METHODS OF PLANKTON RESEARCH. 509 



When the boat is rolling very much, a clever winch man 

 can so work as to wind the net up only as the ship's side 

 descends, while the ascent serves to haul up the net. For 

 systematic plankton work of this kind a steam winch is 

 very desirable. On the German Investigation Steamer 

 ■" Poseidon " there is a very complete set of winches, and 

 the rope used for the net is of thin wire. This wire passes 

 over a very essential, though small, piece of apparatus 

 fixed to the deck, and consisting of a wheel about one foot 

 in diameter, so geared that it records every metre of rope 

 that passes over it, and therefore the exact depth of the 

 net can be seen by simply looking at the figures on the 

 meter face. 



Furthermore, the gradual changing of the figures, 

 as metre after metre passes out or in, can be easily timed, 

 and from this the speed at which the net is being hauled 

 in can be adjusted. In order to determine the volume of 

 water filtered by the net, a formula has been calculated, 

 depending on the size and shape of the net, the filtration 

 capacity of the silk, and the speed at which the net is 

 hauled through the water. The net must be hauled Up 

 at a speed of half a metre per second in order that the co- 

 efficient determined for this type of net can be used. One 

 has then only to multiply the number of organisms found 

 in the catch or the volume of the catch by 80 to give the 

 number or volume present in a column of water of the 

 length the net has been hauled through and of area equal to 

 one square metre. 



In any case, in quantitative work, where the results 

 of different catches are to be compared, and whether the 

 number of organisms in a particular quantity of water is 

 needed or not, it is necessary that the net should always 

 be hauled up at the same speed, otherwise the pressure in 

 the net will not be the same and a greater or less quantity 



