480 HYDROGRAPHICAL SURVEYING [chap, xxl 



Figs. III. and V., Ca>:e 1. 



Three ships present, X, Y, and Z, X being the conducting 

 and preferably the largest vessel, and the scheme made known. 

 No shore or otlier objects then in sight. 



2. X drops Beacon I in a position where it is known, or 

 may be reasonably anticipated, that the shore objects will be 

 visible in clear weather, especially at night. 



X then remains under way, keeping a close but safe distance 

 from Beacon 1. 



3. Ships Y and Z drop Beacons 2 and 3 respectively in their 

 approximate prearranged positions, 2 miles in opposite direc- 

 tions from Beacon 1, and for which purpose the masthead angle 

 and compass bearing of X will suffice. 



Obtaining the Scale. 



Before proceeding to lay out further beacons we now require 

 to obtain a scale of as accurate a nature as immediate cir- 

 cumstances permit, without which we cannot ensure the 

 extension of the triangulation in the required direction and 

 distance. 



Upon this scale and the compass it will very likely be neces- 

 sary to depend for some days, before clear skies and favourable 

 weather enable us to obtain the necessary connection with the 

 shore objects, and true bearing or observations by twilight 

 stars for astronomical position, or whatever the nature of the 

 cu"cumstances may be upon which we ultimately depend. 



4. On Beacons 2 and 3 being dropped, Y and Z take up 

 positions at anchor equidistant from X, on opposite sides, and 

 on a line of bearing from X approximating to that at right 

 angles to the general direction of these beacons. The distance 

 of Y and Z from X is governed by the sensitiveness of the angle 

 subtended by the extreme length of X, situated near Beacon 1 

 when presenting her broadside ajDproximately to Y and Z. 

 Near the bow and stern of X well-defined marks, such as cones 

 or balls, are exhibited about the same height; the " ensign " 

 and " jackstaff " or '•' bowsprit end " generally serve for this, 

 the desiderata being to obtain, if possible, a receiving angle at 

 the extreme beacons subtended between X and each ship when 

 in position of not less than about 40 degrees each. 



