168 TRAXSACTIOXS LIVERPOOL BIOLOGICAL SOCIETY. 



catch. This only applies in the case of the more minute 

 organisms which were present in great abundance, such as the 

 Diatoms and Diiionagellates, where samples of the catch have 

 to be taken for counting, and estimations made. In the 

 case of the Copepoda and Sagitta and other larger organisms 

 this source of possible error is excluded, as these forms were 

 picked out directly from the entire preserved catch with the eye 

 or a hand lens and were all counted. Sampling and estimation 

 were not applied to the macroplankton, and yet the irregularity 

 is as great there as in the case of the estimated microplankton. 



The experimental error to be expected in the case of the 

 chief groups of organisms, and also in the case of a typical 

 common species of each, has been calculated with the following 

 results. 



The total number of Diatoms on April 3rd varied in the 

 six hauls from 3,880 to 10,020, the mean or average being 

 8,055. Two of the hauls are below the average and four above. 

 The smallest haul is as much as 52 per cent, below the average 

 and the largest haul is 2-1 per cent, above. The question is — 

 do these variations in the catch come within the limits of the 

 probable error of the experiment 1 



If we assume that the estimation of the number of Diatoms 

 in each haul is correct, then the possible errors are those 

 inseparable from all such collecting at sea — slight movements' 

 of the boat, unknown currents in the water, irregularities in 

 the verticality of the line, etc. In this case of the Diatoms 

 on April 3rd the " probable error " is found to be = 1,458, 

 and the "range" is the mean ± the probable error, that is from 

 6, COO to 9,500.* Comparing this range with the estimated 



* Obtained as follows : — We may assume that the small unavoidable 

 causes of error are not correlated, and that no single one has much greater 

 effect than any other ; also that they tend to make the individual catches 

 more than the mean as often and as much as they tend to make them less 



