The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

*ompass on shore is not disturbed ~ by the iron of the ship; care must bé taken that no magnetic influence exists in the vicinity of the compass to attract it, or the work performed by it will be in error. MUST BE FAR DISTANT, : It must be remembered that the ob- ject selected for the purpose of taking the bearings from must be at such a distance from the ship that the diame- ter of the space through which she re- volves will make no sensible difference in its real bearing. If the ship is swinging to her anchors a well defined object from 6 to 8 miles distant will answer the purpose, but if a nearer ob- ject be used the bearing in each case will be affected by parallax, which in a case of this kind, may be~-defined as the difference in the bearings as seen from two different stations. STEADY HER -HEAD ON EVERY POINT. The requisite means being prepared, the ship is to be gradually swung round so as to bring her head _ successively upon each of the thirty-two points, or directions, of the compass; and as her head approaches each of these points ner motion should be gently checked. When the ship is quite steady and her head ex- actly on one point the direct bearing of the object is to be observed 'with the standard compass and registered ac- cording to the form shown in the first two columns of the accompanying specimen : FORM FOR REGISTERING THE PROCESS BY DIRECT BEARING, Correct magnetic bearing of Chimney from ship N. 82 degrees W., distant 7 miles. 1 2 8 ' Bearing of Ship's head Chimney by Deviation by Standard the Standard of the Stand- Compass. Compass. ard Compass. North N 70° W 12° Wly ~ N by E N 75 W Coe NNE N 80 W Dae NE by N N 8 W 3 Ey. NE N 89 W Tet NE by E N 91 W Qe et ENE N95 W 1S E by N N 97 W 150 East N 98 W 162 5 And so on with every point of the compass. The ship's head is: to -bée gently brought round in -the same manner to the next point and, when duly stopped and steadied there, the bearing of the Satine object. is to be again set and again recorded, and so on, point after point, till the exact bearing of the one object has been ascertained with ship's head on every separate or alter- nate point of the compass. Then the difference between the correct magne- tic and compass bearings on each point will be the deviation. Thus, the difference between N. 70 degrees W. and N. 82 degrees W. is 12 degrees and it is named Wly because the correct magnetic bearing (N. 82 ae- grees W.) is to the left of the compass {ac TAE Marine. REVIEW bearing (N. 70 degrees W.). Compare each compass bearing with this same correct magnetic bearing, because this is really the compass bearing it would have from on board if the compass was not disturbed by the attraction of the iron in the ship. The difference be- tween this correct magnetic bearing of the object and the bearing observed with the standard compass on board when the ship's head is on any particu- lar point will show the deviation on that point which was caused by the ship's iron, or, in other words, the de- viation of the standard compass accord- ing to the direction in which .the ship's head was placed. The deviations thus found are named Ely when the compass card is drawn to the right hand by the attraction of the ship's iron, and Wly when it is drawn to the left hand of the magnetic meridian. The employment of the terms Ely and Wly being precisely analogous to that of Ely and- Wly variation. TO DETERMINE THE DEVIATION BY THE BEARING OF A DISTANT OBJECT, THE CORRECT MAGNETIC BEARING BEING UNKNOWN. This may generally be effected by taking the mean of all the observed bearings, or of four or more bearings observed on equi-distant points. Take the bearings of the d'stant ob- ject on 6 equi-distant points; divide 'the sum of the bearings by 8; the result will be the (approximate) correct mag- netic bearing. Then the difference be- tween the correct magnetic and com- pass bearing on each point will be the deviation. Thus: Ship's Head by Standard Com- Bearing of Distant Object by Standard pass Compass. Sr oa N 98° W Soul 6 N 94° W BW ee N 83° W West 22. ce N 66° W NW ee ae N 58° W North. NO AW NE ee N 89° W. ete Pe eee N 98° W 8) 656° : N 82° W=correct Magnetic bearing. Correct Magnetic Bearing. Deviation. Ne 16° Ely ee io Be ee ee ae i Re ee 16° Wly i eee a A ee ee oe Bo eee ey Scie ly CS it The deviations thus obtained being simple process. 31 laid down on Napier's diagram, the devi- ations on the. other points of the com- pass can be obtained, as also the course tO Speer: This method of determining the cor- rect magnetic bearing by adding the com- pass bearings and dividing this sum by the number of observations taken, is the- oretically correct, but not practically cor- rect, for the reason that there is nearly always a small error of constant effect which does not vary either in name or amount, no matter what may be the di- rection of the ship's head. regarded as a sort of compass index- 'error, and is due to one or other of sev- eral causes, such as errors of observa- tion, want of symmetry in the disposi- tion of neighboring masses of iron, needles not being truly parallel to the north and south line of the compass card, incorrect estimate of the variation, and a slightly misplaced lubber-line. There is another error that plays a prominent part in this matter, and is the result of magnetism, induced when the ship's head was on one point of the compass, being retained when her head is swung to some other point. It makes a difference which way the ship is swung, whether to the right or to the left (with or against the sun), being plus in one case and minus in the other. The value of these errors is greatest when the ship is swung quick- est. We have already referred to this matter in this chapter (in the run from Chicago to Pt. Betsey when the compass did not settle to the course at once) and we will further explain it under Compass Adjustment. Experience and theory, says Towson, have convinced us that a more satisfac- tory result might be obtained by a more Instead of adding the eight observed bearings together and di- viding by 8, add together the bearings with the ship's head east and west, and _ divide the sum by 2: 98° +66° ----_---- = 82° 2 In this case the two examples agree, but they will not in every case. ° The latter method is founded on two reasons. First, because the deviations on all points of the compass, but east and west, are affected by the ship's heel, pro- ducing heeling error. But when ship's head is either east or west there is no heeling error. Secondly, the error due to retained magnetism (from the fact that the ship's iron producing mag- netism does not gain or depart with its magnetism at once, and varies according to. circumstances). the ship's head east or west the error due to retained magnetism is of an opposite name to the index error of the compass, and, therefore, cance! c1ch other. It may be the e But, however, with.