492 HYDROGRAPHICAL SURVEYING [chap. xxi. 



With the three ships engaged, and dependent upon the visi- 

 bihty of the distant shore hghts as triangulated points, a suit- 

 able method would be to place two shijjs at anchor near by 

 beacons marking the two extremes of the triangulation, and 

 with which they can be connected by compass bearing, the 

 distance fiom each beacon being obtained by a sensitive mast- 

 head angle or range-finder, whilst the ships occupy such posi- 

 tions as will enable them to connect the beacon triangulation 

 by sextant angle with at least one shore object before darkness 

 sets in, when the shore or other objects upon which we rely for 

 our fix, such as hghthouses or Ughtvessels, will then become 

 visible. 



It would be weU to so arrange matters that the searchlights 

 or cruiser arc lamps of these two ships may be intervisible, 

 whilst the third ship occupies, before dusk, some suitable central 

 position that will ensure that the two extreme ships and as 

 many beacons as possible are visible, whilst as a secondary con 

 dition she is able to keep within visible range of as many shore 

 objects as possible. 



The selected positions for each vessel should be understood 

 beforehand by all concerned. 



Vessels so disposed afford the best means of connecting the 

 triangulation with the shore objects and checking all previous 

 work. 



Connecting the Ship with a Beacon near by for 

 THE Purposes of Triangulation. 



With no range-finder available, it may be necessary to send 

 a boat to the beacon to obtain the masthead angle before dusk, 

 or that subtended by two hghts, whose vertical or subtended 

 horizontal distance apart is known. 



If the weather is unfavom-able for this, a rehable distance 

 can be obtained by observing simultaneous angles to a parti- 

 cular part of the beacon by two observers intervisible, situated 

 at the ends of the ship at some known distance apart, whilst 

 the ship remains under way or is anchored, so as to have the 

 beacon approximately abeam, and at a safe swinging distance 

 when the shij) has taken her cable. 



