124 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



security. The external net and ropes also protect the silk net 

 from accidental injuries. 



The apparatus is used in the following manner. To the upper 

 ring of the headpiece are attached three bridles and to these is 

 shackled the rope or steel wire by means of which the net is 

 hauled. A light derrick is rigged out from the mast, and to the 

 end of this is fastened a block over which the rope passes out. 

 When the net is used from a heavy vessel, or in rough seas, the 

 block over which the rope passes is not directly fastened to the 

 derrick but is connected with the latter by means of an accumu- 

 lator, or dynamometer, in the same manner, and for the same 

 object, as in the case of deep-sea trawling or sounding. The rope 

 may be marked as in the case of a deep-sea sounding line, or it 

 may pass out over a " metre-wheel," that is a wheel provided with 

 a counter which indicates the number of metres of rope which 

 have passed over it, and therefore the depth to which the net has 

 been sunk^ When the apparatus is to be used the ship is stopped 

 and brought round so that if a sea is running the net may be 

 lowered over the windward side. It is then lowered when the 

 ship rolls over to windward, so as to avoid the possibility that 

 it may go under the keel and in hauling foul the latter. If the 

 ship drifts too much to leeward while the net is being lowered 

 then the rope will lead out at an angle from the side and the net 

 will not fish through a vertical column of water. But if a sea is 

 running it may not be advisable to use the net at all. 



The apparatus is hauled by the steam winch so that it is 

 brought up to the surface at an even speed, and the time of hauling 

 is taken so that the amount of water passing through the net 

 may be calculated. The net is swung in on deck and the catch is 

 removed. In the Hensen procedure the cock at the bottom of the 

 bucket is opened and the catch is run out into the '' Filtrator.'* 



^ On board a small ship and in shallow water the gear may be simplified. 

 In the Lancashire fishery steamei', John Fell, the net is hauled from an ordinary- 

 boat's davit specially mounted for the purpose. This is swung out and in by 

 means of guys in the ordinary manner. Both metre-wheel and accumulator are 

 unnecessary in a small ship and in shallow water. If a steel wire rope is used 

 this may be marked simply by painting with coloured paint at every five fathoms. 

 When this paint is allowed to dry into the strands of the rope it is not easily 

 rubbed off. 



