NA VI G A TION. 1 1 



diminished brilliance ; then the coloured glasses in 

 front of the unsilvered mirror must be put in 

 requisition. Lastly, I must not omit to tell you 

 that a portion of the glass which I have been 

 speaking of as the unsilvered glass, or unsilvered 

 mirror, is actually silvered ; but this portion is 

 advantageously put out of the way (by means 

 of a sliding piece and screw) in almost all 

 ordinary uses of the instrument. It is useful for 

 star observations when the ghostly image in the 

 unsilvered part of the glass is too faint. 



10. THE AZIMUTH COMPASS. Before describ- 

 ing the azimuth compass, I must tell you what an 

 azimuth is. It is simply a horizontal angle. The 

 azimuth of one object relatively to another, as 

 you see the two from any particular place, is the 

 angle between the two horizontal lines verti- 

 cally under the directions in which you see the two 

 objects. In navigation, azimuths, or " bearings," as 

 they are commonly called by sailors, are generally 

 measured from the true north, or from the magnetic 

 north, point of the horizon. 



11. The true north is found, whether at sea or on 



