438 HYDROGRAPHICAL SURVEYING [cHAr. xx. 



is detected at once; but with a fresh breeze the drifting of the 

 ship through the water cau;es the sweep to grow out from 

 the ship, and renders it difficult to judge if it touches anything. 

 The intermediate depth-hnes should be kept in hand. 



The negative results given by the sweep may be advantage- 

 ously supplemented by stationing leadsmen at intervals along 

 the ship's side, keeping their leads up and down on the bottom, 

 and using heavy leads with |-inch wire rope, and calUng 

 soundings as required. 



In sweeping by drifting, it must be remembered that tidal 

 streams splitting on a rock have a tendency to carry the ship 

 clear of it. 



It is not easy to manoeuvre the ship so as to pass over fresh 

 ground at each sweep. Tidal streams vary in direction even 

 at short intervals, and it cannot be depended upon that the 

 ship will make a sweep regularly parallel to any preceding 

 sweep, and angular " hohdays " thus appear between the sweeps. 

 Nor does the ship take a straight course when making a sweep 

 for any considerable distance. 



Suitable weather for drifting does not often occur, and then 

 only a portion of each day is available. 



It is useless to attempt it if the wind is contrary to the tide or 

 obUquely directed. If the fore and aft hne of the ship does not 

 remain at right angles to her general track through the water, 

 only a narrow area is covered by the sweep-line or by the sound- 

 ings ; nor does the sweep get a f au' chance of catching in any 

 obstructions, owing to the oblique direction of its movement. 

 Alterna- In the method above described the sweep wire should be 

 Method, theoretically taut, but in practice this does not always appear 

 to be the case. 



In order to obviate this defect. Commander F. A. Rayne 

 used six steel angle bars, 2| inches x 2^ inches x I inch, each 

 25 feet in length, and weighing about 187 pounds, to take the 

 place of the sweep wii'e. 



With these bars slung from each extremity point uppermost 

 (see Fig. 96), there was no sag in the centre of the sweep 

 thus formed. 



7^ 



Fig. 96. 



