551 



cisms ('97b) upon the Hensen method, concludes that the loss is 

 voliimetrically of some importance, and numerically as large 

 as that indicated by my determinations of the leakage in our 

 field of operations, indeed, slightly larger in some instances. 



The table beginning on page 552 gives the volume of water 

 filtered, the total catch, the computed catch per m.', the esti- 

 mated per cent, of silt, and the computed volumes of silt and 

 plankton (filter-paper catch) per m.^ The volumes of plankton 

 per m.'' taken at the same time and place by the pump and silk 

 net are also given, and the averages at the end of the collec- 

 tions for each year refer only to the coincident catches of this 

 table. The omission of several silk-net catches in the data of 

 1896 and 1897 in this table makes its averages differ slightly 

 from those of Table III. 



The filter-paper catches, as in the case of the silk-net 

 catches, represent the plankton from a vertical column of water 

 from bottom to surface. Prior to the autumn of 1897 this wa- 

 ter was collected by hose and pump, either in equal quantities 

 at a series of levels from bottom to surface or during a continu- 

 ous transit of the hose through this region, due precautions be- 

 ing taken to secure proportionate amounts from all levels. Af- 

 ter this date a water-trap, consisting of a brass tube six feet in 

 length and four inches in diameter, with a gate at the bottom, 

 was used for securing the water for filtration. The water was 

 taken to the laboratory, and after thorough agitation of the col- 

 lection a sample of the required volume was withdrawn for 

 filtration. The paper used was a hard-.pressed filter-paper from 

 Schleicher and Schiill, No. 575, and filtration was hastened by 

 washing the filtering surface frequently by a spray from a rub- 

 ber hand-bulb. The penetration of the small organisms among 

 the fibers of the filter-paper, and perhaps even through it, causes 

 some loss of material, so that the volume of the catch is some- 

 what reduced thereby. The estimates of silt in the table were 

 not revised with reference to the silk-net records or after tab- 

 ulation. 



Recognizing the fact that these volumetric data are to an 



