260 
and Graybill state. The real credit for first centrifuging plank- 
ton belongs evidently to Kraemer. In view of the statement 
above quoted it may be well to add that our experimental use 
of the centrifuge was begun in the autumn of 1896. It was at 
once adopted, ard our plankton thus far collected was meas- 
ured by it at that time. The measurements made are those 
used in the present paper. Our experiments with and adop- 
tion of the centrifuge were independent of and without knowl- 
edge of similar work elsewhere. 
SILT ESTIMATION 
In Tables III1-IX. the amount of the actual catch of 
the plankton net will be found, and in subsequent columns the 
estimated percentage of silt, and computed volumes of silt, 
plankton, and total catch are given in cubic centimeters per 
cubic meter of water. In all our discussions of the plankton 
the amounts used are those of plankton only, unless otherwise 
stated, that is, of the total catch /ess the estimated amount of silt, 
and they are always quoted in terms of cubic centimeters per 
cubic meter of water. 
The determination of the amount of silt has been of ne- 
cessity a matter of personal estimation, and involves a source of 
error of uncertain extent. The estimates have been made large- 
ly by myself, with some aid from Mr. R. E. Richardson, and no 
effort has been spared to maintain a uniform standard of esti- 
mation so as to distribute, as far as possible, the error incident 
to the process. Accordingly the estimates were revised and cor- 
related after the qualitative and numerical analysis of the col- 
lections at Station E, and they consequently rest upon a com- 
parative basis upon the examination of the catch as it appears 
in the Rafter cell.* In the case of the collections from the Illinois 
River only, they have been controlledin some degree by the re- 
sults of the numerical analysis. The estimates of silt were made 
*Closer study of the quantitative and qualitative data since the above was written 
leads me to suspect that in some cases the silt estimates are too high, especially 
where there is much light flocculent debris, which occupies considerable space in the 
Rafter cell but may be compressed considerably in the centrifuge. 
