44 ON MAGNETISM. 



(The jimbal-rings, by which the compass-card retains 

 its horizon tali ty in all motions of the ship, are omitted 

 in these Figures.) It must be remarked that in ships' 

 compasses the compass-needle is never exhibited naked, 

 but is inclosed in the thickness of a * compass-card/ a 

 circular card on which points of the compass and degrees 

 of azimuth are engraved, and which, being firmly con- 

 nected with the compass-needle, is so directed by the 

 magnetic power of the earth acting on the compass-needle 

 that the N, E, S, W on the card point truly to magnetic 

 north, east, south, west. In the compass, nothing touches 

 the circumference of this card, but there are, rising from 

 the compass-box, two small frames carrying vertical 

 wires; by directing the eye-view along the two wires 

 and turning the box till that eye-view sees a distant 

 object in the same line as the two wires, the line of 

 wires is made to coincide with the direction of the 

 object : and then the observer must read the gradua- 

 tions of the card which correspond to points in the 

 box below the two wires. In this way he obtains the 

 azimuth of the object as referred to the Local Magnetic 

 Meridian. 



In the best modern instruments, a horizontal ring 

 is expressly provided to carry the vertical wire-frame: 

 and, instead of having a wire next to the eye, a glass 

 prism acting by internal reflection is placed there; so 

 arranged, that one half of the pupil of the eye can 

 observe the wire on the further side of the horizontal 

 ring, and the distant object; and the other half of the 

 pupil can see the graduations of the compass-card by 



