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THE VARIOUS CASES OF MERCATOR SAILING ARE SHOWN

IN THE FOLLOWING TABLE.

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Dep.
Distance

One lat.

distance
and de-
parture

Diff. of lat.
Diff, of long.

Diff, lat.
Diff. long

Distance x cos. course
Mer. pts. x tan. course

CORRECTION OF COURSES

DEFINITION

VARIATION OF THE COMPASS.—When off Gravesend, or off the Isle of Wight, if you look at the Pole Star (as showing the true north very nearly),* and note its position by a correct compass you will find it to bear nearly N. by E. Į 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. 17° W. How is this ?

Every bar magnet or magnetised needle has two poles, one at each end; each pole also differs in quality, or property, from the other; this may be tested by presenting a magnet end on, first to the N. end of the compass needle and then to the S. end of the same needle, when it will be found that there is repulsion of one end, and attraction of the other end of the needle ; on the principle that

Like poles repel, and unlike poles attract, one another. Now the earth being a great magnet, with two magnetic poles, it acts on this law, and attracts to its northern part the end of the needle that has magnetism unlike to that of the northern hemisphere; hence, when speaking of the compass, the end of the magnet that turns to the north is the north-seeking end.

In neither hemisphere, however, do the magnetic poles occupy the same place as the true poles of the earth. The magnetic pole of the northern hemisphere is about 1200 miles south of the earth's true N. pole, and the magnetic pole of the southern hemisphere is about 990 miles north of the earth's true S. pole. But the magnetic needle points to the magnetic, not to the true poles.

In the annexed fig. is a part of the N. hemisphere on the stereographic projection, with the terrestrial meridians radiating in straight lines from the true north pole (N.); and the position of the magnetic pole is shown at n; the magnetic meridians (dotted lines) trending towards the magnetic pole n cut the terrestrial meridians at an angle, which is the Variation of the Compass.

The direction of the magnetic needle at any place is the magnetic meridian, hence the angle that the magnetic meridian makes with the true meridian is the Variation of the Compass; the angular value (large or small) is the measure of the variation, and if the direction of the needle

* The Pole star (Polaris in the Little Bear, which is easily found by means of the pointers in the Great Bear or Charles' Wain) is 11° off the true celestial pole, and therefore 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.

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trends to the right of the true meridian, we say the variation is east ; if it trends to the left, we say it is west ; and if it trends in the same direction as the true meridian, we say there is no variation.

There are two meridians, trending from one magnetic pole to the other, where the variation is oo (nil); roughly speaking, one of these meridians crosses the Atlantic in a diagonal direction and thence through N. America; the other crosses Australia, Western Asia, and Eastern Europe ; between them, over one part of the globe, as in the North Atlantic, in the greater part of the South Atlantic, and in the Indian Ocean, 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.—The accompanying Chart of the World, showing the lines of equal magnetic variation illustrate these latter remarks. The plain lines indicate W. variation; the dotted lines, E. variation; and the thick lines no variation.

As illustrative of secular magnetic change, it may be noted that the line of no variation passed through London in the year 1657; the variation had previously been easterly ; since that date it has been westerly, and attained its western maximum (243°) in 1816; since the latter date it has been decreasing at the rate of 7' annually, and is now 163° W. The magnetic needle will again point true north about the year 1976, the cycle of change being about 320 years.

DEVIATION OF THE COMPASS.—Ships called composite (partly wood and partly iron), and such as are built wholly of iron, are strongly magnetic—due to the magnetic direction in which they have been built, and the amount of hammering and twisting to which the iron has been subjected while in that position. The effect on the compass when in its place on board is to cause the magnetic needle to deviate from the magnetic meridian; not that this 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 is an error that might lead 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 of the Compass.

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 de riation is called casterly, 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 Deviation Table, p. 339, 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. and W.S.W. Thus the deviation differs, not only in amount but in name, for different directions of a ship's head. Every iron ship's compass has deviation peculiar to itself, the direction and amount

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CHART OF THE WORLD, showing LINES of EQUAL MAGNETIC VARIATION Easterly Variation is shown by Broken Lines; Westerly Var., by Continuous fine Lines; No Var., by thick black Lines.

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