116 ADVANCED ELECTRICITY AND MAGNETISM. 



heeling corrector, the heeling error is assumed to be due entirely 

 to the permanent magnetism of the ship. 



Assuming the heeling error to be due entirely to the permanent 

 magnetism of the ship, that is, to be due to the component P r 

 (perpendicular to the deck) of the field which is produced at the 

 compass box by the permanent magnetism of the ship, it is 

 evident that the heeling error is a maximum when the ship heads 

 north or south, and zero when the ship heads east or west; for, 

 when the ship heels over with its head to the east or west, the 

 part of P' which is projected upon a horizontal plane is directed 

 towards the north or south and does not deflect the compass, 

 whereas, when the ship heels over with its head north or south, 

 the part of P' which comes into a horizontal plane is directed 

 towards the east or west and it deflects the compass. 



The heeling corrector is a vertical steel magnet placed directly 

 beneath the compass box, and adjusted up or down until it pro- 

 duces at the compass box a vertical field which is equal and 

 opposite to P'. The practical method of adjusting the heeling 

 corrector is explained in Art. 73. 



72. Compass errors due to magnetic lag. The temporary 

 magnetism of the ship's iron tends to lag behind the magnetic 

 field which produces it. Thus, after a ship has been standing 

 for some time in one direction the magnetism which is produced 

 by the earth's field does not at once disappear when the ship 

 turns around, but tends to persist. This magnetic lag produces 

 a compass error which is known as Gaussin's error. This error 

 is never compensated in practice. 



73. Directions for adjusting the correctors of a ship's compass. 



(a) Adjustment of semicircular correctors. The quadrantal 

 error is zero with ship's head north, east, south, or west. There- 

 fore any deviation of the compass which exists on these headings 

 is due to the semicircular error. With the ship's head north 

 (magnetic), place one or more athwartship magnets in one of the 

 semicircular-corrector trays and move them up or down until 



