50 



TERRESTRIAL MAGNETISM. 



[Sect. II. 



^w^ 



m 



^ ■ - 

 ■J 



principal points, they may be confined to the eight principal points. If 

 tiiose observations are carefully made, the results derived froni them 

 •will have nearly equal value with the results derived from observations 



J ^' 



maae ou 



fonaul 



for calculating A, B, C, D and E become much simplified. Using the 



before, and 5, Sg 5^^ S,^ 5,o 5^^ S,g and 5^ being the 



same notation 



eight deviations observed, we have 



A 

 B 



J ih + h + Kz+ 5,s + 5^0 + hi, + »28 + 5^} 

 •1768 (its log = 1-2474) {s, - S^ + 5,, - S.J 



+ i{s 



02^: 



.\ 



c 



1768 (its log = 1-2474) {b, + S^ 



S 



ijt 



D 



E 







K) 



n 



iS 



s 



12 



} 



i{s 



32 



+ §16 



S 



§^} 



The ..._mcients being known, the deviation on any point of the compass 

 may be computed by the toll owing equation : 



sin 5 =r A + B sin C' + C cos C + D sin 2 f + E cos 2 f. 

 A, D, and E may be expected to remain constant, or to have the same 

 values in whatever part of the globe the ship may be, whilst uo material 

 alteration is made in the distribution of her iron. They may, therefore, 

 be regarded as determined, once for all, by the deviations observed 

 when the ship is first swung, though they may possibly be obtained 

 more : ctly by taking the mean of the values obtained on all occasions 

 when that process is repeated. B and C are variable, and depend on 

 the dip, and aisc on the proportion of the iron which changes its mag- 

 netic stat-e cotemporaneously with changes in the geographical position 

 of the ship, to the permanently mag>ietic iron, or to iron of an inter- 

 mediate quality to ihe two which have been named, and of which the 

 magnetism is neither permanent on the one hand, nor, on the other, are 

 its changes cotemporaneous with changes of the dip, but are consequent 

 ou such changes, and require a greater or less interval to conform to 

 them. It is on account of the uncertainty of the law according to 

 which the^c: two coefficients B and C vary in different ships, that when 

 a ship has changed considerably her geographical position, it is desirable 

 to repeat the process by which the values of the coefficients may be 

 re-deterniincd ; and that it is still more desirable that a full and suffi- 

 cient trial should be made of a very simple method suggested by Mr. 

 Arclr-ald Smith, in No. VIII. of the 'Contributions to T.irestrial 

 Magnetism,' for djjrmining the variable coefficients at anytime that 

 may be wished, either at sea or in harbour, by deflections of the 

 Compass-needle with the ship's head successively on aay two opposite 



r - fit 



C 



o^f" 



con 



Vv* 



U 



serve 

 only s 



i 



sU- 

 observ 



and th( 



S] 



in whi 



;iul 





by the 



tion of 



« B 



reqmn 



tables, 

 follow i 



of the * 



Let] 

 "0 mul 



45^ an 



- ^ 



