SHIP'S COMPASS. 



combined temporary magnetism of ship and correctors may be 

 made the same with the ship's head north (or south) as with 



Fig. 79a. 



Fig. 796. 



ship's head east (or west) , and when this condition is reached the 

 quadrantal error of the compass is eliminated as explained in 

 Art. 68. 



70. Compass error due to the magnetizing action of the ver- 

 tical component of the earth's magnetic field. The vertical 

 component V of the earth's field produces a temporary magne- 

 tism of all the vertical iron in the ship; this "temporary" 

 magnetism remains unaltered as long as V remains unchanged, 

 the ship being supposed to stand on even keel; and therefore 

 the "temporary" magnetism due to V merges with the per- 

 manent magnetism of the ship in the production of the semi- 

 circular compass error. 



The temporary magnetism due to V is distinguishable from 

 the permanent magnetism of the ship, however, because it changes 

 when the ship goes from one port to another where the value of 

 V is different. Thus, if the semicircular error is completely 

 compensated at the home port by means of the semicircular 

 9 



