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Sect. II.] 



TERRESTRIAL MAG:%ETISM. 



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angles of deflection, cither with weights or deflectors. 

 cannot be satisfactorily ascertained by a careful observvi. 

 It iS necessary that a spot should be selected for the 

 observations to be made on board ship, which should have 

 as little iron as possible within 5 or 6 feet of it ; and that 

 the instrnraent should always be us^d in the spot so 

 selected. The mode of investigating and of eliminatin 

 (when these precautions are raken) the influence on the 

 results of the iron contained in the ship is explained in 

 Appendix No. 4* It must be carefully borne in mind, 

 that the inverse proportionality of the sines of the angles 

 of deflection to the variations of the eartl 

 onlv true when the magnetism of the needle has not 

 varied ; and althijugh the needles made by Falmouth 

 artists, under Mr. Fox's own superintendence, have 



1 B magnetism, is 



generally proved most remarkable in preserving their 



magnetism unchanged for years and in all climates, it is 

 desirable that reference to a base station should be made 

 as often and with as short intervals as may be cxmvenient ; 

 and evidence must always be furnished that the magnetism 

 of the needle has not changed in a certain iiiterval, before 

 the relative determinations made during that interval can 

 have weight. The more frequently references are made 

 to base stations at which the value of the magnetic force is 

 known, the less damper exists that the labour bestowed on 



observations at sea will pro . . anproductive ; and the more 

 stations are multiplied which afford opportunities of such 



reference, the greater become the facilities for accurate 

 determinations at sea. 



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