126 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



All the water which enters the net does so through the smaller 

 opening of the headpiece. This is much smaller than the great- 

 est diameter of the net, and its area is from l/28th to l/46th 

 of that of the total filtering surface of the net. The object of this 

 arrangement is to allow the water which enters the net to pass 

 through the pores of the latter with as little pressure as possible, 

 and so avoid the destruction of delicate organisms, or the 

 lodgement of the smaller things caught in the meshes. No 

 filtration takes place through the fabric of the headpiece which 

 is made of impervious fustian. The practical advantage of the 

 headpiece is that if the net should touch the sea bottom when it 

 has been lowered the larger iron ring rests on the bottom deposits 

 and none of the latter enter through the opening. The head- 

 piece has also several other objects. The net has now been 

 hauled through a column of water the height of which is equal to 

 the depth to which the apparatus has been lowered, and the area 

 of cross section of which is that of the opening of the net. But 

 it will be obvious that all of this column of water has not been 

 filtered through the meshes of the apparatus because of the 

 resistance offered by the fabric of the net. How much of the 

 column of water does actually pass through the meshes and is 

 filtered ? It is obvious that only a fraction does so pass and it is 

 next necessary to determine what is this fraction. We have to 

 find now what is the filtration capacity of the net, and this is 

 calculated from a knowledge of the amount of water which 

 passes through the silk at a pressure which can be determined 

 from the size and form of the net, and from a knowledge of the 

 rate at which the apparatus is hauled up through the sea, a rate 

 which can very easily be observed^. Once the constants of the 

 net are known the volume of water which is filtered in each haul 



The latter is then turned out, the plankton adhering to its sides, and it is then 

 rinsed into a quantity of preservative contained in a wide-mouthed glass jar. 

 The last trace of plankton remaining on the silk is washed off from the latter 

 by a fine stream of preservative directed on it from a wash-bottle. A very simple 

 filtrator is used by the Heligoland naturalists : this consists of a brass ring 

 three inches in diameter and three inches deep. On one end of this a piece 

 of silk miillergaze, similar to that used for the net, is fastened by a strong rubber 

 ring or band. The contents of the bucket are run into this apparatus ; the silk is- 

 then detached and folded up and put into a bottle containing the preservative. 

 ^ See Appendix. 



