12 



a pointer which reads against a graduation on the base 

 dial. 



The edges M and N are perpendicular to each other, 

 and the line P is parallel to the line of sight and 

 may, therefore, be used as an alidade. 



The above description covers that portion of the in- 

 strument which is used upon a plane table either for 

 ordinary compass work or for mapping on the plane- 

 table sheet. The instrument is, however, provided 

 with a ball-and-socket attachment so that it may be 

 used upon a Jacob staff, tripod, or more conveniently 

 held in the hand if used as a hand compass for rough 

 cruising. These parts are shown in the illustration ; Q, 

 a cone-bearing containing the spindle, which may be 

 clamped by the screw R; the ball 8 is held by the socket 

 cover T, which screws upon the mounting U. 



When this instrument is used on the plane table the 

 proceeding is as follows: 



The sights having been raised and the instrument 

 laid on the table, the table is leveled by observing the 

 bubbles. The variation having been set off, the table 

 is oriented with the compass needle, which should read 

 zero at the north end. Then sights may be taken upon 

 all the objects to be mapped, using the edge P, or the 

 opposite parallel edge, as an alidade. The distances 

 may be measured with the scale. 



When used as a surveyor's compass the leveling is 

 done by means of the ball and socket 8 and T, and the 

 compass is revolved in azimuth by loosening the clamp 

 screw R. 



As a clinometer for measuring vertical angles, the 

 edge M may be laid upon a slope and the pendulum 



