SHIP'S COMPASS. 115 



compensation holds whatever the value of V may be. Flinders' 

 bar is usually about three inches in diameter and from 6 to 24 

 inches long, according to the amount of iron in the vessel, and 

 it is usually placed forward or aft of the binnacle. 



Figure 8oa shows the north polarity NNNN, etc., on the deck of an iron 

 vessel due to the vertical component of the earth's field. This north polarity is 

 distributed symmetrically with respect to the ship's keel (ship's iron being sym- 

 metrical with respect to the ship's keel), and it produces, at the compass, a magnetic 

 field of which the horizontal component is represented by the arrow a which is 

 parallel to the keel. Flinders' bar is placed in the position shown, and its north 

 pole N r (upper end of bar), which is on a level with the compass box, produces at 

 the compass box a field b which is equal and opposite to a. Figure 806 shows a 

 side view of Flinders' bar F (the compass box is supposed to be placed at the 

 point p). Flinders' bar is magnetized by the vertical component V of the earth's 

 magnetic field, a is the horizontal part of the field which is produced at the com- 

 pass box by the vertical temporary magnetism of the ship's iron, and b is the field 

 produced at the compass box by Flinders' bar. 



The method of adjusting Flinders' bar is explained in Art. 73. 



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

 and quadrantal errors have been completely compensated by 

 means of the semicircular correctors and quadrantal correctors, 

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

 tions 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. The dependance of heeling error upon 

 the ship's permanent magnetism may be explained as follows: 

 The horizontal field P, Fig. 70, is annulled by the semicircular 

 correctors, but the vertical component of the field produced at 

 the compass box by the permanent magnetism of the ship is left 

 unaltered by the semicircular correctors. By vertical com- 

 ponent is here meant that component which is perpendicular to 

 the ship's deck; as the ship rolls this component turns out of 

 the true vertical and has a horizontal component at the compass, 

 which deflects the compass. In the following discussion of the 



