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 (797) finds 
that the decrease amounts on the average to S80 per cent. Both 
of these records deal only with a smal] 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- 
- n Cp? — 9? j . . . . 
ing to the formula 0 | | : in which ois the density 
= 
of the contents of the tube; », the angular velocity in ra- 
dians per second; 7 the distance in centimeters from the 
axis of rotation to the bottom of the tube; and nm, 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 @ 7 a . - 
The formula for » is ~g5—. in which » is the number of 
revolutions per minute. 
