DECEMBER 3, 1897.] 
were challenged, and in that time, so far as 
I have learned, no single implement has 
been reported from the gravels, although 
the exposures are as extensive as they ever 
were. The first chapter in the prolonged 
search for glacial man at Trenton may, 
therefore, be regarded as practically closed ; 
but some new evidence furnished by ex- 
amination of certain superficial deposits of 
sand come up for consideration. My re- 
marks upon this subject will appear in a 
subsequent number of SciENcE. 
W. H. Homes. 
U.S. NarronaL Museum. 
ON SOME IMPORTANT SOURCES OF ERROR 
IN THE PLANKTON METHOD. 
Tue Hensen method of plankton collec- 
tion consists essentially in drawing a silk 
net vertically through the water. <A part 
of the column of water traversed by the net 
is pushed aside, hence the actual catch 
must be multiplied by some factor to obtain 
the amount of plankton present in the given 
column of water. This factor, ‘ the coeffi- 
cient of the net,’ has been calculated by 
Hensen, for a series of velocities, from 
empirical data, and is applied uniformly to 
all catches without regard to the character 
of the plankton. The coefficient of the net 
used at the Illinois Biological Station, ac- 
cording to Hensen’s formula, is 1.32 (veloc- 
ity 0.5 meter persecond). A series of field 
tests in which a column of water, similar to 
that traversed by the net was pumped and 
strained, indicates that the coefficient of the 
net varies with the amount and constitu- 
ency of the plankton, ranging in the case 
of our net from 1.5 to 5.7. This variation 
is, in part at least, due to the increased clog- 
ging in the case of heavy planktons. The 
effect of the progressive clogging upon the 
coefficient is shown in a series of horizontal 
hauls of 5,10, 15, 20 and 25 meters, which 
were made successively in similar water. 
The coefficient rose from 1.5 in the 5-meter 
SCIENCE. 
829 
haul to 4.83 in the 25-meter haul. A com- 
parison of 15-meter hauls with those of 30 
meters indicates that from 84% to 96% of 
the 30-meter catch is taken in the first 15 
meters of the haul. Four places of decimals 
in a computed coefficient can hardly offer 
compensation for an error so fundemental 
as the variation in the straining capacity of 
the net. This error can be avoided by 
adoption of the pumping method and strain- 
ing of a known quantity of water. 
The plankton method as elaborated by 
Hensen and others depends upon the effi- 
ciency of the finest silk bolting cloth in re- 
moving the contained organisms from the 
water which it filters. It hasbeen accepted 
by planktologists that the use of this cloth 
furnishes a satisfactory basis for the volu- 
metric determination of the plankton and 
the enumeration of its constituent organ- 
isms. Hensen* (p. 75) states that the 
openings in the silk are so small that not 
many organisms can pass through them. 
Apstein f (p. 235) says: “‘ With nets of this 
cloth almost all organisms are caught, only 
a few diatoms, which happen to meet an 
opening with their long axis, escape.” 
Again { (p. 35) he maintains that almost all 
organisms are removed from the water by 
the use of No. 20 bolting cloth. No pro- 
test has been raised by our American 
workers § || to these claims of the founders of 
the plankton method. The leakage of the 
plankton through the silk has thus been 
minimized or ignored, and without tests of 
the extent to which it occurs. 
*V.Hensen. Methodik der Untersuchungen bei 
der Plankton-Expedition. Kiel und Leipzig. 1895. 
TC. Apstein. Uber die quantitative Bestimmung 
des Plankton in Stisswassers: in Die Tier- und 
Pflanzenwelt des Stisswassers. Dr. O. Zacharias. 
Leipzig. 1891. 
{Das Stsswasserplankton. Keil und Leipzig. 
1896. 
¢J. Reighard. A Biological Examination of Lake 
St. Clair. Lansing. 1894, 
|| H. B. Ward. A Biological Examination of Lake 
Michigan. Lansing. 1896. 
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