SOME PROBLEMS OF THE SEA. 9 



Nothing in the economics of the sea could be more 

 important than snch conclusions if we could feel certain 

 that they are correct, or even that they are reasonable 

 approximations, for, of course, in dealing with such very 

 large numbers it is not possible, and it is not necessary, 

 to have absolute accuracy. 



These elaborate and highly ingenious methods of 

 Hensen and his school, devised both to capture and to 

 estimate the living contents of the sea, have naturally 

 been subjected to a certain amount of criticism. This has 

 appeared chiefly in Germany and in the United States, 

 and a fair example is seen in Professor Kofoid's paper 

 " On Some Important Sources of Error in the Plankton 

 Method " (see " Science " for Dec. 3, 1897).* 



It has been pointed out by Kofoid and others that the 

 net does not, as a matter of fact, filter the whole of a 

 column of water through which it passes; a part of the 

 water is pushed aside. There is for every net — probably 

 for every net on every occasion when it is used — a 

 co-efficient by which the result must be multiplied. It 

 is difficult enough to determine this co-efficient (some 

 number such as 1'32, which has sometimes been used) 

 when all the factors in the case are known, but even when 

 it has been correctly determined for a special form of net 

 and mesh of silk it must not be assumed that it will 

 remain constant. On the contrary, it will vary with the 

 rate at which the net is hauled and with any current 

 tidal or other in the water; and, furthermore, there are 

 two changes which take place in the mesh of the net that 

 will affect it, viz., shrinkage with use and clogging with 



* See also his " Plankton of the Illinois Kiver " in Bull. Illin. 

 State Lab. Nat. Hist., vi., Nov. 1903, where Kofoid explains why he 

 relinquished the Hensen and Apstein nets for the pump-plankton 

 method. 



