Page 337 equipment and instruments 4414 



Variation is called east or west according to whether magnetic north lies to the 

 east or to the west of true north. Deviation is called east or west according to whether 

 the north indication of the ship compass lies to the east or west of magnetic north. 



Deviation varies for different compasses, different ships, different headings of a 

 ship, for different geographic localities, and also if the ship is not on an even keel. 



In converting compass courses to magnetic, and magnetic to true, easterly devia- 

 tions and variations are always additive, and westerly deviations and variations are 

 always subtractive for compasses graduated from 0° to 360°. In converting true courses 

 to magnetic, and magnetic to compass, the signs are reversed. A convenient way to 

 keep the signs straight is to think of the words — compass, magnetic, true — or true, 

 magnetic, compass. If the words are in alphabetical order, apply the signs directly, 

 if they are in the reverse of alphabetical order, reverse the signs before applying. 



4414. Compensation for Deviation 



Compensation of the compass is the process of counteracting the effect of the 

 ship's magnetism so that the residual deviations will be small. It is based on the general 

 principle that the magnetic effect of the iron and steel of the vessel can be counter- 

 balanced by magnets and soft iron placed near the compass. 



Detailed instructions for compensating the compass are contained in Special 

 Publication No. 96, and consequently the necessary operations are, only briefly outlined 

 here. Compensation should be performed only in comparatively calm waters, with 

 the ship on an even keel, and with all movable iron or steel in its customary position as 

 at sea. This should be done in a locality where the variation is accurately known. The 

 operations enumerated below are for a Navy standard binnacle, but the principles are 

 the same for any other type of binnacle: 



(1) Set the spherical (quadrantal) correctors in middle position unless some other position is 

 known to be more nearly correct. Place the heeling magnet in its tube, with its north (red) end up, 

 unless it is known that the south end should be up, and lower it to the bottom of the tube. The 

 magnet trays should be below the middle position. 



(2) Steer magnetic north. Enter the athwartship magnets in their trays, placing the same 

 number on each side, with their north (red) ends to starboard to correct easterly deviation, or to 

 port to correct westerly deviation. Move the trays up or down until the ship's heading is north 

 by compass. 



(3) Steer magnetic east. Enter fore-and-aft magnets on both sides of the axis of the binnacle 

 with their north (red) ends forward to correct easterly deviation and aft to correct westerly deviation. 

 Move the trays up or down vmtil the ship is heading east by compass. 



(4) Steer magnetic south. If any deviation is found, compensate for half of it by changing 

 slightly the position of the trays containing the athwartship magnets. 



(5) Steer magnetic west. If any deviation is found, compensate for half of it b\- moving the 

 trays containing the fore-and-aft magnets. 



(6) Steer magnetic northwest. Move the spherical correctors toward or away from the compass, 

 keeping them at equal distances from the center, until the ship heads northwest by compass. 



(7) Steer magnetic northeast. If any deviation is found, compensate for half of it bj- moving 

 the spheres slightly. 



(8) Follow with a complete ship swing to determine the residual deviations. If the compensation 

 is carefully and accurately performed, the maximum deviation for any heading should not greatly 

 exceed 1°. A skilled navigator will set this as his aim. A ship swing before compensation is of value 

 only for the information obtained with reference to the deviation and the amount of compensation 

 required. 



(9) At the first opportunity when the vessel is* rolling freely, head on a northerly or southerly 

 course. If heeling error exists, it will be most apparent on these courses. It will be indicated by 

 abnormal oscillations of the compass. Raise the heeling magnet until the oscillation decreases or 



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