264 
The coefficient has been computed in each case on the ba- 
sis of measurements by the gravity method, by the centrifuge, 
and by enumeration of all the larger and quantitatively more 
important constituents of the catch. An examination of the 
table will indicate that the relation and direction of the dif- 
ferences of the various coefficients do not materially differ by 
the three methods. Theresults by the enumeration method 
give the largest coefficient—probably as a result of the elimina- 
tion of the silt factor in some instances, and possibly by reason 
of the large margin of error involved in the method. 
It is evident from the table that an average of a number 
of catches, not only by the net but also with the pump, should 
be used if empirical coefficients are to be established with ac- 
curacy. It is probable that the low coefficients seen in a few 
instances result from insufficient pump catches, or from some 
error in paralleling the catches. Since the coefficient problem 
was eliminated in our later work by the use of the pump, fur- 
ther efforts to establish empirical coefficients were abandoned 
for lack of time to carrv on more elaborate tests. 
Three alternatives were thus before us. First, to adopt 
the coefficient computed according to Hensen’s formula, and 
use this one factor, 1.32, for all catches irrespective of the age 
of the net and of seasonal, local, quantitative, and qualitative 
differences in the catch. This method Apstein (96) and other 
European planktologists have adopted. Reighard (’94), Ward 
(95), and Juday (’97) have also followed this plan, but in each 
case they were dealing only with catches taken in midsummer 
from the same or similar bodies of water, and the resulting 
error thus introduced was much less than would result from 
the adoption of a uniform coefficient for our varied catches. 
Furthermore, we had the evidence of the probable extent of 
this errror which the pumping method afforded. 
A second alternative was to ignore the coefficient question 
entirely ; but this involves even greater distortion of the prob- 
able seasonal and local fluctuations in the plankton. 
A third method, and the one finally adopted, was that of 
