22 



HYDROGRAPHICAL SURVEYING 



[chap. I. 



Reading 



both 



Verniers. 



Coloured 

 Shades to 

 Eye- 

 pieces. 



equally and give three readings, which is often sufficient. The 

 other method can be reserved for taking single angles, as, for 

 example, a flash from a distant station. 



If both verniers are read, any error arising from bad centring 

 should be eliminated for any given position of the plates. 



For practical hydrographical purposes if one vernier is read 

 with the zero in several positions, submultiples of 360°, it is 

 as a rule sufficient. 



Different to the sextant, the theodolite has no index error 

 to apply to horizontal angles, but to the vertical arc there is 

 a correction to be found and applied, which will be mentioned 

 in discussing the method of ascertaining heights. 



A theodolite for hydrographical purposes should be fitted 

 with coloured shades to the eye-piece of the telescope for 

 observing the sun for true bearing. 



Theory of 



Station 



Pointer. 



STATION POINTER. 



This useful instrument is of hourly service in nautical sur- 

 veying. 



Either in sounding, coast-lining, or topographical plotting, 

 the position of the observer depends mainly on it. 



The station pointer is used to plot a position on the chart, 

 by means of angles taken at it, to other objects already fixed. 



Its construction depends upon the fact that the angles sub- 

 tended by the chord of the segment of a circle, measured from 

 any point in the circumference, are equal. (Euclid III. 21.) 



Fig. I. 



Thus, in the figure, the angles A D B, A E B, A F B are 

 all equal, so that if we have observed the angle subtended by 

 A B, we know at any rate that we are somewhere on the cir- 



