334 POPULAR LECTURES AND ADDRESSES. 



of the compass-card. The degree of illumination 

 may be made less or more, according to faintness 

 or brilliance of the object, by holding a binnacle 

 lamp in the hand at a greater or less distance and 

 letting its light shine on the portion of the 

 compass-card circle seen through the lens. 

 Indeed, with the azimuth mirror it is easier to 

 take the bearing of a moderately bright star by 

 night than of the sun by day ; the star is seen as 

 a fine point on the degree division, or between 

 two, and it is easy to read off its position instantly 

 by estimation to the tenth of a degree. The 

 easiest as well as the most accurate of all, 

 however, is the sun when bright enough and high 

 enough above the horizon to give a good 

 shadow on the compass-card. For this purpose 

 is the stout shadow-pin which you see mounted 

 on the framework of the azimuth mirror per- 

 pendicularly to the glass and through the central 

 bearing point of the compass. 



(2) Bearings can also be taken with this instru- 

 ment by looking direct at the object over the top 

 of the prism which is the second method referred to 

 above. The degrees of the card reflected in the 

 prism are then seen close below the object. This 

 method is applicable to objects on the horizon, 

 and is more particularly useful for taking bearings 

 of distant landmarks which are too indistinct to 

 be seen when reflected in the prism. 



For taking bearings by this method the prism 



