114 ADVANCED ELECTRICITY AND MAGNETISM. 



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 throughout a 

 wide range of latitude, it is necessary to compensate, by means 

 of the semicircular correctors, only that part of the semicircular 



Jf 



I, 



Fig. 80a. Fig. 80&. 



error which is due to permanent magnetism, the remainder of 

 the semicircular error (which is due to vertical temporary mag- 

 netism) 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 be explained as 

 follows: When V changes in value, the magnetism of Flinders' 

 bar and the vertical temporary magnetism of the ship change 

 together. Flinders' bar being once for all adjusted to compensate 

 the effect of the vertical temporary magnetism of the ship, the 



