262 



Hensen ('95). This coefficient— computed for us by Instruc- 

 tor W. C. Brenke, of the University of Illinois — is 1.320 or 

 '1.303, according to the area of the silk in the bucket of the 

 net, for the velocity of one-half meter per second, which 

 we uniformly employed. Our net has an area of 81.72 square 

 centimeters in the opening; of 1,847.5 and 22.66 square cen- 

 timeters respectively in filtering cone and windows of the 

 bucket; and its coefficient is 1.32. With a disk of silk clamped 

 to the lower end of the net, — a method used prior to the adop- 

 tion of my detachable bucket, — the area of the silk of the buck- 

 et falls to 15.2 and the coefficient is accordingly reduced to 

 1.303. A second net frame which we used, with an opening 

 two millimeters less in diameter, has a coefficient of 1.318. 

 Apstein's ('96) net has an opening of 92 square centimeters, a 

 filtering cone of 1,730, and the silk of the bucket measures 62 

 square centimeters. The coefficient of his net is computed to 

 be 1.39. 



Observations on the operation of the net in the field 

 through the seasonal changes of the plankton led me to be- 

 lieve that a uniform coefficient, and, moreover, one founded 

 on the operation of the net in filtered water, would not ade- 

 quately correct the error, since it takes no account of the sea- 

 sonal changes in the quantity and kind of plankton, and does 

 not recognize the effect of the progressive clogging of the net 

 by the catch, or the change of the net with use. 



Upon the adoption of the pumping method of collection a 

 number of tests were made for the comparison of the amount 

 of plankton taken by the drawn net and that by the pump and 

 filtering net along parallel courses of thirty meters — our usual 

 haul. The results amply justitied my belief that the coeffi- 

 cient fluctuates with the conditions above named as well ag 

 with the condition of the net. We therefore sought by this 

 empirical method to determine the coefficient under a variety 

 of conditions representative of our environment. These tests 

 were not carried beyond this point, since we had adopted the 

 pumping method for the later and major part of our work. 



