216 



HYDROGRAPHICAL SURVEYING [chap. viii. 



Sound- 

 ings 



Experience has shown that in coral waters the edge of a 

 bank is a favourite spot for the growth of a small danger. 

 This part of a bank should therefore be closely searched. 

 Spacing of In depths of 2,000 fathoms soundings at intervals of 7 miles, 

 run in parallel lines 7 miles apart, enclosing areas of 50 square 

 miles between any four adjacent soundings, should effectually 

 clear up the ground and lead to the discovery of any shoal. In 

 depths over 2,500 fathoms the soundings might be more widely 

 spaced. 



In depths of 1,500 and 1,000 fathoms shoals are very 

 unlikely to occur within a radius of 3| miles and 2 miles re- 

 spectively from such soundings ; but as the depth decreases 

 the angle of slope rapidly increases, and a shoal might 

 occur within three-quarters of a mile or even half a mile 

 from a sounding of 500 fathoms. 



Shoals are not usually equally steep on all sides. In some 

 directions the slope is generally more gentle, and thus a better 

 chance is afforded of picking them up by sounding systema- 

 tically. 



An appreciation of these facts will indicate the distance 

 apart at which it is proper to place soundings in squares 

 suitable to the general depth of water. 



Contour lines will soon show in which direction to prosecute 

 the search if any irregularity of depth is manifested. When 

 once a decided indication is found, it is not difficult to follow 

 it up by paying attention to the contour lines as developed by 

 successive soundings. 



The submarine sentry, now supplied to all surveying ships, 

 towed at a depth of 40 fathoms, is here invaluable, and may 

 save hours of hunting. Discoloured water, ripplings, fish jump- 

 ing, or birds hovering about may assist in locating a shoal. A 

 handsome reward to fishermen on the spot is often effectual. 



Many banks in unsuspected positions have been discovered 

 by means of the submarine sentry, which should be constantly 

 towed, set to about 30 fathoms, when on passage. It sometimes 

 fails to strike on a very soft bottom, but it is liable more fre- 

 quently to give false alarms, especially if the ship is pitching 

 heavily and going at slow speed. 



With kites fitted to float with the ship stopped, the chances 

 of fouling the screw are much diminished. As the wire generally 



Use of 



Contour 



Lines. 



Sub- 

 marine 

 Sentry 

 Valuable 



