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Even a celestial fix cannot be depended upon to guarantee the safety 

 of a ship in Arctic or Antarctic waters. The inaccuracy of charts, 

 which in many instances may be several miles, makes it necessaiy and 

 more important to keep a ship's position plotted in reference to ad- 

 jacent land rather than in reference to true latitude and longitude. 



MAGNETIC COMPASS 



The directive force on the ordinary mariners' magnetic compass is 

 derived from the horizontal component of the earth's magnetic field. 

 Although the total intensity of the earth's field remains fairly con- 

 stant in all latitudes, the horizontal component decreases as the mag- 

 netic poles are approached. Within about 20° of the magnetic poles, 

 the directive force is so weak that the compass is sluggish and unre- 

 sponsive. Conversely, the vertical component increases and may give 

 rise to large heeling errors. Although the horizontal component of 

 the earth's field, and hence the induced magnetism in horizontal soft 

 iron, decreases as the magnetic poles are approached, the field of 

 subpermanent magnetism of the ship's structure retains its absolute 

 value, and therefore becomes relatively much more important in 

 causing deviation. Small uncompensated deviations due to subper- 

 manent magnetism thus may attain very large values in high latitudes. 



To obtain the best performance from the magnetic compass in polar 

 waters, the ship should be swung and the compasses adjusted in high 

 latitudes, preferably just before entering the pack ice. If the Flinders 

 bar has not been permanently set at the magnetic equator, it should 

 at this time be adjusted to the position indicated by computation, 

 and the horizontal and heeling magnets should be carefully placed 

 to produce minimum deviation. On completion of the adjustment, 

 the ship should be swung again and a new deviation table constructed. 

 This procedure will provide the navigator with a more satisfactory 

 instrument than if he attempts to use a compass compensated in low 

 latitudes. 



Even if this recommended procedure is followed, changes in mag- 

 netic latitude may cause large deviations to reappear. Likewise the 

 magnetic variations will change rapidly with locality and may un- 

 dergo large diurnal changes, particularly if auroral activity is pres- 

 ent, so that the navigator must undertake frequent azimuth 

 determinations. If large compass errors are found, and if it is 

 uncertain whether these are due to variation or to deviation, swing- 

 ing the ship again to see whether the error persists on all headings 

 will establish the cause. 



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