8 POPULAR LECTURES AND ADDRESSES. 



practice, and sometimes, though rarely, leads 

 to temporary error through inadvertence on the 

 part even of a careful and skilled navigator. It 

 will torment us repeatedly in the course of the 

 present lecture. 



7. As the sextant is used for the measurement 

 of angles, the doubles of those turned through 

 by the movable arm, its arc of sixty degrees is 

 divided into 120 equal parts, and each of these 

 parts is divided into three equal subdivisions. 

 Thus, the subdivision which is actually 10' of arc 

 measures 20' turned through by the reflected ray. 

 The main divisions are numbered by fives from 

 o to 1 20, and, being actually half degrees of arc, 

 measure whole degrees turned through by the ray. 

 When the minutest accuracy is aimed at the scale 

 is read by aid of a vernier attached to the movable 

 arm ; but in cases which frequently occur, where an 

 error of four or five minutes of angle is of no moment, 

 the reading is more easily taken directly by a single 

 division marked on the movable arm. This single 

 division is the zero line of the vernier, as illustrated 

 by the drawing before you. The fractions of a sub- 



