NAVIGATION. 



the direction in which I must look to see the sea- 

 horizon. This gives me what is called the apparent 

 altitude of the sun. 



The framework of the instrument used in old 

 times to be made of ebony, and the divided arc 

 of ivory inlaid in it, as in one of the instruments 

 before you. In the best modern sextants the 

 framework is a light structure of brass, and the 

 graduated arc is of silver, or of platinum, or of 

 gold, inlaid in it. It is of silver in this other instru- 

 ment before you (one of Troughton and Simms'). 



6. Now as to the divisions of the circular scale, 

 I must tell you that, for the purpose of the 

 measurement of angles, the complete circumference 

 of a circle is divided into three hundred and 

 sixty equal parts called degrees, so that a quarter 

 round is measured by ninety degrees. The sixtieth 

 part of a degree is called a minute of arc or of 

 angle, and the sixtieth part of a minute is called 

 a second of arc or of angle. The ambiguity of 

 the names minute and second, sometimes used for 

 angles, and more often, as you well know, for the 

 reckoning of time, is perniciously troublesome in 



