CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 



127 



is easily calculated, and the error resulting from this estimation 

 probably does not amount to very much. Now, at this stage of 

 the investigation, we are able to say that the catch obtained has 

 been contained in a water column extending from the surface to 

 the sea bottom and of a definite cross section. This cross section 

 is always a fraction of a square metre and it is easy to calculate 

 the amount of plankton which has been contained in a column 

 of sea of the depth in situ and of one square metre in cross 

 section. 



One condition limits the use of the apparatus just described. 

 It is necessary that it should be used from a vessel which is stopped, 

 and therefore it can generally be used only on vessels which are 

 intended primarily or exclusively for scientific work at sea. But 

 it would be a very great advantage if some form of plankton net 

 could be devised which could be used from a steamer travelling at 

 full speed, for in this way it would be possible to utilise the 



Annuiai' 

 openi ng 



vy vy V . ' , ^ ^ V ■ A ^ ^'v ■. q-TTTl Y*"^^^^'^ 



Fig. 26. The Korbnetz. 



services of merchant ships traversing all seas. Several such 

 methods of collecting plankton are in common use, but it is difficult 

 to devise a means of quantitative fishing such as would yield the 



