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, ar;d 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 IIL-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 less 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 controlled in 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. 



