DAMPER 



PINS 

 ARRESTING GEAR 



BT ELEMENT 



FIGURE lO. f 

 PRESSURE-OPERATED 

 PLANKTON NET 



understanding of the vertical distribution of plankton is hampered 

 by not being able to sample at specific depths. Various methods 

 of closing the net have been proposed and some of them have been 

 moderately successful. However, in the long run, all of them have 

 been shown to have some basic inadequacy. Pumping water from 

 depths is not entirely satisfactory largely because some planktonic 

 animals can avoid the existing pumps. At our laboratory we have 

 considered designing sampling devices which can be actuated at 

 depth by either pressure, electrical, or mechanical action. An 

 example of a PRESSURE-OPERATED PLANKTON NET (fig. 18. 7) 

 has a pressure mechanism for opening and closing a plankton 

 sampler using a spring-loaded damper actuated by the pressure 

 element from a conventional 900-foot bathythermograph. The 

 sampler fishes between depth intervals preselected by inserting 

 pins of different lengths into the arresting gear. The first pin 

 governs the point at which the sampler will open as the BT ele- 

 ment is compressed by the water pressure. The sampler remains 

 open until further pressure releases the second and longer pin. 

 Larger plankton nets are not as easily opened and shut, one of the 

 means that we have considered is to use so-called strangle lines 

 which are successive attachments to the net for pursing it off. 

 Pressiire pistons can be used to sever the strangle attachments. 



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