-. H, ■_ _ 



^ 



^. 





Q 



a1 





'.lUs . 



6 1 



Appendix. 



1 



TERRESTEIAL MAGNETISM 



1 



points of the compass. By the addition of a brass bar attaelied at right- 

 angles to the prism and si^/* t-vane of the asimuth ring of the standard 



comn-assj a ^'. Meeting ma.. .A or magnets may be leniporarily fixed at a 

 conveni ..t disti?5ce from the conipass-needlej and the deflection ob- 

 served -with the ship's head on opposite points by a process requiring 

 only a very few minutes, and independent of the visibility of the son or 

 stars, or of any distant object. If the points taken be those which the 

 observations in harbour have shown to be points of no disturbance and 

 if Vj v^ be the angles of deflection on the respective points 



fe 



^r 



-«.! 



V^ 



i ' 



A **. 



na. 1. .';- 



IIP*' 



lybe 



O a. 



t * 



1 





t 



I 



^ \ 



% 



f. 



J t- 



r --. 



iQu 



ii^ 





. i 



kr. 



'-'* 



i>'' 



1. 



'. - 



A^' iT 



1 ^. 



^J 





..Ai 



I 



V 



f B^' + C^ 



sm L'l "»- sin y 



^, 



sm i?! + sin i\j, 

 t;j — f^ 



tan -^ — ^ 



2 



tan 



^'i + v^ 



2 



and the deviations on the several points may be computed bv 



sin 5 



Jb^ 



sin 2 C^ + E cos 2 C: 



in which a is the easterly azimuth of the line of no deviation. 



Should this method of determining the variable part of the correction 

 formula be found to succeed ou trial, the correction of the disturbances 

 by the officers of a ship might be still further simplified, by the forma- 

 tion of tables of each term for every probable value of the coefBcio-itSj 

 requiring merely the addition of the quantities to be taken out from the 

 tables. In the meantime the calculations may be facilitated by the 

 following table, extracted from Mr. Smith's Memorandum in No. YUL 

 of the * Contributions to Terrestrial Magnetism/ 



Let Bi, Bj, . . . , B;, Ci, C^, . . . . Cj represent the values of B and 

 C multiplied by sin 11 o 15', sin 22^ 30^ &c. ; and let D,. D,, D,. E^ 

 E„ E, represent the values of D and E multiplied by sin ^2'^ 30' '^ia 

 45^, and sin 67^ 30' j we have then-^-^ ' ^ 



a 

 5 



32 



A + C + E 



16 



A 



C + E 



5i 

 831 



A + B, + C, + D, + E, 



A 



5 



B, + C, 



IS 



S 



17 



s. 



A +B, 



A - Bj 



A + B, 



C 





E, 

 E, 



Cr + D, + E, 



S 



S 



s 



30 



14 



18 



A 



B. + C, 



A + B 



A 



B. 



C. + D^ + E^ 



- I>4 + E, 

 Cb - D, + E, 



Co + D, + E^ 



d2 





^ 



