14 PLANKTON OF WISCONSIN LAKES 



three to four times as great as that in the material retained by the net. 



Following these experiments some tests were made with a milk 

 clarifier and these proved to be very successful. While this machine 

 did not remove all of the organisms, yet it removed such a large part 

 of them that the experiments were continued with a larger and more 

 powerful centrifuge which gave such satisfactory results that this type 

 of machine was selected for the investigation. This centrifuge is the 

 De Laval clarifier and filter which is intended for clarifying oils and 

 varnishes; the belt style, size A, was used and in this machine the 

 water is first subjected to a strong centrifugal action and then filtered 

 through filter papers. During the progress of the investigation the 

 effluent from this machine was tested frequently in order to ascertain 

 what proportion of the organisms that were lost by the net, were being 

 recovered by the centrifuge. These tests were made with the small 

 centrifuge employed for the sedimentation of the material on which 

 enumerations were made ; the results obtained in these experiments indi- 

 cated that the large centrifuge recovered substantially 98 per cent of 

 the algae and protozoa lost by the net. In addition to these organisms, 

 plate cultures made on gelatine and agar showed that between 25 per 

 cent and 50 per cent of the bacteria were also removed in the centri- 

 fuging process. This efficiency is maintained for quantities of water 

 up to 1,500 liters, or for as much as 10 grams of dry plankton material. 



Figure 3 is a sketch drawing of the laboratory showing the equip- 

 ment used in making these studies. In this sketch C is the centrifuge ; 

 M is the electric motor by which the centrifuge is driven through an 

 intermediate; T is the tank into which the water is pumped from the 

 dock by a pump marked P in the figure. 



Figure 4 shows a sectional view of the bowl of the centrifuge. The 

 water enters the bowl at A and passes down to the clarifying com- 

 partment, B, where some of the material is deposited. Then it passes 

 out to the periphery of this compartment, C, where the centrifugal 

 force is at a maximum. By far the greater portion is deposited here. 

 The water next flows upward and toward the center of the bowl be- 

 tween conical discs which divide it into thin layers and subject it to 

 further centrifugal action. This removes the last portion of material 

 that is obtained in the centrifuging process. This material is de- 

 posited on the under side of the discs and most of it passes down and 

 out to the pocket at C. 



The centrifuged water passes to the center of the bowl and is then 

 forced upward and outward to chamber D from which it passes on to 

 the filter compartment. The latter is filled with a series of horizontal 

 corrugated plates, F, nineteen in number, which possess perforated 

 retaining rings at their outer and inner margins. The filter papers, 



