THE SHIP AND ITS EQUIPMENT 



Zl 



he generally uses sackcloth, but a better fine-meshed material 

 would undoubtedly be more desirable. 



Hensen evolved various forms of apparatus for a quantitative 

 study of the pelagic organisms, that is to say, for estimating 

 the relative amounts of plankton organisms present in a given 

 volume of water. He recommends vertical nets of the finest 

 silk cloth, such as is used in the milling 

 industry (see Chapter VI.). 



In actual practice, however, it has 

 been found impossible to capture pelagic 

 organisms of every sort with the same 

 net ; for the larger forms may escape the 

 net altogether, while the smallest forms 

 may pass through the meshes of even 

 the finest silk. There are other objec- 

 tions to the method, for it is an almost 

 impossible task to ascertain the total 

 quantity of floating organisms in deep 

 and shallow water where there are 

 strong currents ; and it is hardly likely 

 that the larger organisms at any rate, 

 even though the nets succeed in cap- 

 turing them, are uniformly distributed 

 throughout the water- masses over large 

 areas, so that an estimation of their 

 total number could not be arrived at 

 with our present appliances. Still, 

 Hensen's theoretical analysis of plank- 

 ton problems has been of great service 

 to oceanic research, and so, too, has 

 his plankton net (Fig. 19), whose co- 

 efficient of capture naturalists have 

 attempted to calculate. It has been of 

 the utmost value, for instance, in investigating certain uni- 

 formly distributed minute species in less extensive areas. The 

 apparatus consists of a filtration net of miller-silk, with a brass 

 cylinder at the lower end of the net, and a large conical part 

 made of canvas, the object of which is to control the amount of 

 water entering and so enable the silk net to filter it. 



Hensen 

 plankton net. 



Fig. 19. — He.nskn's Large 

 Plankton Net. (From Chun.) 



The steamer "Michael Sars " was built in 1900 by the The "Michael 

 Norwegian Government to undertake researches in connection Sai^." 

 with the Norwegian fisheries, and to study the natural con- 



