261 



without reference to, and in most cases prior to, the prepara- 

 tion of tables and plots of seasonal distribution. Estimates 

 made by others, not accustomed to judging the plankton, re- 

 veal a wide divergence in percentages, and independent esti- 

 mates which I have made of the same material on different oc- 

 casions show some divergence, though, as a rule, quite within 

 the probable error of plankton method. The silt estimation 

 does not, I believe, essentially vitiate any of the conclusions 

 drawn in this discussion of quantitative results, and in no way 

 enters into the qualitative analysis. 



THE CLOGGING OF THE NET. 



The collections made with the silk net drawn from the 

 bottom to the surface of the water in vertical or oblique hauls 

 are all diminished in volume to some extent by the resistance 

 of the silk to the rapid passage of the water. As a result of 

 this, the net pushes aside some of the water in the column which 

 it is supposed to traverse. As its meshes clog with the accu- 

 mulating catch the amount pushed aside is increased progres- 

 sively during the haul. The actual catch of the net is there- 

 fore only a portion of the total contents of the column of water, 

 whose length is that of the haul and diameter that of the 

 mouth of the net. The volume of plankton actually present 

 in this column can be computed if we can determine the factor 

 of correction. Hensen ('87 and '95) has sought by experiment 

 with filtered water to determine the mathematical formula 

 which will give this correction for a net of known silk and di- 

 mensions drawn at given velocities. This factor he calls 

 the "coefficient of the net." Reighard ('94) attempted to de- 

 termine the coefficient of his net by using a miniature model 

 in water in which Lobelia seeds had been placed, but ultimately 

 adopted the formula of Hensen. 



It was necessary that in our final computation of the volume 

 of plankton we should make some correction in all catches of 

 the drawn net for this loss due to the pushing aside of the 

 water. Since our net was constructed after the Hensen- Apstein 

 model it was possible to apply the mathematical method of 



