SHIP'S MAGNETISM. 307 



is to say, T is usually greater in Fig. 1 3 than it is in Fig. 1 2, or the 

 coefficient k z is usually greater than the coefficient k r There- 

 fore by placing the quadrantal correctors in the positions shown 

 in Fig. 17 and moving them closer to or farther away from the 

 compass, the weakening of the magnetic field at the compass by 

 the combined temporary magnetism of ship and correctors, may 

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

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

 quadrantal error of the compass is eliminated as explained above. 



13. Compass error due to the magnetizing action of the vertical 

 component of the earth's magnetic field. The vertical compo- 

 nent V of the earth's field produces a temporary magnetism 

 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 " tem- 

 porary" magnetism due to V merges with the permanent mag- 

 netism of the ship in the production of the semicircular 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 com- 

 pensated at the home port by means of the semicircular correctors 

 (permanent magnets), then a perceptible amount of semicircular 

 error will appear when the ship sails to a distant port where the 

 value of V is different. In order to overcome this difficulty, 

 that is, in order to compensate the semicircular error so that the 

 compensation may hold good on a long cruise, it is necessary to 

 compensate, by means of the semicircular correctors, only that 

 part of the semicircular error which is due to permanent magnetism, 

 the remainder of the semicircular error (which is due to vertical 

 temporary magnetism) being compensated by means of a vertical 

 soft iron rod properly placed near the compass box. The use of 

 this rod was proposed originally by Captain Flinders and it is 

 usually called Flinders' bar. The action of Flinders' bar may 



