\ppendix.] 



TERKESTiilAL MAGNETISM 



53 



ic 



ea t 



-"' u. 



sli- 



hS 



i 



or tan ff — c 



d 

 cos C + -- tan 



COS C + B 



cos C ' 



! 



obserYing that the first must be used when the ship's course is l>etween 

 N^ E. and S. E., or N. W. and S, W, ; and the second when her course 

 is nearer the N. and S. points ; and that the values of ^ should be calcu- 

 lated by means of the declination-coefficients, 



9. The coefficient A', for the correction of the intensities observed at 

 sea, may be computed by the subjoined formulse, from the observations 

 of the intensity with Fox's apparatus on board with the ship's head 

 the different points, compared with those of the same instrument on 

 shore, and witu the absolute value of the tc^tal tbrce on shore (jf) ob- 

 tained in the manner described in App. 3. Tables for the correction 

 of the intensities observed at sea may also be constructed by the same 

 formulae ; 







A> 



sin ^ = c cos 6 Qos ( + d sin 0, 



/ 



or -TT— cos 0- sin C' 



(1 — D) cos B sin ^; 



the first to be used when the inclination is large, and the second when 

 the inclination is small. 



10, If the disturbances of the compass-needle, at the spot where the 

 gimball-table for Fox's apparatus is fixed, differ materially from those 

 at the spot where the standard compass is fixed, the standard compass 

 may be removed temporarily to the gimball-table, and the deviations 

 observed on the eight principal poiiits, which will give the values of B 

 and D for that spot, as well as determine the points on which there is 

 no disturbance : if these are opposite points^ the value of the variable 

 coefficient B can always be ascertained experimentally for the correc- 





"j-J 



It? 



r 



tion of the observations of inclination and intensity, by the angles o 

 deflection produced in a compass-needle placed on the gimball-table by 

 the method described in 7, p, 19. 





■^' 



*T 



