SHIP'S MAGNETISM. 299 



gives a very small frictional resistance at the supporting point. 

 The compass card with its attached needles is supported in a 

 copper bowl which is supported on gimbals, so that the compass 

 remains horizontal in spite of the rolling motion of the ship. 

 The complete instrument is supported on a column which con- 

 tains or supports the compensating devices which are explained 

 later, and the entire arrangement is called the binnacle. 



When a ship contains no iron or steel the compass points in 

 the direction of the magnetic meridian, and a chart like Fig. 3 

 enables a navigator to infer the true heading of a ship from an 

 observed reading of the compass. When, however, the ship is 

 made of iron or steel, or, when it carries a cargo of iron or steel, 

 the compass is usually deflected by the magnetism of the ship or 

 of its cargo. In order that a compass may be used for purposes 

 of navigation under such conditions the errors of the compass 

 may be determined by a careful set of observations, or the in- 

 fluence of the ship's magnetism may be compensated, thus 

 reducing the compass errors approximately to zero. The latter 

 method is the one which is usually employed, and in some cases 

 the residual errors which remain on account of incomplete com- 

 pensation are determined by a careful set of observations and 

 allowed for in the use of the compass. 



8. Ship's magnetism. A ball of iron which is devoid of per- 

 manent magnetism, is weakly magnetized by the earth's field. 

 This magnetism, which is not in a fixed direction in the ball, but 

 which is always in the direction of the earth's field however the 

 ball may be held or turned, is called the temporary magnetism 

 of the ball, and it is proportional to the intensity of the earth's 

 field. If the ball is elongated like an ellipsoid its temporary 

 magnetism is not in general parallel to the earth's field, and in 

 the case of a long slim iron rod its temporary magnetism is in 

 the direction of its length and proportional to the component of 

 the earth's field which is parallel to it inasmuch as that part of 

 the earth's field which is at right angles to a slim rod produces 

 no perceptible magnetism. 



