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BULLETIN OF THE UNITED STATES FISH COMMISSION. 
larger, but being lighter, are in part deposited in a thin layer on the top of the mass 
of larger organisms. We may consider separately the errors which are introduced into 
the volumetric method from the three sources above mentioned. 
(a) Errors due to clogging of the net.— This depends principally upon the area of 
the filtering surface of the net as compared to the volume of plankton present in the 
water. If the net surface is large and the volume of plankton in the water filtered 
small, there is but little clogging. The net employed by Kofoid was 25 cm. in diam- 
eter at the base and 40 cm. on one side. The plankton appears to have been unusually 
abundant (Kofoid gives no data) and the conditions otherwise un suited to the use of 
any sort of net. The net employed by Reigliard and Ward in the work above referred 
to had a diameter of 00 cm. and a slant height of 100 cm. Its filtering surface was 
thus about six times that of the net used by Kofoid, while the plankton in the water 
in which it was used was very little. In the work done by Reigkard not more than 
4.5 c.c. of plankton was taken in the net at one time and in the work of Ward not 
more than 11.9 c.c. In a majority of the hauls not more than a fraction of these 
volumes was taken. The net used by Hensen was much larger (Hensen, loc. eit., p. 6), 
while that used by Apstein was about the size of Kofoid’s net, but it was probably 
used under more favorable conditions. Clogging, then, does not seem to me to be an 
important factor with nets of the size used by Hensen, Reigliard, and Ward. It 
becomes important only in case a small net, such as Kofoid’s, is used under unsuitable 
conditions. Some measure of its extent is desirable. 
( b ) Error due to shrinkage. — This error is largely if not wholly eliminated by 
previous thorough shrinking of the net. The cloth used by Reighard and Ward was 
several times dampened and ironed before it was made up into the net and was thus 
presumably thoroughly shrunken. The net was also many times wet and dried before 
it was used for quantitative work. As may be seen from the table on page 57 of 
Reighard’s report, the cloth of the net used by him and later by Ward differed but 
little after a summer’s use from new cloth which had been once wetted and then dried; 
the cloth in the two cases being measured under as nearly as possible the same condi- 
tions. Whether the nets of other workers were similarly shrunken before use does 
not appear. I have not encountered any such enormous shrinkage as that recorded 
by Kofoid, in which the average size of net openings was reduced from .000024 to 
.00001 sq. cm. Everything here depends on a uniform method of measuring the cloth. 
( c ) Errors due to permeability of the cloth. A large number of the smaller plankton 
organisms escape through the pores of the cloth. According to Kofoid “the silk 
net retains from £ to of the total solid contents of the water.” “The amount 
escaping through the silk bears no constant relation to the amount retained.” These 
statements are certainly very startling, but one must reserve final judgment concerning 
them until the conditions of the experiments upon which they rest are made known. 
This degree of leakage through the net may be due to the peculiar constitution of the 
plankton examined. The extent to which this source of error vitiates previous work 
can only be determined by tests of the nets used by previous workers in comparison 
with other methods and in the waters in which the nets were used. In volumetric 
determinations most of the smaller plankton organisms are packed between the larger 
organisms in such a way as not to affect the total volume of plankton in the measuring 
tube. Some of them, however, remain in suspension longer than the larger and heavier 
organisms, and when they settle lie at the top of the whole mass measured, and so 
increase its volume. 
