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260 direction of magnetic north, and mani- fest themselves by introducing a uni- form error affecting the deviation on every point of the compass. The placing of a binnacle out of the fore- and-aft line amidships leads to an arrangement of "soft" iron that is unsymmetrical with respect to the compass, which an inspection of Fig. 7 will show causes a constant easterly deviation on all headings. Many cases arise, however, in which a purely fictitious "constant" exists. The: United States revenue cutter Bear, after a serious collision off the Farallons in 1904, began to make landfalls on the port bow, continually fetching up to the right of her des- tination. It was assumed that the forward part of the vessel had been sprung in some manner, and this was confirmed by the fact that she car- ried a taut starboard helm. An ar- bitrary quarter-point "constant" east- erly deviation applied to the devia- tion table was found to correct all courses very nicely. Theory of Deviation Many years ago the pioneers of compass compensation evolved a very complete and elaborate mathematical theory of deviation based. on the hypothesis that the magnetism of a ship is partly the permanent mag- Metis . of «hard iron and partly transient magnetism induced in soft iron; that the latter. is. propor- tional to the intensity of the in- ducing force, and that the. length of the compass needle is infinitesimal compared to the distance of the near- est iron. From this standpoint their formulae are exact; but however neces- sary these intricate studies may have been to the development of the sub- ject, the fact remains that a knowl- edge of them cannot enable one to compensate a compass more accurate- ly than by the slip-shod tentative methods that are in vogue today. With the old "starboard angle" type of binnacle a mathematical analysis of deviations was often necessary, but where rectangular correctors are used all theory beyond that already given may safely be ignored. The obvious manner of effecting compensation is to place magnets near the compass, wrong end to, so as to counteract the magnetic forces of the ship, and in the old starboard angle method this was literally done. The vessel was swung through 360 degrees, the deviation on every other point determined, and after much. tedi- ous computation resolvedinto its com- ponent parts. The computed angle between ship's head and the axis of an imaginary magnet whose effect separately. THE MARINE REVIEW would be equivalent to the sub-perma- nent magnetism of the hull deter- mined the position of the corrector. It was then necessary to place the corrector in the binnacle at the com- puted angle, head the vesesl on a magnetic cardinal point and raise or lower the corrector tray until the compass gave the correct magnetic reading. Sub-Permanent Magnetism In the rectangular system the sub- permanent magnetism is resolved into its fore-and-aft and thwartship com- ponents, and each of these corrected The vessel is headed on one of the magnetic cardinal points, say north, by means of a range on shore whose magnetic bearing is known, or by a table of time azimuths of the sun previously prepared for the particular latitude and local ap- parent times at about ten-minute in-. tervals. The quadrantal deviation on cardinal points being zero, all error observed is semi-circular. Also the fore-and-aft component of the semi- circular deviation is inoperative be- cause it is acting north and south. Any difference between the ship's head by compass and north must, then, be due to the thwartship com- ponent and can be corrected with a thwartship magnet. If, instead of heading north by compass the card is found to read N. % E., there is half a point westerly deviation and a mag- net placed in the tray with its south end to starboard will, when raised or lowered in the binnacle to a suff- cient distance from the compass, draw the compass north to starboard until its north mark is opposite the lubber's point. Then, heading the vessel mag- netic east, the fore-and-aft component is compensated out. Should the com- pass read E. ¥% N., the. half point easterly deviation can be removed with a fore-and-aft magnet, north end forward to drive the compass north off where it belongs, or until the compass: reads east. The compensa- tion should be checked on south and west. It is not necessary that the correctors be placed in the binnacle itself, but the thwartship vertical plane passing through the center of the compass must bisect every fore-and- aft correcting magnet, and the vertical fore-and-aft plane passing through the center of the compass must bisect every thwartship magnet; otherwise the two poles of the correctors would exert different influences on the com- pass, spoil the symmetry of the semi- circular curve and set up new semi- circular deviations of their own. Correction of the quadrantal devia- tion is a more simple process. The The July, 1913 semi-circular error out of the way, it is usual to head the ship on some intercardinal point where quadrantal deviations are maximum. The princi- ple of the non-continuous soft iron bar is utilized in the form of the two iron balls or rods which are fitted to the arms of the binnacle. correction consists simply of moving the correctors in or out, each the same distance from the compass bowl, until the deviation is removed and the vessel heads by compass ex- actly on the intercardinal point on which she was laid by the shore range or sun azimuth. As quadrantal deviation does not change with either latitude or time, the correction once made may be relied on as long as the disposition of iron about the vessel remains unaltered. Residual Deviation There will always be some residual deviation after these corrections have been made, but there is rarely any excuse for their exceeding one-fourth of a point on any heading if the work has been carefully done. After the first compensation has been made and the ship shows considerable devia- tions while being swung for residuals, a few patient readjustments of cor- rectors and compromises between the balls and permanent magnets will al- ways greatly reduce them. | A constant deviation cannot be con- veniently' corrected in a_ standard compass. A real constant is extreme- ly rare, but when it does occur it is the same for every heading and can easily be allowed for. In steer- ing compasses, which are sometimes unavoidably placed in bad positions as regards surrounding iron, large constants are frequently observed. They may be corrected, however, for steering purposes, by altering the lubber's point, to starboard if the deviation is easterly and to port if westerly. A vessel in north latitude has in the upper ends of its vertical ele- ments a south polarity. Thus the frame ends, davit heads, stanchions, etc., continually exert a force tend- ing to draw downward the north end of the needles in bridge compasses. So long as the ship is on an even keel there can be no resulting devia- tion, but the moment she heels over some of these vertical elements. will swing off to one side of the still horizontal bowl and exert a sidewise pull which can cause deflections. Also all the beams and other horizontal elements become inclined and acquire magnetism from the vertical compon- ent of the earth's force. Their upper ends become strong south poles and