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Appendix.] 



TERRESTRIAL MAGNETISM. 



47 



ifPPENDIX No. 4. 



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Observations kkquired to determine the Effect of the Shix-'s 



Iron on the Magnetic Instruments ttseb on boarb. 



1. When the ship has got her guns, shot, and iron stores on board, and 



in their places, and has her boats, stanchions, and all other iron-work 



in the positions in which they are to remain at sea, her head must be 



^nccessiTely placed upon each of the sixteen principal points of the coni- 



)as=:, as indicated by the standard compass used in its proper position in 



'^^ ship. When the ship is quite steadyj and her head is exactly on the 

 oint on -which it is first to be placed, the bearing of some distant and 

 'ell-deliised object must be noted by the standard compass, the distance 

 f the object being such that the space through which the ship revolYes 

 n being swung round shall make no sensible difference in its bearing : 

 he magnetic inclination by the direct observation with Fox's apparatus 

 aust also be noted, and the angle of deflection produced by a second 

 leedle used as deflector N, in the manner described in the directions for 

 he obser%'ations of the magnetic force with that apparatus (§ 4, page 42)- 

 2. Repeat these three observations (viz., the bearing by the standard 



L * 



ompass of the distant object, the inclination, and the angle of deflection 

 or the ratio of the force), when the ship is steady v, ith her head placed 

 ^ -, jsively upon each of the sixteen principal points. 



L Determine the real or true magnetic bearing of the distant object 



from the ship, by taking the standard compass to some place on shore 



ffhere no iron may be near, and fruiii whence the position of the standard 



;ompass on boai'd and the distant object shall either be in oue, or in 



irections exactly opposite to each other. The bearing of the distant 



bject observed from this spot will be the ti'ue magnetic bearing, or that 



hich the compass should have shown in eaoh of the sixteen observa- 



ons on board, had it not been for the iron in tlie ship. The differences 



etween the true magnetic bearing and the successive bearings on board 



.^iil show the amount of the error occasioned by the ship's Iron, when 



16 ship's head was placed on each of the slxtec^'^ points. Call the 



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rror East, or 



3 



in the cases in which the North end of the ueedle is 



rawn to the eastward by the ship's attraction, and West, or +, when 

 -v/n to the westward. Observe also and record the magnetic inclina- 

 *n on shore with Fox's apparatus by the direct observation and with 



.e face of the circle to the East, precisely as when on board, as well 



3 the angle of deflection produced by the second needle used as de- 

 lector N. 



4. The obsorvations which have thus been detailed should be repeated 



