128 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



results given by the vertical nets. In 1888 Hensen devised the 

 *' Korbnetz," an apparatus which could be so employed. The 

 Korbnetz (Fig. 26) consists of a strong metal case shaped like an 

 inverted cone, and pierced with two small holes at its base. The 

 truncated apex of the cone has a rim, and to this rim is screwed 

 a metal ring which is covered with fustian. A hollow metal 

 cone is attached to the ring so as to leave an annular opening 

 between it and the ring. The net is towed by a rope which is 

 shackled to the cone. When it is being towed the water passes 

 through the annular opening into the net, distending this and 

 then passes out again into the sea through the two small holes in 

 the base. 



The Petersen-Hensen quantitative net is a very complicated 

 apparatus and I can refer to it only very briefly. It is a 

 cylindrical net which is thrown into deep lateral folds, which folds 

 are produced by metal rods which are contained in the case in 

 which the net is placed. This form of the net gives it a very 

 large filtering surface. All the water which enters the net does 

 so through an opening in the cover of the case, and in this opening 

 is placed a small metal propeller which is actuated by the in- 

 draught of water, and also a revolution counter. From the 

 number of revolutions made by the propeller the amount of water 

 which enters the net can be calculated in an analogous manner to 

 the estimation of the distance traversed by a vessel which tows a 

 "patent log." The net is towed astern of the ship while the 

 latter is travelling at an ordinary speed, and because of the small 

 area of the opening through which the water enters there is not 

 much pressure on the walls of the net, and the latter is not rup- 

 tured even if it is towed at a very considerable speed. 



In the case of each of these apparatus the net, w^hen the haul 

 has been made, is removed from the case and is washed out and 

 the catch estimated in the same way as will be described for the 

 vertical net. The volume of w^ater which has been filtered is 

 found directly from the recorded revolutions of the counter, in the 

 Petersen-Hensen net ; and it is calculated in the case of the 

 Korbnetz from the length of time during which the apparatus 

 has been towed, the area of the annular opening, the speed of the 



