'f 



' 



46 



TERRESTRIAL MAGNETISM. 



[Beet. II. 



If then m he the magnetic moment of the deflecting maguet, and * the 



total lorce m absolute measure : 



m 



<P 



J r^ X sin u. 



The Inclir ueters s^^-i :''ed to the expeditions under Sir Jaaies Ross 

 and Sir John Richardson are furnished with the additional apparatus 

 for this purpose. The dipping-needles are 3-5 inches in length, and the 

 deflecti:ig magnets are those of 3*67 inches, belonging to the iinifilar 

 magnetometers, which also make a part of the equipment of these 

 expeditions. 



The chief practical difficalty which appears to present itself hi the 

 ^niployineut of this nietliod consists in the small distance apart at which 

 it is necessarj' tc place the two magnets^ viz,, the deflecting magnel and the 

 dipping-needle, in order that the angle of deflection may be of a suffi- 

 ciently large amount. The angle consequently is very considerably 

 influenced by the law according to which the free magnetism is distributed 

 in the particular magnets, and a long process of observation and of cul- 



niiired to eliminate this effect. The formula by which 



dilation is re 



the total force is deduced becomes in effect 



m 



<t> 



-. \ . 



>*4 



f v sm u 



1 



i + 



P 



Q ^ Iv 



-, &c.^ . 



>fi 



whe 



P, Q, R, &c., being coefficients depending upon the distribution of the 

 free magnetism in the deflecting magnet and dipping-needle, and of which 

 the values must be determinea by experiments at different distances. 

 This process may, however, bf^ gone through in England before the in- 

 struments -ire supplied to the officers, or after they are returned ; it has 

 been partially accomplished at Woolwich for the magnets supplied to 

 the Arctic expeditions aboTcnamed. Tl -^ rernaining observations re- 

 quired for a determination by this method of the total force in localities 

 re the dip approaches 90^ present no particular difficulties, and may 

 be made on shore or on the ice whenever the er^pedition is stationary tor 

 a few hours. It is necessary, however, to be very particular in observing 

 the temperature of the deflecting magnet whenever it is employed in 

 producing deflections, and to know very accurately its temperaturt: co- 

 efficient at dillerent parts of the thermometric scale; and it is also 

 necessary to combine in some satisfactory manner the observations 

 (described in App. 1) by which the magnetic moment of the deflecting 

 magnet is examined, with the observations in which it is made to deflect 

 the dipping-needle. 







App 



en^i^'^ 





C 



y^ S' 



.vV 



th< 



111 thei" V-; - 

 .-.. as in>^i2a' 



"-- ^hip. 



11 ^n^ 



f the omect b 





nu. 



ae ^nm 



r*^'' u 



1 ^ 



■^ \A^* 



\ 





UV V 



/u,.,rvatiOD 



2. Repeat f^' 



onr^^^ofthe 

 or the ratio of 



^rom the ship, 

 "J iron 1 





'ifipass oa V- 



^1 



. * 



■ -ms e 



' ■o'oservt: 

 '^A tne com 



■V 



- 3 r h 



it 



^^^^on th. . 



% 



'ill sb 



m 



^^'^^ the 



^^'-.. , 



^ ' -^d 



'^'^^F-.- 



; to the e 



' '"'^^ -^f t] 



J^ 



01 



4 



^ 



**tt 



Ob' 



