452 HYBROGRAPHICAL SURVEYING [chap, xx 



A winch was fitted in the bows of the lighter for hauHng her 

 forward or backward. The diameter of the drum on this winch 

 was only 6 inches, so the rate of progress over the grovmd was 

 extremely slow. 



The time that was found suitable for this work was from 

 2 to 2| hours before low-water to 1 hour after low-water, on 

 account of the strong tide. 



To determine the depth at which to set the bar a tide -pole 

 was constantly read, and the bar set to 26 feet, plus the tide- 

 pole readmg, by means of the graduations on the wii-e pennants, 

 the zero on the tide-pole being L.W.O.S., so that when the 

 Ughter was hauled over the ground, if there were any inequali- 

 ties or lumps with a def)th of less than 26 feet L.W.O.S., the 

 bar would catch on the bottom and the spot could be immedi- 

 ately fixed. 



A method by which a man did not have to handle the wire 

 pennants to feel when the bar bumped was devised. 



A piece of wire seizing was secured to each end of the bar 

 and brought up to the lighter through a block (caj^able of being 

 raised or lowered) made fast to a boat-hook stave lashed to one 

 of the spars ; to the inboard end of this wire a 20-pound shot was 

 secured and an electric wu'e connected to this shot. Directly 

 under the shot was a clean brass contact piece, connected by 

 wire to a bell and battery (see Fig. 107), so that if the bar was 

 Ufted through coming in contact with an obstruction, the shot 

 would drop, and, completing the circuit, the bell would ring. 

 This was very successful. The shot could always be adjusted 

 so as to be within a fraction of an inch of the brass contact plate 

 so that the shghtest hft could immediately be detected. 



The rate of travelling over the ground was very slow indeed, 

 about 100 yards being covered in half an hour — that is, about 

 900 square yards being thoroughly swept. 



Excellent results were obtained when towed alongside a tug 

 at a very slow speed. 



This method of sweeping was very effective in the places 

 where the bottom was hard, but for the muddy patches it was 

 of practically little use. 



The work was continuously checked by soundings being taken 

 and recorded. 



Fine weather was essential for this work, as, if not smooth, 



