204 HYDROGRAPHICAL SURVEYING [chap. viii. 



Sweeping An effective method of sweeping for a rock on a broad front 

 over a jg niuch to be desired. Havine sounded out a rocky channel 

 Area on a in the usual manner, it would be a great satisfaction to be able 

 Y^°^t ^^ sweep over it rapidly to a depth of about 6 fathoms, to 

 ensure no danger being left undetected. 



It is possible that a solution of the difficulty may be found 



by towing from two steamboats submarine sentries set to a 



depth of G fathoms, and connected by a piece of 7-stranded 



wire 100 yards long. The boats should steam on parallel 



courses, and maintain their proper distance apart by means 



of a distant line. Lieutenant Helby, R.N., in a lecture 



delivered at the United Service Institution, has proposed the 



use of the Otter Trawl in connection with submarine sentries. 



Experiments on these lines are much required. 



Sweeping A long heavy iron bar suspended horizontally at the re- 



over a qviired depth under the boat or barge will indicate boulders 



Area. lying on the bottom that might be missed by the lead. This 



procedure is particularly useful when sounding a rocky area 



that has been dredged. 



An effective sweep is formed by a couple of railway irons, 

 each 15 feet long, placed end to end, and joined by a third 

 railway iron in the centre. 



A 30-foot spar should be placed across the gunwale of the 

 boat, with distant lines marked in feet, run through a block 

 at each end, and connected with the ends of the iron sweep, 

 by means of which the required depth is ensured throughout 

 the length of the sweep. Any deviation from the horizontal 

 due to the boat heehng over on one side is detected by the 

 mark on the distant line dipping below the water, and the 

 corresponding mark on the other distant line being raised by 

 an equal amount. 



The operation of sweeping is most conveniently carried out 

 by allowing the boat to drop down slowly on transit lines 

 running in the direction of the stream, and controlling her by 

 lines secured to buoys suitably placed on each bow and 

 quarter. 



An examination of the bottom by divers is sometimes 

 desirable. 

 Anchor- An anchor should not be placed on a chart unless the officer 

 ages. jjj charge is assured that sufficient examination has been made 



