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ship, in effect, becomes another magnet. We do not intend to give a detailed explanation of how this force affects  a  magnetic  compass,  but  where  deviation  exists, it must be taken into account. Although it remains a constant amount for each compass heading, it gradually increases, decreases, increases, and decreases again as the  ship  swings  through  a  complete  360°  circle. Deviation  must  be  considered  in  correcting compass error, consequently the deviation for any given heading of a ship must be known. Before the ship puts to sea, it is swung through the complete circle from 0° to 360°, and the amount of the compass deviation is noted  at  every  15°  swing.  Compass  deviation  is calculated by various methods, generally by comparison with the gyrocompass, or by reciprocal bearings on a compass on the beach, which would be unaffected by the metal in the ship. The results are compiled into a table called the deviation table (table 5-2). Every 15° is considered close enough, and in using the table, you should use the deviation for the heading nearest the heading you are checking. In other words, if you look in this table for the amount of deviation for a 17° heading, you would select the deviation for 15°, a deviation of 10°W. When  studying  the  compass,  a  coxswain  should have  the  following  definitions  memorized. Deviation— The error caused by the magnetic properties  of  any  metal  in  the  immediate  vicinity  of  the compass. Deviation for a particular compass should be shown  on  the  deviation  table  mounted  near  the instrument. Variation—  The  angular  difference  between  true north and the direction of Earth's magnetic field. Variation is marked on charts because it changes from year to year and from place to place. Compass error— Deviation and variation together are known as the compass error. True course— The angle between true north and the path along the ocean floor over which the boat is traveling,  measured  from  true  north  in  a  clockwise direction. Magnetic  course—The  angle,  measured  clockwise, from the magnetic meridian to the track. Correcting the magnetic  course  for  variation  gives  the  true  course. Compass course— The reading of a particular boat's compass when the boat is following a definite track. Correcting the compass course for deviation gives the magnetic  course. Correcting—  The  process  by  which  both  deviation and variation corrections are applied in converting a compass course to a true course. CORRECTING COMPASS COURSE The course you take from a chart usually is a true course. You must convert the true course from a magnetic compass to a compass course. To do this, you apply the compass error (variation and deviation) to the true course. Changing a true course to a compass course is called UNCORRECTING, and changing a magnetic course to a true course is CORRECTING. You can Table 5-2.–Typical Deviation Table DEVIATION TABLE SHIP'S SHIP'S SHIP'S HEADING DEV HEADING DEV HEADING DEV MAGNETIC MAGNETIC MAGNETIC 000° 14°W 120° 15°E 240° 4°E 015° 10°W 135° 16°E 255° 1°W 030° 5°W 150° 12°E 270° 7°W 045° 1°E 165° 13°E 285° 12°W 060° 2°E 180° 14°E 300° 15°W 075° 5°E 195° 14°E 315° 19°W 090° 7°E 210° 12°E 330° 19°W 105° 9°E 225° 9°E 345° 17°W 360° 14°W 5-17

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