192 



HYDROGRAPHICAL SURVEYING [chap. vii. 



Let annexed diagram, Fig. 55, represent an island of this 

 kind. 



A long measured lead-line, say of 500 feet, is provided. 

 This is taken by an officer we will call B, who has a prismatic 

 compass. Another officer, A, is provided with theodolite, or 

 sextant, or micrometer, and prismatic compass, according to 

 circumstances, sextant and compass being quite sufficient. 



Starting at a, B remains there while A walks to h. B 

 stretches his line out at right angles to a, &, and plants a flag 



Fig- 55- 



at the extremity. A observes angle subtended by flag and 

 A a, with his micrometer or sextant, and both A and B 

 observe the bearing oi ah. 



A waves to B, who goes on to c, when the operation is 

 repeated. 



A then moves on to d, B pivoting his line round c, so as to 

 be rectangular to c d ; and so on, until / is reached. We will 

 here suppose that, from a to /, we have been able to triangu- 

 late, the reef being broader. We have therefore the correct 

 bearing and distance of a /. 



To plot this, the mean compass bearings and distances 

 a, b, c, etc., will be put on a separate sheet of paper on a larger 

 scale than the chart, and the positions a f being joined on 

 both, the other stations will be squared in on to the chart. 



Marks will be left at each station, if required for sounding. 



