144 HYDROGRAPHICAL SURVEYING [chap. v. 



All complete triangles in which the ship is involved should be 

 tested before plotting. 



This use of the ship is permissible because the points fixed 

 by means of her angles lie between regularly triangulated 

 stations, and thus cumulative errors are avoided. 



Example XI. — An inaccessible object visible from both ter- 

 minal astronomical positions, which are invisible from each 

 other, ivith an intermediate station visible from one of them, and 

 from which the inaccessible object is also visible. From the ship 

 at anchor midway betiveen the astronomical j)ositions all points 

 are visible. 



In Fig. 34, A and B are the terminal astronomical positions 

 from which the inaccessible object C is visible. 



D is an intermediate coast-station from which B and C are 

 visible. 



From the ship tautly moored without the swivel at E, all 

 points are visible. 



Fig- 34- B 



A 



Observers being at the masthead and provided with Galton's 

 sun-signals for flashing, if necessary, other observers with 

 theodoHtes and hehostats are stationed at A, D, and B. 



At a preconcerted signal (the dipping of a flag, letting fall 

 square sails, or at a given moment by comparison of watches 

 beforehand) simultaneous angles to the ship are observed at A, 

 B and D, and the observers at the mast-head take angles to all 

 the points. 



