282 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



important to know, for each variety of the silk bolting 

 cloth of which the net is composed, how mnch water for a 

 given pressure and in a given time passes through a square 

 centimetre. Hensen has with this object in view con- 

 structed a very ingenious apparatus and by means of 

 numerous experiments has calculated the filtration 

 capacity of each net-substance (1. page 12). In addition 

 to these experiments another set is necessary in order to 

 determine the filtration capacity of the net itself, and in 

 order to make this calculation easier the net must 

 be exactly conical, and to that end great care must be 

 exercised in its construction. Even then an exceedingly 

 laborious and difficult calculation is necessary for each 

 separate net. From the depth of water to which the net is 

 sunk, from the quantity or volume of water through which 

 it passes, from the speed with which it is drawn up, and 

 from the filtration capacity of the net substance and of the 

 net itself, Hensen has finally calculated what fraction of 

 the water column through which the net has been hauled 

 passes through the net itself, and what fraction has escaped 

 over the edge of the ring. The number of organisms 

 captured in the net is thus brought into relation with the 

 column of water through which the net has passed, and so 

 with any given portion of the sea. 



By means of a closeable net, observations can be made 

 at any depth required. The net is lowered vertically to 

 a given depth while open, but in a collapsed condition, 

 so that practically nothing gets into it through the ring. 

 The water that passes in at first does so through the 

 meshes of the net. Then it is hauled up vertically 

 through a given height, and by means of a weight which 

 runs down the rope it becomes closed, and is then hauled 

 on board so that the organisms in the water at the required 

 depth, say from 1,000 to 500 metres, are captured. 



