838 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



litres of water have passed through. The turbine gives 

 relativelv true volumes of water when used at fairly 

 similar velocities. The absolute volume may be estimated 

 if the head piece be placed in water and through the open- 

 ing a measured volume of water be poured. The amount 

 for one rotation is 0'44 litre. 



The net is so manipulated that it cannot quite sink to 

 the bottom. The use of the weight mentioned above 

 obviates this. The weight itself sinks to the bottom, and 

 if it be allowed to remain there the net itself cannot sink 

 so far. It is possible to estimate when the weight touches 

 the bottom by trying with the rope or by greasing the 

 weight, and making several trials for the depth. The 

 haul is a diagonal one, and its length may be calculated 

 if the following factors are known : — the velocity with 

 which the net sinks, the distance the ship has travelled 

 from the point where the weight touched bottom, and the 

 angle between the rope attached to the net and the 

 vertical. 



A number of drawbacks stand in the way of the exacti- 

 tude of a quantitative estimation of all the organisms 

 which are retained (usually) in the net. These are — 



1. The ship frequently does not remain in the same 



situation, and undercurrents influence the net, so 

 that possibly more of the upper layers is fished 

 through at one time than another. 



2. The variability of the pull on the net is in marine 



investigations uncontrollable. 



3. It happens on account of the length of time the net 



remains damp, and the subsequent drying in the 

 sun, that a shrinkage is set up. This happens 

 earlier when the net is mishandled, but is sooner 

 or later unavoidable. 



