CHAPTER XX. USE OF NAPIER'S DIAGRAM. ART. 93. In the list of questions proposed to candidates for Masters' Certificates in the Board of Trade Regulations, Nos. 11, 12, 13, and 14 refer to the use of Napier's Diagram. Example. No. 11. Having taken the following compass bearings of a distant object, find its Correct Magnetic Bearing, and thence the deviation. Correct magnetic bearing required: S. 8° E. } S.790 W. bearing Dev. 12 E. 8. 91 W. Cor. Mag.or N. 89 W. Comp. N. 8° W. W. No. 12. With the deviation as above, construct a curve of deviation on a Napier's Diagram, and give the courses you would steer by the Standard Compass to make the following correct magnetic courses : Magnetic courses: S.S.W. W.N.W. N.N.E. E.N.E. S.S.E. Compass courses required: S. 14° W. N. 77° W. N. 2910 E. N. 79° E. S. 20° E. No. 13. Supposing you have steered the following courses by the Standard Compass, find the correct magnetic courses from the above curve of deviations : Compass courses: W.S.W. N.N.W. E.N.E. S.S.E. Magnetic courses required: S. 82° W. N. 23° W. N. 57° E. S. 25° E. No. 14. You have taken the following bearings of two distant objects by your Standard Compass as above, with the ship's head at W. S. by compass: find the correct magnetic bearings. Bearings by compass: W. by S. N. W. Magnetic bearings required: N. 89° Ŵ. N. 3° E. Explanation. In No. 11 the Correct Magnetic Bearing is the 'mean of the eight observed bearings. (a) When the bearings all read one way from S. or N., the mean is found by adding the eight bearings together and dividing the sum by 8. (b) When the bearings read different ways, viz. some towards E. and some towards W., then the mean is found by summing up the bearings of the same Name separately (as in the above example), and dividing the difference of the totals by 8. (c) The deviations are the differences between the correct magnetic bearing and each of the eight bearings by Compass, it being E. dev. when the correct magnetic reads to the right of the compass bearing, and W. when to the left. ART. 94.-Napier's Diagram and Curve of Deviations. The deviations found for the eight equidistant courses are taken with a pair of dividers from the middle line, which is graduated like a compass card, and set off from the graduated line on the dotted lines for the given courses-to the left for W. dev., and to the right for E. dev. A curve is then drawn, as in the diagram (Fig. 145). The deviation for any course (compass or magnetic) may now be found by simply measuring the part of the line (dotted or plain) intercepted by the curve. For No. 12, place one foot of the dividers on the given course, and extend the other on the plain line to the curve. Keeping one foot on the middle line, turn the other up or down according as the line inclines upwards or downwards, and read off the required compass course. For No. 13, proceed exactly in the same way as in No. 12, except that the dotted lines only are to be used. NOTE. The advantage of the diagram and curve is that it affords a ready mechanical way of changing magnetic courses to compass courses, and vice versa, and so avoids the risk of error in applying deviation by adding or subtracting. For No. 14, find the Deviation for the given direction of ship's head, by measuring the part of the dotted line intercepted by the curve, and apply it in the usual way to the bearings by compass, viz. E. to right, and W. to left. Example. (a) Magnetic bearing required: S. 53° 45′ W. NOTE.-The magnetic bearing required is the mean of all the bearings by compass, found by adding them and dividing by eight. The deviations are then marked on the dotted lines of a Napier's diagram, and a curve drawn. (b) Magnetic courses: N.N. W. S.S.E. W.N.W. Compass courses required: N. 1° W. S. 31° E. E.S.E. N. 53° W. S. 68° E. P 53° 45' Explanation.-Place one leg of the compasses on the given course, and extend the other along the plain line to the curve, and turn upwards if the line tends upwards, but downwards if it tends downwards, and read off the required course. (c) Compass courses given: N.N.E. E.N.E. S.S.W. W.S.W. Magnetic courses required: N. 6° E. N. 62° E. S.W. S. 83° W. Explanation.-Place one leg of the compasses on the given course, and extend the other along the dotted lines to the curve, and turn up or down, as the line tends. (d) Ship's head by compass: N.E. § E. Compass bearings: S.E. by S. "N.N.W. Magnetic bearings required: S. 44° E. N. 32° W. Explanation.-Find the deviation on the given compass course from the dotted line, and apply it to both bearings (east to right, west to left). EXERCISES ON NAPIER'S DIAGRAM AND CURVE. 1. (a) Correct magnetic bearing required : Find the correct magnetic bearing and the deviations. (b) Correct magnetic courses: N.E. E. E. by N. S.E. by S. 1S. N.W. Required courses to steer. (c) Courses steered: N.W. W. S. E. by S. N. by E. Required correct magnetic courses. (d) Ship's head by compass: W.S.W. Bearings of distant object: S.S.W. and N.E. 2. (a) Correct magnetic bearing required Required the correct magnetic bearing and the deviations. (b) Correct magnetic courses: E. N. E.S.E. S. W. N.W. Required courses to steer. (c) Courses steered: N. S.E. S. S. W. W. N. 35° W. Required correct magnetic courses. (d) Ship's head S. 65° E. Bearings by compass, W. 10° S. N. 40° E. Required the deviation and the correct magnetic bearings. (c) Courses steered by compass: W.N.W. N. N.E. N. S. E. N. 40° W. Correct magnetic courses. (d) Ship's head at W. by N. by compass. Bearings by compass (of distant objects): S. W. W. N. W. by W. W. Correct magnetic bearings. 4. (a) Correct magnetic. (b) Correct magnetic courses: S. 50° W. N. E. 5° S. E. by N. Courses to steer by compass. (c) Courses steered by compass: W. by S. N. by E. E. S.E. S. S. by W. Correct magnetic courses. (d) Ship's head at N.N.W. by compass. Bearings by compass (of distant objects): W.N.W. S.S.W. 5. (a) In the following table give the correct magnetic bearing, and thence the deviation. |