SHIPS MAGNETISM. 309 



The method of adjusting Flinders' bar is explained in Art. 1 6. 



14. The heeling error. Let us suppose that the semicircular 

 and quadrantal errors have been completely compensated by 

 means of the semicircular correctors and quadrantai correctors, 

 the ship being all the time on an even keel. Under these con- 

 ditions a deflection of the compass is produced when the ship 

 rolls, or heels over, at sea. This deflection of the compass is 

 called the heeling error, and it is due in part to the variation of the 

 temporary magnetism of the ship which accompanies the change 

 of direction of the earth's magnetic field with reference to the ship's 

 iron as the ship rolls, and in part to the permanent magnetism of 

 the ship, as follows : The horizontal field P, Fig. 9, is annulled 

 by the semicircular correctors, and the vertical component of the 

 field at the compass box which is produced by the permanent 

 magnetism of the ship is left unaltered by the semicircular cor- 

 rectors. By vertical component is here meant that component 

 which is perpendicular to the ship's deck, and which, as the ship 

 rolls, turns out of the true vertical, and has a horizontal compo- 

 nent, at the compass, which deflects the compass. In describing 

 the action of the heeling corrector, the heeling error will be 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 

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

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

 dent 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. 



temporary magnetism of the ship's iron, and b is the field produced at the compass 

 box by Flinders' bar. 



