128 



ON MAGNETISM. 



needle, and the quotient of the second by the first will 

 give the tangent of the inclination of the whole force 

 to c. 



55. Composition of the disturbing force in the hori- 

 zontal plane with the terrestrial horizontal force. 



In the case of Figure 39, the horizontal force pro- 

 duced by A is in the direction at right angles to c: in 

 Figures 40, 41, 42, 43, it is in the vertical plane which 

 contains c ; and in Figure 44, it makes a definite angle 

 with c, depending only on the magnitude and distance 

 of A and its inclination to c. If the disturbing magnet 

 A rotate in the horizontal plane (as for instance when 

 it is part of a ship revolving in azimuth, the compass in 

 these figures being the ship's compass), in Figure 45, 



let BF with length f represent the force which, as found 

 above, is produced by the magnet A acting on the red 



