278 Birge — The Crustacea of the Plankton. 



and I have therefore put out the suggestions of the final sec- 

 tions of each part of my paper, with the hope that they will 

 stimulate others to similar attempts and thus lead to an en- 

 largement of our knowledge and to the correction of whatever 

 errors may be present in my conclusions. 



THE COEFFICIENT OF THE DREDGE. 



One of the most difficult and unsatisfactory portions of plank- 

 ton investigation has been the determination of the coefficient 

 of the dredge. It is well known that the net when raised 

 through the water offers a certain resistance to the passage of 

 the water, so that a part only is filtered by the net, while an- 

 other fraction is displaced. The determination of the relative 

 amounts of water filtered and displaced is the determination of 

 the coefficient of the dredge. Many attempts have been made 

 to determine this quantity. The most elaborate investigations 

 have been made by Hensen (Hensen, '87, p. 11, and Appendix; 

 '95, pp. 67-86). Reighard ('94, p. 57) has also devised and car- 

 ried out another method of determining the coefficient. Hensen 

 has attempted to work out a formula by which the coefficient for 

 a net of given cloth and given area could be determined, and 

 has finally given the best and easiest method of determining the 

 coefficient in lakes abounding in vegetable plankton ('95, p. 92). 

 Reighard's method depends upon mixing with the water a known 

 number of particles and determining the relation between those 

 caught by the net when drawn through the water and the num- 

 ber known to be present. This method was entirely inappli- 

 cable to a net constructed like mine, and it was impossible for 

 me to enter upon any elaborate investigation of the coefficients 

 of the cloth which I used. I confined myself, therefore, to a de- 

 termination of the coefficient of my net under the conditions in 

 which it was used. In the serial investigations which formed 

 the greater and more essential part of my study, the dredge 

 was raised through a distance of three meters. The speed was 

 approximately one half meter per second, although ordinarily a 

 little less, the total time occupied by raising the dredgelhrou^n 

 3 meters, being from 6.5 to 6.75 seconds. In order to ascertain 

 the coefficient of the dredge I determined to ascertain the num- 



