SHIP'S MAGNETISM. 311 



corrector magnet may be inferred as follows : Suppose that the 

 north end of the compass is deflected to the east when the ship 

 rolls to the west. Then it is evident that the perpendicular-to- 

 the-deck component P of the field which is produced at the 

 compass box by the permanent magnetism of the ship is down- 

 wards, because the part of it which is projected into a horizontal 

 plane is to the east when the ship's masts roll to the west. In 

 this case the north end of the heeling- corrector magnet is to be 

 placed upwards so as to produce an upward field at the compass 

 box. 



(d) Adjustment of Flinders' bar. Having carefully adjusted 

 the semicircular correctors at the home port so as to annul com- 

 pletely the semicircular error, the ship is taken to a distant port 

 and the semicircular error is observed with the ship's head east 

 or west (magnetic). Let this error be represented by </> ; let 

 V and H' be the vertical and horizontal components of the 

 earth's magnetic field at the home port and let F t and //"/ be 

 the vertical and horizontal components of the earth's field at the 

 distant port as determined by observation, or as taken from mag- 

 netic charts. The forward (or aft) component of the magnetic 

 field which is produced at the compass box by the vertical tem- 

 porary magnetism of the ship, is proportional to the vertical com- 

 ponent of the earth's field and it is therefore equal to aV at the 

 home port and equal to a V v at the distant port. The deviation 

 of the compass which is produced by this field is proportional to 

 its intensity and inversely proportional to the horizontal intensity 

 of the earth's field. Therefore this deviation is equal to bVJH 1 

 at the home port and equal to b V^H{ at the distant port, where 

 a and b are proportionality factors. Therefore the observed 

 compass deviation (j> is equal to b(V]H' Fj ////), and the 

 total compass deviation, <>,, which is due to the vertical tem- 

 porary magnetism of the ship at the distant port is equal to 



V Iff' 

 .... / TT I pjr X < With the ship's head east at the distant 



'/** '\l**\ 



port (the condition under which (j> was observed), put Flinders' 



