6 THE VOYAGE OF H.M.S. CHALLENGER. 



coral, a strain less than sufficient to break the dredge-rope breaks the stop, and the 

 dredge alters its position, and probably sets itself free. In case of the dredge taking in 

 a greater load of mud than the rope can bring u]3, the stop likewise gives way^ and 

 allows it to fall into such a position that a large part of its contents fall out. 



The dredge-bag, which is of hand-made net of strong twine, the meshes ^-inch to 

 the side, was attached to strong welded iron rings, passed through holes perforated in the 

 thick edges of the scrapers, at distances of about an inch. So open a net-work would 

 let many of the smaller things wash through, and to avoid this, the bottom of the bag, 

 to the height of about 9 inches, was lined with " bread-bag," a light open kind of 

 canvas. 



In many of our dredgings in the Poecupine at all depths, we found that while few 

 objects of interest were brought up within the dredge, many Echinoderms, Sponges, and 

 Corals came to the surface sticking to the outside of the dredge -bag, and even to the first 

 few fathoms of the dredge-rope. 



This suggested many expedients, and finally Captain Calver sent down half a dozen of 

 the " swabs " used for washing the decks, attached to the dredge. The result was marvellous. 

 The tangled hemp brought up everything rough and movable which came in its way, 

 and swept the bottom as it might have swept the deck. After various experiments, we 

 came to the conclusion that the best plan was to attach a long transverse bar to the bottom 

 of the dredge-bag, and to fasten large bunches of teazed-out hemp to the free ends of the 

 bar (fig. 2). We have used the " hempen tangles " ever since, and we now regard them 

 as an essential adjunct to the dredge, scarcely less important than the dredge itself, 

 and often much more conspicuous in its results. 



The length of the dredge-rope used in the Porcupine was 3000 fathoms. Of 

 this, 2000 fathoms were " hawser-laid " of the best Russian hemp, 2^ inches in circum- 

 ference, with a breaking strain of 2 J tons; the 1000 fathoms next the dredge were 

 " hawser-laid," 2 inches in circumference. 



The accompanying diagram (fig 6) will give an idea of the method of dredging adopted 

 by Captain Calver, and the various relative positions of the dredge and vessel at different 

 stages of the process of " paying-out." 



On the 22d of July 1869, lat. 47° 38' N., long. 12° 8' N. in the Bay of Biscay, the 

 depth was ascertained by a careful sounding to be 2435 fathoms, with a bottom of 

 grey ooze, and at about 4.45 p.m. the dredge was let go, the vessel drifting slowly 

 before a moderate breeze (force = 4) from the N.W. The 3000 fathoms of rope were all 

 out at 5.50 P.M. — A represents the position of the vessel when the dredge is let go, and 

 the dotted line A B the line of descent of the dredge. When the dredge is going down 

 the vessel drifts gradually to leeward ; and when the whole (say) 3000 fathoms of rope 

 are out, C, W, and D might represent respectively the relative positions of the vessel, 

 the weisfhts attached 500 fathoms from the dredge, and the dredge itself. The vessel 



