CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 121 



But if instead of towing a net along the surface of the sea we 

 sink it down to the bottom and then haul it slowly up again we 

 get rid of most of these disturbing factors. The net can be 

 hauled by means of a steam winch, the speed of which can be 

 exactly regulated, and thus we ensure that the same quantity 

 of water always passes through the net. It is possible to calculate 

 very approximately what this volume of water is, so that we can 

 say that the catch made was contained in a certain portion of sea. 

 The haul can be made very quickly, and while it is being made the 

 ship is drifting with the wind and tide so that the effects of these 

 are eliminated. Further the net fishes through the water from 

 the bottom up to the surface and thus it does not matter in what 

 horizontal stratum of the sea the plankton is contained. Also the 

 amount of plankton taken in such a net is usually small, and this 

 is an advantage, for we can then work through it exhaustively 

 without considerable labour. This is the principle of the Hensen 

 method. The net is very exactly constructed ; it is hauled up 

 vertically from the bottom of the sea to the surface ; and its 

 "constants" are determined so that it is possible to calculate what 

 volume of water passes through its meshes \ 



The Hensen net. In Fig. 24 I give a diagram of the newest 

 form of the Hensen net. The apparatus measures about 7 feet 

 from the opening to the extremity of the bucket. It is made of 

 three parts, the headpiece, the net, and the bucket. The basis of 

 the whole is an iron ring of 1 metre in diameter. Three iron 

 stays connect this with a second iron ring which is 36 cm. in 

 diameter : between these two rings is stretched a piece of strong 

 white fustian, and this part, which has the shape of a truncated 

 cone, is the headpiece. 



The bucket has various forms. Sometimes it consists of a 

 strong glass cylinder which is about 35 cm. in length and about 

 20 cm. in diameter. This is contained in a strong iron holder, at the 

 upper end of which is a brass ring which is provided with three 



1 The net and the subsidiary apparatus are described in Hensen's first paper, 

 "Ueber die Bestimmung des Planktons" cfec, in 5 Ber. Komm. Wiss. Untersiich. 

 dcutschen Meeres, BerHn 1887; and in "Methodik der Untersuchungen," Ergebnisse 

 Plankton- Expedition, Kiel, 1895. 



