426 Birge — Vertical Distribution oj Pelagic Crustacea. 



passes through holes on each side of these plates. The upper 

 two plates are pierced by the branches of the fork, and the stem 

 passes through the bottom plate. To the stem below the frame 

 is attached a side wire — the weight-pin — which passes up 

 through a hole in a lateral projection of the bottom plate 

 for about half an inch. This is the hooK to which are hung the 

 weights for moving the cover. 



The fork for the opening weight is narrower and shorter than 

 that for the closing weight. Its branches are 6 cm. apart, and 

 project 2 cm. above the frame. The branches of the other fork 

 are 9 cm. apart, and project 6 cm. above the frame. The top of 

 each branch of the forks is bent at right angles across the 

 frame, projecting inwards, so that the messengers which release 

 the weights have something more than the ends of the wires to 

 rest upon. Both forks are held up by rubber bands, stretched 

 from the junction of the branches to the middle plate. They 

 can be depressed just far enough to bring the top of the weight- 

 pin upon a level with the plate through which it passes, and thus 

 allow the weights to become detached. The messengers for re- 

 leasing the weights are discs of lead, each with a slot in the 

 side and a wire catch, so that it can be slipped on the dredge 

 line from the side. The opening messenger is 7 cm. in diame- 

 ter, and weighs 275 g. (10 oz.). The closing messenger is 10 

 cm. in diameter, is perforated with holes to sink more rapidly, 

 and weighs 450 g. (16 oz.). The opening messenger is of such 

 diameter that it passes between the ends of the closing fork 

 and strikes the top of the opening fork, depressing it so as to 

 release the opening weight. The larger — closing — messenger 

 when it sinks rests on the broader closing-fork and depresses 

 it, thus releasing the closing-weight. 



The weights. — The weights used in working the cover of the 

 dredge were at first made of ordinary lead fishing sinkers, 

 weighing 4 to 6 ounces each. These were chosen because the 

 total weight could easily be altered by adding or withdrawing 

 sinkers, as was indicated by experience in using the dredge. 

 When the weights had been adapted they were cast into one 

 piece. The opening weight weighs 425 g. (15 oz. ) and the 

 closing weight 1025 g. (36.5 oz.). The closing weight has to 



