478 HYDROGRAPHICAL SURVEYING [chap. xxi. 



In the general survey of the North Sea, conducted generally 

 on the 1-iticli scale, with three ships engaged simultaneously, 

 this number of beacons, which has not often been exceeded, has 

 been found to give the best practical results, bearing in mind 

 the special conditions — i.e., prevailing haze, continued presence 

 of fishing fleets and passing vessels, desirabiUty of detaihng 

 an independent area for each vessel which is readily under 

 the supervision of the controlling ship, and within which area 

 the beacons may to a certain extent be safeguarded by the 

 presence of that vessel; also, that information relating to those 

 beacons can thereby be kept up to date and circulated. 



Fig. III. illustrates a triangulation by beacons connected 

 to shore objects by angles taken simultaneously from two 

 ships at Nos. 1 and 13 to prominent Ughthouses visible after 

 dusk near their extreme range, the searchlights of the two 

 ships being intervisible and true bearings obtained. The area 

 to be surveyed is shown embraced by the pecked line. 



Fig. IV. illustrates the floating triangulation connected to 

 the shore objects, where it is desu'ed to fix the position of the 

 hghtvessels in addition to examining the special area covered 

 by the beacons. 



The triangulation will be completed by three ships, X, Y, and 

 Z, during one day and two successive evenings, X being the 

 controlhng ship. 



Lightvessels and lighthouses visible at night in clear weather. 



Assistants on board the four lightvessels. 



Shore objects not visited. 



Oftlfne of General Procedure, 

 First Day. 



X drops Beacon 1 (also Mark Buoy, being controlled by Z 

 at Beacon 1), and finally Beacon 2 (being controlled by Y at 

 Mark Buoy). 



(«) X, Y, and Z determine provisional scale for distance 

 between Beacons 1 and 2 (for w^hich see Section III.). 



X drops Beacons 5, 7, 9, revisiting them in reverse order; 

 also 1 and 2. 



Y drops Beacons 3, 4, 6, 8, revisiting them in reverse order. 



