162 



HYDROGRAPHICAL SURVEYING 



[chap. v. 



Fig. 44. 



In order to obtain the least water on such a ridge, it must 

 be crossed by hnes of soundings at close intervals at right 

 angles to the direction of the ridge. Lines of sonndings 

 radiating from the ship do not fulfil these conditions. 



Mooring the ship between one end of the shoal and the peak, 

 observe the bearing of the latter. Knowing the distance of 

 the peak approximately, calculate its change of bearing due 

 to a movement of 1 mile at right angles 

 to its direction, and call this angle a. 



In Fig. 44 from the ship at A draw 

 A D at right angles to A G, the line of 

 bearing of the peak. On A D set off 

 A C = 1 mile on the required scale. At 

 a point B near the edge of the paper 

 draw B H parallel to A D. At C lay 

 off angle A C F = 90" - a , intersecting B H 

 at E. Subdivide A C and B E into the 

 same number of equal parts at close 

 intervals, and join the corresponding 

 points on each line. 



The lines thus drawn converge in the 

 direction of the distant peak, and, 

 spreading out fan-like on the paper, 

 they can be used for fixing equally well 

 as the peak itself, the limits of the paper 

 not permitting the peak to be plotted 

 on the scale adopted. 



The shoal X can now be satisfactorily 

 sounded out, running the hnes of soundings in the required 

 direction, and fixing the boat by observing the elevation of 

 the mast-head and the angle subtended between the ship and 

 the peak. 



Each fix is plotted by drawing the arc of a circle, with the 

 ship as a centre, at the distance corresponding to the mast-head 

 angle ; and setting the observed angle between the ship and 

 peak on the station pointer, with its centre resting on the arc 

 of the circle, bring one leg to pass through the position of the 

 ship, and the other to correspond with, or parallel to, one of 

 the lines radiating from the peak. The point thus found 

 on the arc of the circle is the position required. 



