RECENT FRESHWATER INVESTIGATIONS S07 



from 1.5 to 5.7, where the greatest variation is largely due to 

 the clogging of the net by heavy plankton hauls. Reighard 

 (98) proposed to eliminate all of the difficulties connected with 

 clogging, shrinkage and net coefficient at once by measuring 

 the volume of water that actually' passes through the net in 

 each haul. To this end a small current meter is to be placed 

 in the mouth of the net and the volume calculated from the 

 rate of the current passing through the opening. Experiments 

 in this direction are now in progress. 



As a substitute for the vertical net in obtaining the plankton 

 from a certain quantity of water, several investigators experi- 

 mented almost simultaneously with a plankton pump, so con- 

 structed that a definite amount of water is delivered by a single 

 stroke, the depth from which it comes being regulated by the 

 position of the mouth of the attached hose. The greatest diffi- 

 culty which presents itself is the proper filtration of the water 

 discharged from the pump. The advantages urged in its favor 

 are (Kofoid, 97) greater accuracy in determining the volume 

 strained, the wide applicability of the method in shallow water, 

 in currents, under ice, amid vegetation, for water very rich or 

 very poor in plankton, and the rapidity of the process. The 

 pump used by Kofoid was very large. Frenzel (97) who 

 advances much the same arguments in favor of this apparatus, 

 which he used with particular success in obtaining plankton 

 under the ice, gives no particulars regarding his pump. For- 

 dyce (98) describes a pump which is easily portable and can be 

 used with advantage in small bodies of water. 



A centrifugal apparatus has been used with success by Juday 

 and Kofoid (97) on preserved material in the measurement of 

 plankton volumes. Dolley (96) has employed a larger form, 

 called by him the planktonokrit, in the precipitation and 

 measurement of living plankton. This machine has been 

 used by Field (98) who later (98a) maintains its great superior- 

 ity for volumetric estimation over all other methods yet dis- 

 covered. Jackson (96, 98) found, however, that while good 

 results were obtained with Infusoria and Rotatoria, the reverse 



