Wii] Kofoid: Self-closing Plankton Net. 815 



weights is first lowered to the desired level and then the net is 

 slid down on the cable and the curtain across its square opening 

 is withdrawn and rolled up as the net strikes the terminal 

 weight. After towing, the net is closed by a messenger whose 

 impact unrolls the curtain again across the opening. The 

 amount of weight at the end of the cable can be readily in- 

 creased for increased depths or greater speed in towing, so as 

 to maintain a horizontal position of the net. The bag of the 

 net itself is very short so as not to become entangled during 

 descent with the closing apparatus or weights. This net in 

 its operation on the "Hirondelle" and her successors has, ac- 

 cording to all reports, functioned with accuracy and certainty. 

 It was used also upon the "Pola" Expedition (see Morth, 1892), 

 but not with entire satisfaction. The rather complicated struc- 

 ture of this net and the great cost, over fifteen hundred francs, 

 appear to have interfered with its general adoption. It may 

 also be noted that in hauling in the net some pressure will be 

 exerted through the upper side of the bag against the flexible 

 curtain whose free lateral edges might be pushed outward with 

 resulting loss of contents. Furthermore, there appears to be no 

 device in the net, as constructed, for insuring positively that it 

 is at the bottom and has been opened when the messenger strikes 

 it and brings about a condition indicating closure. The net has 

 also a minor disadvantage in having two vertical bars crossing 

 the opening. 



It is unfortunate that the closing net in process of perfection 

 by Fol (1890) was never described. 



An ingenious device for a closing-net to be opened and closed 

 at any desired depth by the pressure of the sea-water is described 

 by Viguer (1890) of the Algerian biological station. This net 

 would be opened upon reaching the depth to which the pressure 

 apparatus was adjusted and closed again upon leaving it. It 

 does not appear that it was ever actually successfully con- 

 structed. It is obvious that it would be expensive to build and 

 difficult to maintain in efficient working order. 



A modification of the curtain net of the Prince of INlonaco 

 was used by Professor Th. Barrois (1892) in his exploration of 

 Svrian lakes. He substituted for the silk curtain which closed 



