CHAP, i.] THE COMPASS. 52,3 



observations by night, the other two contain each a compass, so as 

 to be under immediate control. 



In the ship's compass the magnetized needle rests on a pivot in 

 the centre of the compass-box, or cylinder of copper. It carries a 

 light disc, or compass card, on which are drawn the various points, 

 and which also makes the oscillations smaller. The compass-box, 

 weighted by a mass of metal, is itself carried in the binnacle by 

 means of Cardan's suspension, or gimbals : so that the plane of the 

 card remains horizontal whatever may be the movements of the ship. 



Fn;. .H.%. Ship's, or iiinviner's, compass. 



A mark or a star on the compass- box in its front side shows the direc- 

 tion of the axis of the ship ; this point is called the head of the 

 compass. 



At any moment the angle which the magnetized needle makes 

 with the head of the compass can be read on the compass card. 

 By adding to this angle the magnetic declination, or by subtracting 

 it, as the case may be, the true orientation of the ship is found. 1 



1 The employment of the eompnss for navigation or geographical exploration 

 supposes, as we have just seen, the knowledge of the value of the magnetic declina- 

 tion of the places where the sailor or the traveller makes his observations, but it is 

 essentially necessary that there should be no disturbance close to him, that would 

 introduce an error of all the more importance as he believes himself protected 

 from it. Now that the number of ships whose hulls are wholly or in part built 

 of iron, goes on increasing, a similar cause of error exists in effect in the ship itself ; 

 and it appears certain that the deviation of the magnet on vessels so constructed 



