404 



HYDROGRAPHICAL SURVF.YING [chap, xviii. 



The hawser is weighted with about 60 pounds, at 5 fathoms 

 from the dredge. 



On a sandy bottom, a net bag is substituted for the canvas 

 bag, which gets full of sand. 



On a rough bottom an iron triangle carrying swabs only is 

 vised, the arms being stopped lightly together so as to carry 



Fig. 82. 



The Dredge. 



aAA'ay, if caught ; or even one large swab at the end of a rope, 

 weighted 2 feet above it to keep it down. These are especially 

 useful on coral banks, where a regular dredge may very likely 

 be lost. 



To dredge, turn the ship away from the wind or current, and 

 drop the dredge from aft A\ith slight headway on, taking care 

 that the bag and swabs do not capsize and foul the mouth of 



