CHAP. XX.] SWEEPING 439 



Holes were bored in the extremities of each bar, through which 

 a wire strop was passed, and to which a shackle, with the hemp 

 lowering line and wire depth-Une on it, was secured. Except 

 at the extremes of the sweep, the shackle was passed through 

 the strops at the ends of the bars adjoining each other, and 

 thus the six bars were loosely joined into one flexible length. 

 The bars were slung over the ship's side from convenient davits 

 or spars specially rigged out, so that the sweep hung in a 

 straight line parallel to the keel. 



The lowering Unes were used solely for raising or lowering 

 the sweep; the wire depth -Unes were only used to regulate its 

 depth, thus placing no undue strain on them. 



This method proved highly satisfactory ; the sweep on touch- 

 ing the smallest shoal patch grew away from the ship's side, 

 and swung back to its vertical position when the shoal patch 

 had been passed. 



Owing to the sweep bars being joined together, if a shoal patch 

 was touched, it caused those bars next to it to draw out from 

 the vertical as well, thus giving an erroneous impression of the 

 size of the shoal. This could be obviated by slinging each 

 bar separately from both its extremities; but as, in this case, 

 twelve depth and twelve lowering lines would be needed, 

 many more hands would be necessary to work the sweep. 

 With the current setting about 2 points from the direction of 

 the length of an area swept, it was found possible to keep the 

 ship at right angles to the direction of the drift down the area. 

 A twin-screw ship with out-turning screws has a great advantage 

 in this respect ; the effect of wind on the he of the ship may be 

 corrected by an a^Tiing triced up aft, or a staysail forward 

 (according to the direction of the mnd), which materially 

 assists the action of the screw. 



The iron bars used by Commander Rayne are very much 

 lighter than railway iron, and he considers they might bo used 

 satisfactorily in sweeping over areas when the bottom is of 

 soft mud. There is also less likehhood of missing a smooth 

 rock, as is sometimes the case when sweeping with wire. When 

 sweeping in difficult areas, such as narrow channels between 

 dangers, and in cases where the current runs across the length 

 of a long narrow area, where it would take a long time to sweep 

 a succession of short lines with the current across that area. 



