206 HYDROGRAPHICAL SURVEYING [chap. viii. 



Unex- boats starring round her ; or, if the shoal is of large extent and 

 pectediy j^^^y ]jq prudently crossed in the ship, it is a good plan to lay 

 Shoal down two beacons on a bearing and patent log distance of 

 Sound- 4 or 5 miles. With another beacon (or mark-boat, carrying a 

 large black flag) fixed by means of this base, and forming 

 an equilateral triangle AAith it, the ship being anchored as a 

 fourth point, soundings may be carried out by the boats, and 

 fixed by station pointer. The ship's position should be deter- 

 mined by observations of twilight stars. 



A convenient scale for Avork of this sort is | inch or 1 inch 



to the mile. 



Large When surveying a large bank, where accuracy is desired, 



ofsfeht"* the beacons should be placed on a regular plan, and nothing 



of Land, is better from every point of view than anchoring them in 



two lines, so as to form equilateral triangles and a series of 



parallelograms, the beacons being about 5 miles apart. This 



distance permits the corners of the parallelograms being seen 



from one another. Bases are obtained by patent log, and 



astronomical observations fix the extreme points. 



If sounding out the triangles by boats, a mark-boat, flying 

 a flag from a bamboo lashed to the mast, can be moored half- 

 May betAA een the lines to aid fixing. 



Eight or nine beacons are desirable to carry out a satis- 

 factory floating triangulation of large extent. The beacons 

 should carry flags distinguishable from each other, and the 

 vertical height of each should be known, in order that the 

 necessary correction for false stations may be calculated and 

 applied to the angles taken for fixing the beacons. 



Beacons are visible from boats to a distance of about 6 or 

 7 miles, and from the ship up to 10 or 12 miles, but they are 

 very difficult to observe at that distance. The bearing of a 

 distant beacon can be observed with a theodolite used as a 

 bearing plate when the beacon is too faint to obtain its direct 

 bearing by standard compass. For this purpose the theo- 

 dolite, \\hen set to zero, should point to a fixed mark in the 

 ship in a direction exactly parallel to the fore and aft line. 

 This should be verified by direct observation in connection 

 with the lubber fine of the standard compass when at a quiet 

 anchorage. The first beacon that is laid down should be left 

 in position as a point of reference to be eventually connected 



