148 



SCIENCE. 



[Vol. IV., No. 81. 



weight of the frame, and varying its propor- 

 tions of length and breadth, the same general 

 shape being always retained. The handles 

 were modified in different ways, and several 

 tangle-swabs were generally attached to the 

 hinder end of the bag. 



The Porcupine dredges weighed from a 

 hundred and fifty to two hundred and twenty- 

 five pounds, and the frames were in some cases 

 four and a half feet long. Discussing their 

 merits, Sir Wyville Thomson states, that in 

 many instances he 

 had evidence " that 

 the dredge, instead 

 of falling upon the 

 surface, and then 

 gliding along and 

 gathering the loose 

 things in its path, 

 has fallen upon its 

 mouth, and dug into 

 the tenacious mud, 

 thereby clogging it- 

 self so as to admit 

 but little more. I 

 mean to try the ex- 

 periment of heavier 

 weights and lighter 

 dredge-frames in the 

 Challenger, and I be- 

 lieve it will be an im- 

 provement." 



It was the fault 

 here mentioned that 

 suggested the con- 

 struction of the 

 Blake dredge de- 

 scribed below, and 

 which is now used 

 b} T both the coast- 

 survey and fish- com- 

 mission for the mud- 

 dy bottoms of deep 

 water. 



The Challenger 

 dredge (fig. 6), as 

 figured in the first 

 volume of ' The At- 

 lantic,' was an elaborate affair ; and much rigid- 

 ity was given to the entire appliance b}^ two 

 iron bars extending back, one on either side, 

 from the mouth-frame to an iron crossbar behind 

 the net. This cross-bar afforded attachment for 

 tangle-swabs and weights, when such were de- 

 sirable ; but its main object, in connection with 

 the lateral bars and three loopings about the 

 net, was to keep the latter distended, and pre- 

 vent its folding over the mouth of the dredge. 



Fig. 5. — The dredging arrangements at the stern of 

 the British ship Porcupine, showing the accumula- 

 tor, THE DREDGE, AND THE MODE OF STOWING THE ROPE 



on the ' Aunt Sallies.' 



(From * The depths of the sea,' p. 248.) 



We might almost be led to consider that in 

 this device we have a faint suggestion of the 

 more recently invented Blake dredge ; yet the 

 two differ radically in construction, and no 

 hint is given, in connection with the former, 

 that a framework might be so constructed as 

 to prevent the undue digging-in of the mouth- 

 scrapers. The dredges used by the Challenger 

 for all excepting the greatest depths were no 

 smaller than those of the Porcupine ; the length 

 of the frame being the same as that above 



given, and the width 

 much greater (fif- 

 teen inches). 



The Blake dredge. 



The difficulty of 

 obtaining good re- 

 sults with the com- 

 mon dredge, on the 

 soft bottoms of mud 

 and ooze which char- 

 acterize the deeper 

 waters off shore, 

 gave rise to many 

 experiments on the 

 steamer Blake dur- 

 ing her first dredg- 

 ing - cruise (1877- 

 78) , resulting in the 

 construction of an 

 entirely new pattern 

 (fig. 7), well adapt- 

 ed to this kind of 

 work. The neces- 

 sity for a change in 

 this direction is well 

 expressed in the 

 above quotation from 

 Sir Wyville Thom- 

 son. The whole ten- 

 dency of the flaring 

 mouth, with so shal- 

 low a frame, is to 

 work downwards as 

 well as forwards ; 

 though in moderate 

 depths this tendency 

 may be more or less counteracted by a careful 

 manipulation of the drag-rope. The dredge 

 becomes clogged, and its farther progress is 

 of no avail in collecting the objects which live 

 upon the surface of the mud. 



The first remedy tried was applied directly 

 to the ordinary dredge, and consisted in 

 ' stopping ' a piece of two-and-a-half-inch rope 

 around the hinder part of the frame, thereby 

 correcting to a certain extent the unfavorable 



