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Sect. II.] 



TERRESTRIAL MAGNETISM 



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The importance^ and in some degree the noveltyj of 

 this part of the subject has made it appear desirable that 



No 



Summary of the Observations to he made. 



11, An officer, therefore, who pm-poses to make mag- 

 netic observations, or to cause them to be made on board 

 his ship, has to attend to the following points :™™He must 

 take care that he obtains the instruments some days before 

 the ship is ready for sea, in order that he 



mav assure 



himself that they are all complete, and that, if inex- 



perienced in their use, he, or the observer 



whom he 



selects, 



may have some preliminary practice with them. 

 lie will then have to determine the constants, index cor- 

 rections, &c. (unless these »hall have been furnished with 

 the instruments), and to make the observations required for 

 a base station, with the needles which are to be em.ployed in 

 the relative determinations of the magnetic force and dip 

 at sea. Positions will then have to be selected on board 

 for the standard compass, and for Fox's apparatus, and 

 tlie pillar for the one, and the gimball stand for the other, 

 fitted accordingly. When the ship is ready for sea, the 

 observations which are directed in the Admiralty Instruc- 

 tions for ascertaining the deviations of the compass caused 

 by the iron of the ship are to be made on board ; and when 

 the ship is swung round for this purpose, the deviations 

 of the dip and of the force must be also observed, on the 

 sixteen, or at least on the eight, principal points of the com- 

 pass, with Fox's apparatus used at the spot selected for it. 

 This completes the preparations to be made before trie 



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