1911] Kofoid: Mf -closing Plankton Net. 317 



towing, and thus to reduce the relative amount of the catch as 

 compared with that taken during the ascent of the net. This 

 ob.jection is less applicable to the improved models in which the 

 only weight to the rear is that in the light pipe-frame for the 

 closing weights. 



Xhe small horizontal closing-net of Fric and Vavra (1893) 

 is of a simple type with two ropes, one for towing and a second 

 for closure by constriction, obviously ill-adapted for work in 

 deep water. 



The idea of using the pressure of the Avater to open and 

 close the net is embodied in the description given by De Livron 

 (1893), Lieutenant of Marine in the Russian Navy, of an ap- 

 paratus with a piston, set to begin movement under the desired 

 pressure, that is. at the desired depth. The lids are connected 

 with the piston and its first movement opens the net. It is 

 closed again by the piston when it moves because of the increase 

 or decrease of the pressure. No figures of the apparatus are 

 given and there is no evidence that it was ever actually success- 

 fully operated. 



The Giesbrecht (1893) self-closing net is devised for hori- 

 zontal towing and is sent down closed, is opened by resistance 

 of the water upon the two spreading wings, when towed hori- 

 zontally, which releases the closing lever. It is closed by mes- 

 senger. The four hinged sides of the square opening of the 

 net spread open by their own weight upon release and closure 

 is accomplished by their folding together again. In its original 

 form the bag of the net is rather complicated and not readily 

 cleaned and the system of opening not sufficiently under control. 

 In operation at sea in rough weather or in deep water it would 

 be impossible to determine with any certainty when the net 

 really opened or to be positive that the closure was complete 

 and continuous, since the net is held closed merely by its own 

 weight. Furthermore, the vertical bar across the opening inter- 

 feres with the capture of certain pelagic organisms. In a modi- 

 fied form (Richard, 1896) the net has been used somewhat 

 extensively in connection with the explorations of the Prince of 

 Monaco, apparently with satisfactory results. 



