246 FOUNDERS OF OCEANOGRAPHY 



sources of error, such as the movements of the ship, may, 

 for all we know, have added to the bulk of the smallest catch or 

 have diminished that of the largest, and so may have actually 

 lessened the evidence in regard to the natural irregularity 

 of the plankton, and the same is true of any possible error 

 there may be in the reading of the catch. The total mean 

 divergence of the average catch has been estimated at 

 32 per cent., and Schiitt attributes 20 per cent, of this to 

 the possible errors of the experiment all combined, and he 

 then deducts this from the 32 per cent, so as to reduce the 

 amount of divergence ; but some of the errors may have to 

 be added, not deducted, or they may neutralize one another. 

 They are quite unknown and it must not be assumed that 

 they tell in all cases, or at all times, in favour of uniformity. 



The Sargasso Sea, and no doubt some other oceanic areas 

 of limited extent, are probably more constant in their 

 physical conditions and more uniform in plankton contents 

 than inshore seas and than many other parts of the ocean ; 

 but it may be doubted whether they are sufficiently uniform 

 to yield results by Hensen-net methods that would enable 

 us to make a census or a quantitative estimate of the 

 whole area. 



Great stress has been laid by some writers upon the 

 efficacy of vertical hauls as giving reliable and therefore 

 comparable samples of the contents of a column of water of 

 known dimensions. I shall therefore discuss in some detail 

 the results obtained from a recent series of such hauls taken 

 in the Irish Sea. 



A few experiments have been made in the past, by Hensen 

 and others, in hauling comparable nets simultaneously or 

 the same net several times in rapid succession in order to 

 estimate the amount of variation in the results or the 

 divergence of each sample from an average. With the view 

 of getting further evidence from a new series of data, taken 

 with all possible care under favourable conditions, I carried 

 out a number of similar experiments at Port Erin during 



