CHAPTER IX 

 DREDGING AND TRAWLING 



BY E. J. ALLEN AND S. W. KEMP 



Dredging. 



Dredges are suitable for the biological investigation 

 of all grounds that are too rough to admit of trawling ; 

 they are very serviceable for obtaining samples of the 

 bottom, and for testing the nature of unknown 

 localities. 



Many different patterns of frame are used for dredg- 

 ing, but these, for the most part, only differ from one 

 another in unimportant structural details. The best 

 type for all ordinary purposes is known as the double- 

 s worded oyster dredge. Fig. 200 shows an example of 

 this type which was used by the ' ' Porcupine ' ' Expedition 

 more than forty years ago, and the pattern has not been 

 modified to any great extent since that date. The 

 essential features of this type of dredge are the 

 obliquely set blades or swords, on both sides of the 

 mouth, the rings by which the net is attached (these 

 prevent the edge of the bag from chafing when working 

 on the bottom), and the loose connection, which allows 

 free lateral movement, between the bridles or arms 

 and the frame. The whole instrument should be 

 strongly made of wrought- iron. Frames about 2 feet 

 long and 6 inches broad are suitable for in-shore work 

 from small boats. For work at depths from 10 to 



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