Plankton Catcher- — Bary et al. 
53 
and the flow-meter registering were this 
permitted. 
To facilitate using flights of catchers on a 
vertical wire, a mechanism for releasing a 
messenger from the after end of each catcher 
was constructed. The impact of the messenger 
on the release arm set free the lower messenger 
which, in turn, triggered the closing mechan- 
ism of the next catcher down the wire, and so 
on. Part of the mechanism was in the rear 
bracket and is shown in Figure 3, o", o'"; 
some of it was also built into the original 
forward bracket, but this has not been con- 
tinued in the present one. The assembly has 
now been dismantled, but there are few dif- 
ficulties to its reintroduction. 
Horizontal Towing 
To convert from the rigging for vertical, to 
that for horizontal tows, takes about five 
minutes. The wire stop, and the after and 
forward brackets are removed. The eye of the 
towrope is inserted into the bracket (Figs. 1, 
2, 4, p) used for horizontal tows, at a point 
close behind the attachment of the release 
arm of the closing mechanism (1, 2, g'). 
Because the catcher is required to be towed 
at depth, a 45-pound depressor (as supplied 
by Scripps Institution of Oceanography) is 
attached (Fig. 7). It hangs on a free wire from 
a bracket on the underside of the forward end 
(1, 4, q). It is most efficient, but in rough 
weather it is difficult to handle, and causes 
further trouble when it snatches into passing 
waves. A removable, adjustable, built-in de- 
pressor would be desirable in a future model. 
The Release Arm ( Closing Mechanism ) 
The same release arm (Figs. 1, 2,4 , g; 5, 6, 
7), of stainless steel tubing, operates the clos- 
ing mechanism in vertical and horizontal tow- 
ing. It is essential that the surface which the 
messenger strikes is approximately at right 
angles to the run of the wire. Provision for 
this is made in the curvature of the arm. The 
messenger (4, v; 6, 7) carries a ring on a 
frame from its base, so that the two sides of 
the arm are struck, wherever the wire is lo- 
cated between them. 
Features which are novel to the plankton 
catcher are its fibreglass construction, the 
sleeve-type valve for stopping the flow of 
water into the net, and the readiness with 
which conversion from a catcher for vertical 
towing to one for horizontal towing is carried 
out (without alteration of the catching power) . 
EFFICIENCY TRIALS 
Trials to determine the efficiency with 
which the catcher accepts the water presented 
to it, and the effects that nets may have, have 
been carried out. The course and nature of 
the flow through the mouth and valve are 
indicated by tests with a two-dimensional 
model. 
Model Tests 
The model of the mouth and valve is a 
full-scale longitudinal, sagittal section, one- 
half inch thick, which is sandwiched between 
sheets of "Perspex,” screwed to the model 
and to separating pieces at the sides. A dif- 
fuser placed between inlet ports and model 
ensures that parallel lines of flow are presented 
to the model. Flow lines were indicated by 
cotton threads attached to a small metal sledge 
(made in the form of a box, but open to the 
flow at the sides) which was manoeuvred 
about the model with a magnet. The width 
of the test channel is 18 inches by one-half 
Figs. 5-7. (5) The catcher before lowering for a vertical haul. The lead sinker just clears the surface of the 
water. (The dark stripes on the body are "Scotchlite” fluorescent tapes which assist during subsurface observations.) 
(6) The catcher, rigged for horizontal towing, hanging in the guardrail crotch. The nose-down position, 
as illustrated, is a steady one when moving between stations. Until the net is withdrawn, however, the nose is 
kept higher than the tail. 
(7) The catcher as recovered from a horizontal tow. It is closed, as shown by the head of the spindle protruding 
from the mouth; a messenger rests on the release arm of the closing mechanism. The depressor hangs below the 
catcher. 
