SHIP'S COMPASS. 105 



of the jewel cap, as shown in the figure. The entire weight of 

 the card, including the magnetic needles, is about 170 grains, 

 and this extreme lightness combined with the relatively large 

 moment of inertia due to the distribution of the mass, insures a 

 long period of free vibration and therefore great steadiness. The 

 lightness of the card also 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 sup- 

 ported on a column which contains 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.* 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 so as to 

 reduce 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 

 compensation are determined by a careful set of observations 

 and allowed for in the use of the compass. 



64. Ship's magnetism. A ball of iron which is. devoid of 

 permanent magnetism is weakly magnetized by the earth's 

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

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

 may be held or turned. It 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 



* Magnetic north. 



