258 



On this basis the pressure in dynes in our use of the centri- 

 fuge, where there were 1 ,000 revolutions per minute with dis- 

 tances of 16.7, and 4.5 cm. to bottom of tube and top of liquid 

 respectively, was 1,420,484. Professor Ward writes me that 

 these distances r and n in the Bausch & Lomb machine he used 

 were 14 and 5 cm. respectively, which with a density of 1 and 

 80 turns of the crank, equaling 1,840 rotations of the axis (on 

 the manufacturer's authority that the machine is geared to 

 give 23 rotations of the axis to one of the crank), gives 2,774,- 

 897 dynes. Mr. Juday ('97) states that the pressure in his cen- 

 trifuge was 391,680 dynes, but he writes me that the density 

 used was the difference between that of alcohol and dried 

 plankton, amounting to only .2 and .25. Reducing his calcu- 

 lations to our basis in so far only as the matter of density is 

 concerned, we find the dynes to be from 1,958,400 to 1,566,720. 

 A further difference between our methods lies in the fact that 

 in our use the pressure was exerted two minutes, and but one 

 minute in that of the other investigators. 



The practical differences may not really be so great as the 

 figures indicate because of the asymptotic character of the 

 curve of reducing volume as the pressure continues or is in- 

 creased. It it desirable that some standard unit of measure- 

 ment be agreed upon for purposes of comparison. 



The use of the centrifuge in the many measurements here 

 recorded ha.s only confirmed the views expressed by me ('97) 

 regarding its utility, greater accuracy, and convenience for 

 volumetric plankton work. Material which has been properly 

 preserved has not suffered in the compression. All of the enu- 

 meration work to be reported in the second part of this paper has 

 been done upon plankton which has been centrifuged at least 

 once and in some cases six times. I have not detected any 

 mutilation or distortion of the constituent organisms unless it 

 be of Leptodora hyalina, an elongated and delicate cladoceran, 

 and several other organisms of great delicacy of organization. 

 These are often crumpled as a result either of the compres- 

 sion or manipulation of the plankton, but the crumpling rarely 



