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 Kottom 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 
