Plankton Catcher — Bary ET AL. 
55 
rigged with no net, with a net of 10 meshes/ 
in., and with a net of 40 meshes/in., are 
shown in Figures 9 and 10. The catcher with 
each rig was towed over a straight course of 
one nautical mile (determined by radar) . Each 
run was timed, and calculated speeds ranged 
between 3 and 9 knots. The flow meter was 
read at the beginning and end of each run. 
Meter readings were highest when no net 
was enclosed— about 390 revolutions/mile 
(Fig. 9)- The 10-mesh net caused a reduction 
to 345 revs. /mile (about 11.5 per cent), and 
the 40-mesh net to 310 revs. /mile (about 20.4 
per cent). Thus, some restriction to flow is 
introduced with the nets. That this is due to 
frictional resistance offered to flow past the 
strands of the net, and not to insufficient 
filtering area, is suggested by two sources of 
evidence. When moderate blooms of diatoms 
are encountered the 40-mesh net clogs at first 
only in the lower 12 to 24 in., indicating that 
filtering is occurring over a relatively small 
area of the net. More convincing evidence is 
illustrated by Figure 9, where revolutions per 
mile of tow are plotted against speed of tow- 
ing. It is apparent that over the range of 
speeds utilized there is no reduction in the 
number of revolutions, for each of the rigs 
investigated. This is an important point and 
shows that the quantity of water filtered per 
unit of distance is constant for each rig, al- 
though the volume presented per unit of time 
increases with the speed. If there were an 
insufficient area to filter the flow (or if there 
were other restrictions, the effects of which 
increased with speed), less water per unit of 
time would be accepted at the higher speeds 
and fewer revolutions recorded for a given 
distance. 
An approximate calibration of the meter 
has been made for several speeds. It was 
towed mounted in a straight tube, 3 ft. long 
and 9 in. in diameter (the same diameter as 
the mouth and tail of the catcher). It is 
assumed the tube accepts all of the volume 
of water in a column with a cross-sectional 
area equal to the tube. The meter recorded 
Calibration tube:--- o 
Catcher: 
no net x 
io-mesh net • 
40-mesh net a 
p 1 1 1 1 i * i i l i i » i ■ i 
2 3456789 10 
Knots 
Fig. 9. Revolutions of the depth-flow meter per 
nautical mile plotted against speed of towing. The 
uppermost curve is from the readings obtained when 
towing a straight tube, 9 in. in diameter; the lower 
three curves result from towing the catcher with no 
net (upper), with a net of 10 meshes (middle), and a 
net of 40 meshes/inch (lower). 
approximately 675 revolutions per nautical 
mile. Against this was the recording of 390 
revs. /nautical mile for the catcher (without a 
net). This represents an apparent acceptance 
value for the catcher of only 57.8 per cent 
(Fig. 10). Such a value suggests a major 
restriction to flow in the catcher, and it is 
probable that this is caused by the narrower 
orifice, namely the valve. This being so, the 
ideal acceptance would be in the ratio of 
area of valve , . , . ^ . . , 
, which is 0.649, i.e., the valve 
area of mouth 
should accept 64.9 per cent of the water pre- 
sented to the mouth. That the actual accep- 
tance (57.8 per cent), by the valve is lower 
than its theoretical acceptance (64.9 per cent), 
indicates other, but less obvious, restrictions 
to flow. Cavitation on the inner face of the 
valve, and some details of the construction, 
are probably factors to be considered. 
The volume of water in a column one nau- 
tical mile long (1.85 km.), of a diameter of 
9 in. (22.9 cm.), is 76.7 m 3 . The meter in the 
calibration tube recorded 675 revolutions for 
this volume, which is equivalent to 0.114 
m 3 /rev. The tail of the catcher is also a tube 
of 9 in. diameter, and it may be assumed the 
flow patterns in it are similar to those in the 
calibration tube. Therefore, a meter reading 
of 390 revs. /mi. (no net in the catcher) rep- 
