The Coefficient of the Dredge. 279 



ber of Crustacea in a column of water 3 m. in length and 10 cm. 

 in diameter and to compare with this number the catch of the 

 net. For this purpose a tin tube was made, of the size indi- 

 cated. This tube was provided at the lower end with a slide in 

 which was placed a carrier bearing a net and bucket. The car- 

 rier and net could be slipped to one side so as to leave the open- 

 ing of the tube entirely free, and by means of a cord reaching 

 to the surface, they could be drawn back so as to hang immedi- 

 ately below the opening of the tube. The slide and carrier 

 were made of brass plates carefully scraped and fitted together, 

 so that no Crustacea could escape between the bottom of the tube 

 and the top of the net, and the net was closely covered when 

 slipped to the side of the tube. 



The tube was lowered into the water with the net moved to 

 one side of the opening and was lowered slowly so that the 

 water within the tube might remain at the same level as that 

 without and no appreciable currents should be set up in the 

 water. The tube was also provided with a close fitting cap on 

 the top, which could be closed after the top of the tube had 

 sunk about one-half meter below the surface. When the tube 

 had been lowered this cap was closed and the slide with the net 

 drawn across the bottom of the tube. There was thus im- 

 prisoned a column of water 10 cm. in diameter and 3 m. long. 

 The tube was then slowly raised to the surface and lifted out of 

 the water so that the contained water might be filtered through 

 the net, leaving behind the plankton. Several successive hauls 

 of the tube were made, and the number of Crustacea so taken 

 was compared with that obtained from a similar number of hauls 

 of the net made at the same time and through the same dis- 

 tance. The number of Crustacea thus obtained was carefully 

 determined, T V to T V of the number being counted where 

 the number was great, and ^ where the number was small. In 

 determining the coefficient of the dredge, it was assumed that 

 the tube took all of the plankton in the column of water which 

 it contained, and the number of crustacea caught by the tube 

 was compared with that caught by the net. Since the opening 

 of the net was four times that of the tube the catch ought to 

 have been four times as great, provided all of the water was fil- 



