236 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
For example— 
| Volume under square metre of 
Area of opening | Speed of pull | surface water calculated from the 
sq. centimetres. cm, per second. | separate catches by the theoretically 
| determined p 
| 
6°1 53 | 284 
12°6 | AT9 | 262 
24-4 | 552 234 
45°7 | 54°3 190 (’) 
90-2 | ipa | 189 
367°7 | 53°6 184 
1226:°0 | HA:9 | 189 
Thus the coefficient @ is not sufficiently great to 
cover the loss in catches made with wide mouthed nets. 
Since the pressure s used and determined in the reckoning 
is correct, something must be wrong with the pressure d, 
and that is the assumption, made at the beginning, that 
it was uniform all over the net. As a matter of fact in 
a plankton net the filtration pressure varies and just as 
at the proximal edge only a portion of the water is 
filtered, so at the following zones only a portion of the 
residue is filtered, until, finally, at the apex of a long net, 
only a minimal quantity of water passes through. This 
fact 1s of great importance in the practical employment 
of the net because too often the upper parts of the net 
(especially under the large ring of the conical head-piece 
in the Hensen nets) are insufficiently washed down and 
a large quantity of plankton caught here where filtration 
is greatest, is left adhering to the silk. Hensen 
attempted to calculate out theoretically, therefore, the 
correction necessary for this variation, but this is 
attended with many difficulties, and is best done 
empirically in the following way. This applies also for 
the conical net to be referred to shortly. Determine first 
a atl 
