16 GENERAL NAVIGATION. 



The three-arm protractor (station pointer of the British serv- 

 ice) consists of a graduated brass circle with one fixed and two 

 movable radial arms, the three beveled edges of the arms, if pro- 

 duced, intersecting at the exact center of the instrument. The edge 

 of the fixed arm marks the zero of the graduation which enables the 

 movable arms to be set at any angles with the fixed arm. 



To plot a position, the two angles observed between the three 

 selected objects are set on the instrument, which is then moved over 

 the chart until the three beveled edges pass respectively and simul- 

 taneously- through the three objects. The center of the instrument 

 will then mark the ship's position, which may be pricked on the chart 

 or marked with a pencil point through the center hole. 



The tracing'-paper protractor, consisting of a graduated circle 

 printed on tracing paper, is an excellent substitute for the brass 

 instrument and in some cases preferable to it, as when, for instance, 

 the objects angled on are so near tlie observer that they are more or 

 less hidden by the circle of the instrument. The paper protractor 

 also permits the laying down for simultaneous trial of a number of 

 angles in cases of fixing important positions. Plain tracing paper 

 may also be used if there are any suitable means of laying off the 

 angles. 



The value of a determination depends greatly on the relative 

 positions of the objects observed. If the position sought lies on the 

 circle passing through the three objects (in which case the sum of 

 the observed angles equals the supplement of the angle at the middle 

 object made by lines from the other two) it will be indeterminate, as 

 it will plot all around the circle. An approach to this condition 

 must be avoided. Near objects are better than distant ones, and, in 

 general, up to 90° the larger the angles the better, remembering 

 always that large as well as small angles may plot on or near the 

 circle and hence be worthless. If the objects are well situated', even 

 very small angles will give for navigating purposes a fair position, 

 when that obtained by bearings of the same objects would be of little 

 value. 



Accuracy requires that the two angles be simultaneous. If under 

 way and there is but one observer, the angle that changes less rapidly 

 may be observed both before and after the other angle and the proper 

 value obtained by interpolation. 



A single angle and a range give in general an excellent fix, easily 

 obtained and plotted. 



Advantag-es of sextant method. — In war time, when the com- 

 pass may be knocked away or rifle fire make it undesirable to expose 

 the person more than necessary, a sextant offers great advantages, as 

 angles can be obtained at any point where the objects are visible. 

 This contingency makes it especially desirable that all navigating 



