SHIP'S COMPASS. 117 



the compass points north. Then head the ship east (magnetic) 

 and place fore and aft magnets in the other semicircular-corrector 

 tray and move them up or down until the compass points north. 



(b) Adjustment of guadrantal correctors. Having corrected the 

 semicircular deviation of the compass, head the vessel northeast 

 (magnetic) or southeast, southwest, or northwest, and if any 

 deviation of the compass exists, place the quadrantal spheres on 

 the side brackets of the binnacle and move them in or out until 

 the compass reading is correct. 



(c) Adjustment of the heeling corrector. With the ship headed 

 north or south in a heavy sea, place the heeling-corrector magnet 

 in its tube with its proper end upwards, and raise or lower it 

 until the slow motion of the compass due to the rolling motion 

 of the ship is nearly eliminated. The proper end up of the 

 heeling-corrector magnet may be inferred as follows: Suppose 

 that the north end of the compass needle 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 downwards, 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 

 completely 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 r be the vertical and horizontal components of the 

 earth's magnetic field at the home port and let Fi and HI be 

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

 distant port as determined by observation, or as taken from 

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

 netic field which is produced at the compass box by the vertical 



