142 MANUAL OF THE MOLLTJSCA. 



The dredges are put overboard on the weather-side, and the 

 ropes made fast to a bulwark or thwart; each dredger holds 

 the rope in his hand, after giving it a single turn round a thwart 

 or "belaying pin," to regulate the strain by means of the 

 spare line. When a sufficient distance has been traversed, or 

 the ropes strain with the weight of mud and stones, the vessel 

 is brought to, and the dredges hauled up and emptied.* 



The length of line required is about double the depth of the 

 water. If the line is too short, the dredge will only skim the 

 bottom ; if too long, it will be in danger of getting fast. When 

 the bottom is loose sand or soft mud, the line must be short- 

 ened, or the vessel have more way, or else the dredge will be 

 apt to get buried. 



The strength of the line ought to be sufficient to anchor the 

 vessel in smooth water, — though not, of course, when there is 

 much way on her, — so that if the dredge gets foul it is necessary 

 to let out the spare line and relieve the strain while the vessel 

 is brought round. The dredge will then usually capsize, and 

 may be hauled up. 



If the bottom is at all rocky, a small strong dredge is best. 

 The line must be shortened, and some additional precautions 

 may be taken, such as fastening the rope to one ring of the 

 dredge, and tying the other with spun yarn, which will break 

 under a sudden and dangerous strain, and release one end of 

 the dredge. 



In dredging on coral-ground, Mr. Cuming employed a 3-inch 

 hawser, and had a patent buoy attached to the dredge by a 

 Ij-inch rope. More than once the hawser parted, and the dredge 

 was left down all night, but recovered the next day. 



Mr. McAndrew's researches on the coast of Norway were 

 conducted in the Naiad, a yacht of 70 tons, and extended from 

 the shore to 250 fathom water. The dredge employed was at 

 least twice as strong and heavy as the one we have represented, 

 and all forged in one piece, instead of folding up. The bag was 

 fastened on the frame with thongs cut from the hide. Before 

 using, it requires to be towed astern for a couple of hours, to 

 soften it. In three months' work only two cow-hides were used, 

 and one of those was torn by accident on sharp rocks. Several 

 spare dredges were on board, in case of emergency, but not used. 



Dredging in deep water (50 to 300 fathoms) can only be done 



* The collector may go out with the fishermen and superintend his own dredge 

 almost any time of the year, although oyster catching is illegal in the summer. The 

 scallop-banks off Brighton are in 15 fms. water, and nearly out of sight of land. It is 

 not always possible to work over them and return the same night. 



