154 



HYDROGRAPHICAL SURVEYING 



[chap. v. 



happens in practice, the curve of the coast is such as to prevent 

 one station from seeing the next, except occasional!}' here and 

 there, in order to avoid placing stations unnecessarily close to 

 each other, it may sometimes become necessary to resort to 

 the principles adopted in the previous example. 

 It is proposed to illustrate such a case as follows : 

 In Fig. 39, stations are erected along the coast about 5 miles 

 apart at A, B, C, D, etc., in such positions that at least two 

 of the stations are visible from each other. Four of these 

 stations are occupied at one time by observers with theodolites 



Fig- 39- 



and heUostats. Beacons are laid down at H, J, K, L, etc., 

 forming approximately equilateral triangles with the shore 

 stations. The ship is then anchored as near each beacon in 

 succession as may be convenient without fouling it, and without 

 obstructing the view of the beacon from the two shore stations 

 adjacent to it. 



At each position of the ship, at a preconcerted signal, the 

 observers at the mast-head are shot up by each of the observers 

 on shore, who immediately afterwards note the angles to the 

 beacons. Simultaneous angles are observed at the ship between 

 the shooting-up stations and the adjacent beacons. 



As the ship moves from one position to another, each ob- 

 server on shore moves on to the next station, where he should 

 find a note from the officer who has vacated it, stating the zero 

 he has used. 



The theodolite lines to the beacons effectively control any 

 small movement due to their being floating objects, and give 

 the means of referring their positions at the moment of making 

 each ship's position to those originally occupied by them when 



