90°.
All the magnetic elements have periodical and secular changes of slight amount, which are only distinctly perceptible after a lapse of time.
In sailing, the vertical force is of no use; we want horizontal direction, hence the horizontal magnetic force is what is utilised in the mariner's como pass, and to counteract the effect of the vertical force, which gives the dip, small copper slides are attached to the needles, or pieces of brass to the under side of the card.
Does the Mariner's Compass give us the true direction of the various points of the horizon?—is the first question to be answered, and that brings us to the
VARIATION OF THE COMPASS.. When off Greenhithe on the Thames, or off Cowes, if you look at the Pole Star (as showing the true North very nearly),* and note its position by compass, you will find it to bear a little northward of N.N.E. (about N. 19° E.): or, if you look at the sun at noon, when it is on the meridian and at its greatest altitude, and consequently true South of you, it will be found to bear by compass S. 19° W. How is this?—I will explain. In neither hemisphere do the true and
magnetic poles occupy the same place. The magnetic pole of the northern - hemisphere is 1200 miles south of the earth's true N. pole, and the magnetic pole of the southern hemisphere is 990 miles north of the earth's true $. pole. But the magnetic needle points to the magnetic, not to the true poles ; therefore, referring to Fig. 3, if we call N. the true north pole, n the magnetic pole, and Na the true meridian of the place a, it will be seen that if the magnetic needle at a points to n, its direction lies at an angle (n a N) with the true meridian ; but the direction of the needle at any place is the
* The Pole Star (Polaris in the Little Bear, which is easily found by means of the printers in the Great Bear or Charles' Wein) is 11° off the true celestial pole, and therea fore true North only twice in 24 hours; but it is always so nearly true North that for practical purposes at sea, when in the northern hemisphere, it may be taken to show true North.