257 



be doubled for comparison with volumes reported in similar regions 

 elsewhere and measured by the gravity method. 



The silt in our catches and the predominance of rotifers at 

 times will tend, I believe, to render the decrease on centrifug- 

 ing somewhat less in the case of our planktons, on the average, 

 than that found in lake planktons where filamentous diatoms 

 and Copepoda abound more generally. Ward and Graybill ('00) 

 find that the decrease ranges from 60 per cent, to 70 per cent, 

 and averages slightly less than two thirds. Juday ('97) finds 

 that the decrease amounts on the average to 80 per cent. Both 

 of these records deal only with a small number of midsummer 

 planktons, while our records cover the whole round of sea- 

 sonal changes. This may be an element which tends to 

 increase the range of the decrease shown in our results. The 

 gravity work in our experiments was done in tubes identical 

 in pattern with those used by Ward and Graybill ('00), and the 

 time for settling was the same. Juday ('97) gives no account 

 of his gravity method. 



The centrifuges which we have severally used are only 

 approximately of the same pattern, and they have not been 

 used in exactly the same way by any two of us. 



We may compute the specific pressure in dynes per square 

 centimeter at a given distance from the axis of rotation accord- 



ing to the formula <*" ~ n \ : in which d is the density 



I J 



of the contents of the tube; <o, the angular velocity in ra- 

 dians per second; r the distance in centimeters from the 

 axis of rotation to the bottom of the tube; and n, the dis- 

 tance from the axis to the top of the fluid. For density I 

 have used the specific gravity of water, since the extractions 

 from the plankton considerably increase the specific gravity 

 the alcohol in varying degrees in which the plankton was 

 preserved. 



2 ~ n 

 The formula for w is 6o , in which n is the number of 



revolutions per minute. 



