138 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



enough to retain these organisms in its meshes, and one cubic 

 centimetre of sea water will usually be a sufficiently large 

 sample. 



Other quantitative plankton apparatus. If then the mesh 

 of the silk cloth employed in the construction of the ordinary quan- 

 titative nets is too coarse to retain the smaller organisms of the 

 plankton it is necessary to devise some other filtering medium. 

 Lohmann employed a hardened filter paper which was fitted into 

 a metal funnel in much the same way as the large corrugated 

 filters of the chemist are used. The sample of water was run 

 through this filter and the catch was then removed from the latter. 

 If the sample of water to be examined is obtained from the surface 

 of the sea then it is a matter of no difficulty to employ this 

 apparatus ; and if small samples are to be investigated then the 

 water can easily be obtained by the use of one of the closing 

 water-bottles used in hydrographic observations, and which can 

 collect a water sample from any desired depth. But it is an essen- 

 tial of the quantitative examination of the plankton that the whole 

 column of water from the surface to the bottom must be filtered, 

 and this requirement introduces serious difficulties. Lohmann 

 followed up Kofoid's suggestion of using a pump, and he devised 

 an ingenious apparatus by means of which a fairly deep column of 

 water could be obtained. A long hose-pipe was coiled on a wind- 

 lass. One end of the pipe was open and the other was connected 

 with a pump. The open end was then let down to the desired 

 depth, and windlass and pump were then worked simultaneously, so 

 that a sample of water was obtained from every level of a vertical 

 column of known depth. This water was passed through the 

 filter and then collected so that its volume could be measured. 

 Thus a sea area of moderate depth can be investigated quantita- 

 tively by a method which does not appear to be open to any 

 serious objection. The apparatus thus devised also provides a 

 means whereby the constants of the vertical net, as calculated by 

 Hensen's method, can be corrected, for we can filter a column of 

 water which has, say, a cross section of two square centimetres, 

 and since the composition of the plankton can hardly change 

 within the limits of a column of water which is not greater than 



