290 



still be floating in the liquid may settle to the bottom; 50 more revolutions are 

 given and the quantity of plankton is recorded. In all the measurements given 

 in the table below only from 20% to 50% of each haul was actually measured. As 

 a check upon this method several hauls were estimated and then the entire quan- 

 tity measured. The differences were so slight that they might be accounted for 

 by the small quantity of sand that is nearly always present. This would settle to 

 the bottom of the graduated cylinder quickly, and would not be included when 

 only a part of the haul is measured. 



One revolution of the drive wheel of the centrifuge causes SIA revolutions of 

 the sedimentation tubes, and a hundred revolutions of the drive wheel are made 

 in one minute. Thus in each case the plankton is subjected to 3,150 revolutions 

 per minute, which is equivalent to a centrifugal force of about 391,680 dynes. 

 In order to compare these results with those obtained by letting the material 

 settle twenty-four hours, eight measurements were made by both methods. These 

 show that the quantity obtained by the centrifuge is, on an average, only one-fifth 

 of the quantity obtained by the other method. So it must be borne in mind that 

 the results tabulated below must be multiplied by five in comparing them with 

 results obtained by letting the material settle twenty-four hours. 



The quantity of plankton taken at each haul does not represent the entire 

 quantity in the column of water through which the net passes. Some of the water 

 will be forced aside and the amount thus forced aside depends upon the velocity 

 of the net. (Reighard, 1893. ) That is, when the net is raised at a velocity of about 

 77 cm. per second, it will strain only half the column of water. In this case, to 

 get the entire quantity of plankton, the amount taken must be multiplied by two 

 which is the co-eflicient of the net for this velocity. If the net is drawn slower 

 more water is forced aside, hence a greater co-efficient. By plotting the results 

 obtained by using the co-efficient for the observed velocity and that for the aver- 

 age velocity of the sixty hauls, it was found that the two curves differ very little 

 except in two places and these represent hauls in which the velocity is very low. 

 So it was deemed best to use the average velocity, 63.5 cm. per second, in com- 

 puting results. The co-efficient of the net for this velocity is 2.215. 



Also, to find the quantity of plankton under one square meter of surface 

 another calculation is necessary. The area of the top of the net is 873.5 sq. cm. 

 or a little less than one-eleventh of a sq. m. (jr^lg) Thus, the quantity taken 

 multiplied by the co-eflicient of the net (2.215) and this by 11.448, or a total of 

 25.357, will give the amount of plankton under one sq. m. of surface. This result 

 divided by the depth of the haul will give the quantity of plankton per cu. m. of 

 water. 



The tabulated results of the sixty hauls are as follows; 



