o 



58 



TERRESTRIAL MAGNETISM. 



[Sect. U 



ApP^ 



• -H 



* 



respectiveivj m X derived from the experiments of vibration, becomes 

 when corrected to the tenipei-ature of the experiments of vibration ; 



m 



X[l+q (t-ty\ 



The absolute horizoutai force determined by the foregoing process will 

 fin-ally require a very small correction, on account of the circumstance, 

 that the magnetic moment of the deflecting magnet is not strictlv idcu- 

 lical in the different positions in -v^hich it is placed in the experiments of 

 deflection and vibration, as it is perpendicular or nearly so to the magnetic 

 meridian in the first case, and coincides with it in the other. In the 

 first ca^e, the magnetic moment is tha.t proper to the bar itself; in the 

 second case it is augmented by the inducing action of the earth. The 

 coeificient for this correction requires a separate apparatus for its deter- 

 mination, and the correction m.ay be applied to the results when finally 

 re-computed at Woolwich, as it requires no special data to be furnished 

 by thf^ observer. 



APPENDIX No. 2. 



DiBKC 



FOR USING Mr. Fox's Apparatus tor observing the 

 Magnetic Inclination ani> Force. 



T, — Gensral Eemaeks^ 



In fixing the gimball table, it is convenient that it should be so arranged 

 that when the apparatus is placed on it, the zero divisions of the horizon- 

 tal circle should coincide with the fore and aft midship-line of the ship. 

 la preparing for an observation at sea, the circle should be turned in 

 azimuth until the vernier of the horizontal circle shows an angle with its 

 ;^ero, corresponding with the difference between the magnetic meridian 

 and the coarse which the ship is steering. The plane of the circle will 

 then coincide with the magnetic meridian, when the ship is steadily 

 steered. When from circumstances of weather, &c., the steerage is diffi- 

 cult, an assistant is required to indicate to tlie observer the times when 



the ship is steady on her course. 



The apparatus is usually furnished with three or four needles, one of 

 which is intended to be used on shore for the determination of the true 

 inclination (when no special instrument is provided for the purpose), by 

 the process subsequently described, Appendix 3, in which the poles are 

 reversed. The other needles, which are intended for the intensity, are 

 never to have their poles reversed, and care is to be taken not to place 

 them inadvertently near other magnets or iron. Besides Oxe needles. 



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angle c 

 be less 

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&c.), 

 nom': .. 

 preserv 

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gimba:: 

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release ■ 

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