CHAP. XXL] SCHEME or TRIANGULATION 477 



approximately, 4 miles. This will be found to give the best 

 radius of action, station pointer fixes bemg the desiderata 

 to be aimed at, unless the scale is less than one inch to the mile, 

 when the compass and sextant may in certain cases be resorted 

 to for fixing. 



Should the area involved be in sight of shore objects, the 

 positions of which are already known, and which are visible 

 under favourable conditions either by day or by night — such as 

 lighthouses, light vessels, or distant mountain peaks — it would 

 be well to arrange the floating triangulation so that it may be 

 possible to obtain a station pointer fix or reUable sextant angles 

 to these objects, combined with observed true bearings from 

 positions near the two extremes of the triangulation. 



It will thus appear very essential to prepare a rough diagram 

 beforehand, especially when two or more ships are co-operating. 

 The diagram should show an outline of the proposed scheme 

 of the triangulation, giving the aj^pioximate positions and 

 names for the beacons, and all other explanatory information 

 required. 



Fig. I. illustrates a beacon triangulation independent of 

 shore or other objects, showing the " disposition of thirteen 

 beacons moored at distances of 4 miles apart in two parallel 

 lines in a north and south direction 3| miles apart. 



The iiTegular-shaped figure embraces an area of 395 square 

 miles (approximately), within which a " station pointer " 

 fix without resort to the compass is everywhere obtainable, 

 with the beacons visible up to 8 miles. 



Fig. II. illustrates the position of thirteen beacons arranged 

 in three parallel lines at the same distances as in Fig. I., the 

 area in this case being reduced to approximately 370 square 

 miles. 



The disposition here shown is generally more suitable when 

 examining a bank <i\ shoal area with the aid of boats, when a 

 necessarily increased number of fixed objects are available in 

 the central portion of the area. 



It may be stated here, as a general guide, that about twelve 

 beacons will be found to be a suitable number to have moored at 

 any one time if conducting a general or extensive survey; 

 but it is recognised that this number is largely dependent upon 

 the varying circumstances and the number of ships engaged. 



