214 



HYDROGRAPHICAL SURVEYING [chap. viii. 



Compass 

 Bearings. 



Deck- 

 Book. 



necessary to recalculate. Altitude azimuths cannot be much 

 trusted in at night. 



When objects are visible from deck at night, and we can 

 rely on the compass, very good bearings can be taken with 

 the standard, if the lighting arrangements are properly 

 fitted. 



A ruled " deck-book," as now supplied, is convenient for 

 ship's sounding. In this everything taken from the ship 

 should be recorded, as rounds of angles when the ship is used 

 as a station in main triangulation ; elevations with sextant, 

 and the corresponding fix ; sketches of little bits of coast, etc. 



Difficulty 

 of Dis 

 proof- 



Area of 

 Search. 



Small 

 Area very 

 Possible 

 without 

 much 

 Indica- 

 tion. 



SEARCHING FOR VIGIAS. 



In searching for a vigia, it is difficult to say when its 

 existence is to be considered as disproved. This is especi- 

 ally the case in comparatively shallow waters, when nothing 

 short of a detailed examination of the ground by the aid 

 of floating beacons, if shore objects are not in sight, is of any 

 avail. 



Although experience shows that nine out of ten of these 

 bugbears and blots on the oceanic charts have been mistakenly 

 placed there, from reports of floating whales, wrecks, and 

 patches of confervce taken for discoloured water near a bank, 

 etc., still, we must be careful not to assume from a hasty search 

 that no shoal-water exists near a given locality. 



The area over which to search must always be large, as the 

 reckoning of the reporting ship, especially as regards longi- 

 tude, may often be considerably in error. In the vicinity of 

 such reefs, also, currents are generally accelerated, and 

 altogether we must allow a large margin in undertaking to 

 search for a danger reported in a particular spot. 



In bright, clear weather coral banks will show some miles 

 with the sun in the right direction ; but under other circum- 

 stances it is quite possible for a ship to pass within a mile, or 

 less, of a bank with as little water as 3 fathoms on it, without 

 its being detected from the mast-head. Soundings are the 

 only effective means of verifying or disproving the existence 



