470 HYDROGRAPHIGAL SURVEYING [ohap. xxi 



depends upon the maintenance of the flags. The risk of 

 damaging the beacon is great should any attempt be made 

 to replace the flag from a ship under way except under the most 

 favourable conditions of weather. 



A boat is not suitable for this, for we should not depend upon 

 a block and haUiards for securing a flag of this nature, which 

 should be stopped along the staff at regular intervals from the 

 metal grommets inserted along the strengthened canvas piece. 



The bamboo being 35 feet in length, it will be found advan- 

 tageous, and facihtate recognition, if one or more smaller sized 

 flags are attached beneath the larger sized flag (see Fig. 116). 



Caution. — The beacons should be distinguished as far as 

 possible by theii- particular flags, bearing in mind that the 

 colours should be arranged in such a manner as will not render 

 them liable to be mistaken as National flags. 



Under favourable conditions it is quite possible to reflect 

 these beacon flags with a sextant when they are ten miles 

 distant and the eye elevated not less than 30 feet. 



A record should be kept in the data book giving full par- 

 ticulars of the dimensions of the bamboo. 



This is often useful for obtaining short distances by subtense 

 angle. 



SECTION II. 



SCHEME OF TRIAKGULATION. 

 See Figs. I., II., III., IV. (at end of Section XXV.). 



Dependent upon the nature of the work in hand and the 

 area to be examined, some considered scheme should be adojjted. 



Generally speaking, if we are relying entirely upon astro- 

 nomical observations we cannot do better than to lay out the 

 beacons in parallel Unes, either in a north and south or east 

 and west direction, the Unes of the beacons being approximately 

 3 J miles apart, whilst the beacons take the form of equilateral 

 triangles, the lengths of the sides being approximately 4 miles. 



We then have the readiest means of checldng the triangula- 

 tion near its two extremes, either by difference of latitude or 

 longitude, or other forms of j)osition lines combined with 

 observed true bearings. 



The distance between the beacons should not exceed, 



