SHIP'S COMPASS. 



109 



ship so that the iron of the ship is symmetrically placed on the 

 two sides of the compass, then the compass error due to the ship's 

 temporary magnetism is zero when the ship heads north, east, 

 south, or west, as may be shown as follows: When the ship heads 

 magnetic north or south, its temporary magnetism is symmetrical 

 as shown in Fig. 73 ; the magnetic field at the compass due to the 



N 



S 



Fig. 73. 



temporary magnetism of the ship is therefore due south, and 

 consequently the compass is not deflected. Figure 74 shows a 

 compass box C placed on the center line of a ship of which the 

 dissimilarity of bow and stern is greatly exaggerated. Imagine 

 the vessel to be made of solid iron and consider the transverse 

 slice of iron which lies between the dotted lines in Fig. 74- 

 When the bow points east or west (magnetic) the transverse slice 

 is magnetized as indicated by the letters A/" and 5; and the 

 curved line Jf , which represents a line of force due to the poles 

 N and 5, shows that the field, produced at C by the 

 magnetism of the transverse slice, is towards the south. What 



