« 이전계속 »
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. A N. is
S.E. $ 8.
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 th& 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. A E. instead of E. £ N., or S. by E. 1 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 67^° 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, but 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 e»rth, 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 meridian; and that the two places, one in each hemisphere, where the needle stands vertically, are 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 compasB, 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 Op The Compass. . When off Greenhithe on the Thames, or off Cowes, if yon 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 S. 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 printer* in the Great Bear or Charles' Wain) is 1 \° off the true celestial pole, and therefore true North only twice in 24 hours; but it is always so nearly true North that for Sractical purposes at sea, when in the northern hemisphere, it may be taken to show true forth.
magnetic meridian, and hence we call the angle that the magnetic meridian males with the true meridian the Variation Op The Compass; the angular value (large or small) is the measure of the Variation, and if the direction of the needle trends to the right of the true meridian, as at a, we say the Variation is East; if it trends to the left, as at c, we say it is West; and if it trends in direction with the true meridian, as at b, we say there is no Variation. There are two meridians, trending from one magnetic pole to the other, where the Variation is 0°; roughly speaking, one of these meridians crosses the Atlantic in a diagonal direction and thence through N. America, the other crosses Australia and Central Asia; between them, over one part of the globe, as in the North Atlantic, in the greater part of the Sonth Atlantic, and in the Indian Oceans, the Variation is Westerly; and again between them, over the remaining part of the globe, as in the whole of the Pacific Ocean, the Variation is Easterly.
Note.—It is possible that on the return of the late Arctic expedition you may have read, in some of the Journals or Reviews, that the course on which the ships were steered through Smith and Robeson straits, on their way to the North Pole, was a Southerly one. This you will at once perceive (by fig. 3) must have been the case, from the relative positions of the magnetic (a) and true (N) poles. Since the N. marked end of the needle points to the magnetic pole of the northern hemisphere, and the expedition passed beyond this pole, leaving it on their left hand, their course to the true N. pole as shown by the arrow (barbed end being N.) was to the Southward, by compass. Had they reached the N. pole, they would at first have steered Northward on their voyage home.
You can now, I think, understand how it comes to pass that at Greenhithe, where the Variation of the compass is 19° Westerly—such being the angle and direction, at that place, of the magnetic needle with the true meridian—the Pole Btar bears by compass N. 19° E., for it is the compass bearing corresponding to true North; and we at once get a—
Rule for Converting Compass Courses and Bearings into their equivalent True Courses and Bearings—and you must remember that every couri-e or bearing by compass is alike affected by Variation, and to the same amount, for the place where you are. Also remember, once for all, that you are to look upon yourself as standing in the centre of the compass—as the centre of the horizontal plane ;—or, as I am about to use the terms right and left, their significance on the face of the compass card is—to the right is as the hand of the clock goes, and to the left the other way. Vabiation Westerly, allow it to the left of the Compass Course, or Bearing. Variation Easterly, allow it to the right of the Compass Course, or Bearing.
We will correct some compass courses and bearings:—
with Var. 2 J pts. W. gives Tr. Co. N. by E. IB. or N. 1| pts. E.
N.W. by W. i W. or N. 5J W.
W. f S. or S. 7J W.
8.E. f S. or S. 3} E.
If you have carefully examined and tested the foregoing examples, you will now know how to make the correction for Variation; and I will proceed to draw your attention to another correction of vital importance which must be applied lo compass courses and bearings, in cases where the ship is of iron, or where any iron, even in a wood-built vessel, is placed or fitted in undue proximity to the compass. I refer to the correction due to the—
Deviation Of The Compass.
So long as ships were built entirely of wood, the effect of iron on the compasses was not much heeded; but there is no doubt that many a fine vessel has, in times gone by, been lost through ignorance on this point. The introduction of iron in the construction of ships drew special attention to the subject, because compass courses that navigators had been accustomed to set often took them wide.of the direction they intended it should do.
I will explain what is meant by Deviation. I dare say you are well aware that if iron is brought close to a magnet, the two come together, as if with a slight force, showing attraction; you already know (p. 15-17) that the ends of the magnetic needle have different polarities, but so has a bar of soft iron when held vertically; and iron, when in any position, if near a compass, will affect it to a greater or less extent. Ships called composite (partly wood and partly iron), and such as are built wholly of iron, are strongly magnetic, and the effect on the compass is that it produces an error called Deviation; not that it is an error, strictly speaking, for the compass is only acting in obedience to a law of magnetism, but for the practical purposes of navigation it may be an error leading to serious consequences. Now, exactly as the magnetic needle, unaffected by local surroundings, points to the magnetic pole, forming an angle with the true meridian, so, in like manner, the same needle, when under the influence of an iron ship's magnetism, forms an angle with the magnetic meridian; and this is called the Deviation Op The Compass.
As I shall have, at a later period, to draw your attention more particularly to the magnetic character of a ship, I shall here speak only of the effect of Deviation on the compass, and the method of applying the error arising from it.
Unlike Variation, which, for any given place, is of the same amount, and in the same direction on every point of the compass, Deviation attains its greatest value on two nearly opposite points of the compass, and also, somewhere between these two points are two other points on which there is little or no Deviation; nearly half round the compass, from one point of no Deviation to the other, the Deviation is called Easterly, because the needle lies to the right of the magnetic meridian; between the same two points, on the other part of the compass, the Deviation is called Westerly, because the needle then lies to the left of the magnetic meridian. A glance at the following Deviation Card will illustrate this. N.E. by N. and S. by W. are the points of no Deviation; between them, round by eastward, the Deviation is Westerly, and it attains its greatest amount at S.E. by E.; from the same two points, but round by westward, the Deviation is Easterly, and attains its greatest amount at W. by S. Thus the Deviation differs, not only in amount but in name, for different directions of the ship's head. Also, bear in mind that every iron ship's compass has deviation peculiar to itself, the direction and amount of which for its various points must be ascertained by what is called swinging ship for the errors of the compass.
But our present purpose is to know how to apply the Deviation, and the rule is somewhat similar to that we use when correcting for Variation of the compass.
I must, however, in the first place introduce to your notice a term we have not as yet required, viz., Correct Magnetic. You can understand that before