152 



SCIENCE-GOSSIP. 



Ill— DREDGING. 



The dredge is an essential implement in marine 

 zoology. Myriads of forms have been discovered 

 by its means which would have remained unknown 

 to naturalists. Dredges of many different sizes are 

 employed, but, when used in deepish water, the 

 large ones necessitate the assistance of several 

 men or of a winch. Deep-sea dredging is quite an 

 art in itself, and is, moreover, a very expensive 

 amusement ; even dredging in medium depths, say 

 from sixty to two hundred fathoms, should not be 

 attempted until considerable experience is gained in 

 shallower depths. Personally, I prefer to be as 

 independent as possible of the aid of hired men, and 

 have found that quite a respectable amount of profit- 

 able work can be done with two or three people in a 

 boat. Those who wish to read about different kinds 

 of dredges, and other practical information, should 

 consult such books as Wyville Thomson's "The 

 Depths of the Sea " (chap. vi.). 



The form now most usually adopted is that 

 known as the "Naturalists' dredge," with two 

 parallel edges. I would strongly recommend that 

 only a small one with a wide mouth be employed. 

 One with a mouth that measures twelve by six 

 inches will be found to be sufficient for all ordinary 

 purposes ; the net should be strong and not less than 

 eighteen inches in length. It is advisable to fasten 

 a flap of canvas over each side of the net, so that 

 on whichever side it falls the canvas will help 

 to protect the net from being torn ; the ends of 

 the canvas flaps should be so secured as to prevent 

 them from blocking up the mouth of the dredge. 

 The mouth of dredge is usually provided with four 

 rods, each of which is hinged by a ring to one of 

 the four corners of the frame ; the free ends of the 

 two bars of each end should be fastened to a ring. 

 The dredging rope is fastened to one of these rings 

 only, the two rings are lashed together by a piece 

 of string ; the object of this method of fastening is 

 to recover the dredge should it get jammed. If 

 this should happen the strain on the apparatus is 

 so great that the string which connects the two 

 rings is snapped, as a consequence the dredge 

 usually turns over and can be hauled up by the 

 dredge rope which is fastened to the other ring. I 

 do not remember ever to have lost a dredge which 

 was provided with this simple safety gear. A 

 strong thin dredging-rope is more serviceable than 

 a thick one, and its length should be three times the 

 maximum depth to be dredged. Never forget to 

 fasten the free end of the dredging-rope to the 

 centre of a thwart. 



The naturalist should provide himself with an 

 Admiralty-chart of the district to which he is going, 

 and this he should study carefully as it will supply 

 valuable information as to depth and nature of 

 bottom. It is good practice for the beginner to 

 plot sections to scale showing the angles the 



sea-bottom has in various places of the district 

 he is going to work in. As a general rule his 

 position when in a boat can be sufficiently 

 accurately determined by means of cross-bearings 

 taken by the eye ; for minutely accurate work a sex- 

 tant or other similar instrument may be necessary. 

 Although the depths are so carefully given in the 

 charts, it is well to have a sounding line with one ; 

 this can be easily made and marked at home, but 

 care must be taken to prepare the line so that it 

 does not sink. It is usually a good plan to tie a 

 weight, almost anything heavy will do, to the 

 dredge line a fathom or so away from the dredge, 

 this serves to prevent the lifting up of the mouth 

 of the dredge by the rope, and thus enables it to 

 fish better. It is a mistake to make the frame of 

 the dredge too shallow. Tow-nets should usually 

 be tied to the rope in such a manner that they will 

 fish just above the dredge; many interesting forms 

 can be obtained in this manner which cannot be 

 secured by the dredge. Tangles are often success- 

 fully employed with a dredge ; these are long fringes 

 of yarn or untwisted rope, and organisms get 

 entangled therein and brought to the surface. On 

 very rocky ground tangles tied to an iron bar are 

 very effective in bringing up specimens which 

 cannot be got at by the dredge. The remarks I 

 made in my previous paper with regard to the jars 

 and receptacles required for shore-collecting apply 

 equally to dredging. 



A little practical experience in the art of dredging 

 will soon teach the beginner far more than can be 

 imparted by writing. Be contented to dredge at 

 first from an ordinary row-boat. Dredge with the 

 tide, not against it, as in the latter case the current 

 brings up the rope and prevents the dredge from 

 biting the ground. It is well to dredge up marine 

 slopes. You can tell when the dredge is fishing 

 properly by the nibbling and slightly jerky motion 

 that is conveyed by the rope ; when there is an 

 unyielding strain then you know that the dredge is 

 jammed ; sometimes a little extra force will free it, 

 but more usually the safety lashing gives way, and 

 then the capsized dredge should be hauled in. 

 Don't let out all your rope; when you have let out 

 enough to enable the dredge to be properly dragged, 

 take a turn round a thwart and keep your hand on 

 the line, between the boat and the water ; if a 

 sudden jerk comes you can relieve the strain by 

 letting out a little more rope. This is especially 

 important when dredging from a sailing boat, and 

 it is not a bad plan to tie a life-belt to the free 

 end of the rope, so that should the dredge get 

 stuck, the rope can be thrown overboard and 

 picked up on the other tack. 



When dredging in shallow water it is best to 

 make a number of quite short hauls of the dredge, 

 rather than to keep it down till the net becomes 

 choked up. It is a good plan to empty the contents 



