magnetic meridian, and hence we call the angle that the magnetic meridian makes with the true meridian the VARIATION OF 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 South 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 (n) 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 Greenbithe, 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 Star bears by.compass N. 19° E., for it is the compass bearing corresponding to true North; and we at once get 8– Rule for Converting Compass Courses and Bearings into their equivalent True Courses and Bearings—and you must remember that every course 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. VARIATION Westerly, allow it to the left of the Compass Course, or Bearing. VARIATION Easterly, allow it to the right of the Compass Cuurse, or Bearing. We will correct some compass courses and bearings :Comp. Co. N.E., with Var. 24 pts. W. gives Tr. Co. N. by E. I E. or N. 14 pts. E. N.W., .. gives N.W. by W. & W. or N. 51 W. 1 N. by E.. 11 gives 11 N. by W. or Ň. 1 W. Nr. S.W. by W.. gives 1 S.W. & W. or S. 41 W. 8: by W. I W. gives South. w.in., gives N W. & S. or 8. 74 W. S E. i s., gives S.E. by E. # E. or 8. 54 E. · E. by S.,' gives 11 E. & N. or N. 74 E. Comp. Co. N.N.E., with V gives Tr. Co. N.E. | N. or N. 31 E. S.W. f W, or $. 48 W. E.S.E. . E. or S. 61 E. gives N.N.W. & W. or N. 24 W. gives 8. & W. N. by W., W., gives North, S.E. by E., gives S.E. I S. or S. 31 E. in W. Š. 11 N gives all W. N. or N. 71 w. . A ACANA 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 COMPA88. 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 bas, 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'magnetism, forms an angle with the magnetic meridian; and this is called the DEVIATION OF 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 MAGNETIO. You can understand that before deviation of the compass, was recognised, a compass course or bearing and a magnetic course or bearing were considered to mean the same thing ; but inasmuch as a compass course affected with deviation is also a magnetic course, since it is indicated by a magnetic needle, it became necessary to distinguish between the two; and hence Correct Magnetic Course or Bearing is used when we speak of direction as referred to the magnetic meridian, while Compass Course or Bearing is considered to be affected with both deviation and variation. We will take examples from the following Deviation Card, in which col. 1 is supposed to represent the various directions of the ship’s head by compass in its proper position, when, by observation, during swinging ship, the Deviations of col. 2 were ascertained; col, 3 gives the courses of col. 1 reduced to Correct Magnetic. Deviation Card of the Standard Compass—Ship “ NAVIGATION." 3 3 Ship's Head Ship's Head (course) by Standard made good. made good. Compass. 1 Deviation, Correct Magnetic Course Deviation. Magnetic Course (course) by Standard Compass. Rule--To correct a Compass Course steered, take the Deviation on that point -then DEVIATION Westerly, allow it to the left to get Correct Magnetic Course. DEVIATION Easterly, allow it to the right to get Correct Magnetic Course, Express the course in degrees, when applying the deviation. Ex.-Compass course being S.E., i.e. S. 45° E., with Dev. 22° W., gives the Correct Mag netic course S. 670 E. Ex.-Compass course being West, with Dev, 18° E., gives Correot Magnetic N, 72° W. Ex.-Compass course being E.N.E., with Dev. 11° W., gives Correct Magnetic N. 561° E. Ex.-Compass course being N. by W.W., with Dev. 8; E. (half sum of 7 and 91), gives Correct Magnetic N. 81° W. You will see the whole of the Correct Magnetic courses in col. 3. Compass BEARINGS (with deviation) require to be corrected, not for the deviation on the bearing, but for the Dev, on the direotion of the ship's head (or course); thus :Object's Bearing (by compass) S.W., with ship's head N.W., the Dev. on N.W. (by card above) being 12° E., the Correct Mag. Bearing will be 8. 57° W. When correcting for Deviation and Variation, and you take them separately, apply the Dev. first, and then the Var. Rule-To set a Correct Magnetic Course by the Deviation Table: DEVIATION Westerly, allow it to the right oi the Compass Course ; DEVIATION Easterly, allow it to the left of the Compass Course ; but, in each case, taking care to allow a proper amount of Deviation, for it rarely happens that the deviation on the required course is the amount to apply; from the following examples you will see what I mean : We wish to make good a Correct Magnetic Course E.N.E.; by the card, the deviation with the ship's head on E.N.E. is 11° W.; now 11° to right of E.N.E. gives E. by N (nearly); but if you steer E. by N., which has 14° W. deviation on it, you will evidently make N. 641° E. or N.E. by E. * E. correct magnetic; so that to make E.N.E. correct Magnetic, you would have to steer a compass course a trifle easterly of E. * N. One of the most accurate methods of setting a Compass Course such that it sball give the Correct Magnetic Course required is to look down Col. 3, for the latter, and then take from Col. 1 the Compass Course that corresponds to it. Let us say that the required Cor. Mag. Course is East; now S. 893° E. (in Col. 3) is the nearest to it, therefore steer by Compass E.S.E. (from Col. i). Or, the Cor. Mag. Course is W. by N.; now W. by N. is N. 7820 W., which, in Col. 3 lies between N. 827° W. and N. 72o W.; midway between these is N. 771° W.; therefore from Col. 1, steer by Compass a little westward of W. 1 s. ERROR OF COMPASS, AND HOW TO ALLOW IT ON Compass COURSE STEERED TO GET TRUE COURSE. If you now have a clear idea of Deviation and Variation, you will understand that their sum when both have the same name, but their difference when they have different names is the ERROR OF THE COMPA88; for example Dev. 20° W. and Var. 21° W. (sum) gives Error of Compass 41° W. Dev. 9° E. and Var. 22° W. (diff.) gives Error of Compass 13° W. and note, that in taking the difference, the remainder will be E. or W. according to the name of the greater quantity. · Now, the Error of the Compass, obtained as here shown, can undoubtedly be 'applied to a Compass Course steered, or to a Compass Bearing taken, with greater facility and with less chance of making a mistake, than if Dev, and Var. be applied separately, one after the other; and I must again remind you that the Dev, must be that on the compass course steered, while the Var. will be the same in amount and name on every course or bearing. The rule for its application is the usual one : s right if Easterly. • Apply Error of Compass to the 3 I left if Westerly.... |