CHAP. XXL] EMPLOYMENT OF FLOATING BEACONS 507 



position, and with a heliostat to assist the observer to distin- 

 guish and observe the more distant beacons. 



In detailed surveys, beacons, in conjunction with shore 

 objects, can also be made to serve as objects for use when 

 fixing, which might be rendered otherwise difficult owing to the 

 trend of the coast or the delay involved in extending a triangu- 

 lation in order to obtain additional points. 



SECTION XXIV. 



THE EMPLOYMENT OF FLOATING BEACONS IN SHALLOW 



WATER. 



Surveying beacons, pattern 1912, owing to the length of the 

 submerged staff, cannot be used where the depths at low 

 water approximate to about 24 feet or less. 



If, however, it is not desired to surmount a bamboo and flag 

 of the full sizes mentioned in Section I., the submerged spar 

 can be reduced in length with due regard to the stability of the 

 beacon when supporting those of the lesser dimensions that it 

 is intended to employ. 



Two 18-gallon casks, floating on their bilge, and lying parallel 

 when firmly secured together by a stout wooden exterior 

 framework and filling-pieces, can be used when it is desired 

 to improvise without delay a beacon of modified form with the 

 materials usually found on board ship. The weighted staff, 

 of convenient length, passes through the centre filhng-piece, 

 being weU secured by close interfitting chocks nailed or bolted 

 to both staff and filling-pieces. Lengths of flat iron boltstaves, 

 about 30 inches in length, being firmly secured to the staff, 

 and used as fishes above and below the middle filling-piece, will 

 ensure the staff being able to withstand the strain that is here 

 brought upon it. Beacons of this description Avill support a 

 flag that is easily visible at 8 miles distance. 



In the delta of the River Niger, where it is necessary to moor 

 beacons, in depths between 6 and 18 feet, and tidal stream of 

 3 to 4 knots, it was found that boats could conveniently handle 

 beacons consisting of lengths of stout planking about 10 feet 

 in length, bolted and dovetailed together in triangular form, 

 with central crosspiece supporting the weighted staff. The 



