COMPOUNDED FORCES OF EARTH AND MAGNET. 129 



end of B ; and let EB with length h, as measured from 

 E to B, represent the earth's horizontal magnetic force 

 acting on the same red end of B. Then EF will repre- 

 sent in magnitude and direction the total composite 

 horizontal force acting on the red end of the compass- 

 needle. As the ship rotates in azimuth, the line / will 

 assume the different positions BF' y BF", &c., the points 

 F'y F", &c. all lying in a circle of which B is the center : 

 and in these different positions, the total horizontal 

 force acting on the needle will be represented in mag- 

 nitude and direction by EF' , EF", &c. It is seen here 

 that when the ship is in such a position that the devia- 

 tion of the compass (which is the same as the angle 

 BEF) = 0, the force is either the greatest possible = h+f, 

 or the least possible = h f. It is also seen that -in the 

 entire revolution of the ship, the compass-needle deviates 

 during half of the revolution to the right and during the 

 other half to the left. The positions of the ship at 

 maximum deviation to the right and maximum devia- 

 tion to the left are not exactly half-way between the 

 positions of no deviation. 



If / be greater than h, the circle will include the 

 point B : and, as the ship revolves uniformly, the com- 

 pass-needle will turn entirely round, but not with 

 uniform angular velocity. 



