METHODS OF PLANKTON HESEAltCH. 



507 



above this two semi-circular brass lids are hinged so that 

 they can fall down, one on each side, and lying over the 

 brass ring completely close the opening. In order to 

 make the closure more water-tight, the net ring bears a 

 rubber ring on its upper surface, and there is further a 

 piece of stout waterproof canvas fixed by screws over the 



hinge itself, so that no 

 water can pass through the 

 crevices into the net when 

 it is being hauled up to the 

 surface. One of these trap- 

 doors (both together re- 

 semble a " butterfly valve ") 

 is perforated to allow air to 

 enter the net when it is 

 raised out of the water. 

 This is necessary because 

 when lifting the hermeti- 

 cally sealed net out of the 

 water, the water contained 

 in it filters out through the 

 silk, and unless air is 

 allowed to enter the filtra- 

 tion is hindered, and, more- 

 over, the whole net collapses. 

 In order that no water shall 

 pass in through the opening 

 for the air , it is provided with 

 a balanced valve, so that 

 it remains closed as long as the net is being pulled 

 through the water, and opens immediately it is above the 

 surface. The essential apparatus for closing the net at 

 any particular moment is shown in the photograph (fig. 3). 

 The apparatus in the photograph differs slightly from that 



Fig. 3. 



-Closing apparatus at mcuth 

 of net. 



