HYDROGRAPHIC SURVEYING 



127 



FIG. 9 



Three-Arm Protractor. The positions of soundings made by 



method 6 can most conveniently be platted by means of the 



three-arm protractor, 

 Fig. 8. The arm / is 

 fixed and its beveled 

 edge is in line with the 

 center and the zero 

 point of the graduated 

 circle. The arms m 

 are movable, and their 

 beveled edges also pass 



D /f*SUI F? ^\\X\\ * through the center of 



the circle. To deter- 

 mine the position of a 

 sounding F, Fig. 7, 

 when the positions of 

 E, C, and D, are platted, 

 set the movable arms of 



the protractor to form the measured angles A and B with /; 



then, with the beveled edge of / passing through C, slide the 



instrument around on g 



the paper until the bev- \ 



eled edges of the arms 



m pass through E and 



D; the center of the 



circle c will then be 



over the point F. 

 The Sextant. The 



sextant is a hand instru- 

 ment for measuring 



angles. With it angles 



can be rap idly measured 



while in a boat when in 



motion. A sextant is 



illustrated in Fig. 9, 



and its essential parts * 



are diagrammatically 



shown in Fig. 10. It has two fixed arms BD and BE, Fig. 10, to 



which are attached a telescope T and a horizon glass A , one-half 

 10 





