POSSIBLE IMPROVEMENTS 



5°i 



PLANKTON 



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It would be a great advantage to be able to use two grades of gauze, one of coarse 

 mesh for the zooplankton and one of much finer mesh for the phytoplankton. Attempts 

 using a ribbon of fine-mesh gauze sewn on to the coarser gauze to entrap the smaller 

 phytoplankton forms have not proved 

 successful. It is hoped later to construct 

 a miniature auxiliary water tunnel which 

 could be fixed to the body of the standard 

 machine, so that a narrow very fine-mesh 

 gauze within it could be wound on at the 

 same rate as the gauze in the main re- 

 corder ; it would be worked by an extension 

 from the main driving gears. Alternatively 

 the main water tunnel could be divided 

 by a vertical partition to separate a narrow 

 tunnel leading to a fine-mesh gauze placed 

 side by side with the coarser gauze, the 

 two fabrics winding on together into the 

 formalin tank. The front opening of the 

 small tunnel could be reduced to a mere 

 £ in. sq. to allow for the greatly reduced 

 filtration with the fine-mesh gauze. 



In the Type I machine with a larger 

 opening a grid of intersecting wires was placed at the front opening to prevent large 

 objects entering the water tunnel; in the Type II machine with the very much smaller 

 opening the grid was dispensed with. Recently two records have been spoiled by pieces 

 of fish, evidently struck off by the nose of the machine, entering the water tunnel and 

 jamming against the slit for the exit of the gauze. It is proposed to reintroduce a pro- 

 tective grid at the front opening. 



A series of vertical vanes might be fixed within the water tunnel to prevent the 

 tendency of the plankton to be carried more to one side of the gauze than the other 

 on account of the slightly asymmetrical path of the water tunnel (see p. 468). 



My colleague, Mr C. E. Lucas, has suggested a modification of the machine so that 

 it could be used in one place for recording changes in the plankton in a time dimension. 

 Instead of the machine being towed through the water, a propeller fitted into the water 

 tunnel behind the gauze and driven by an enclosed electric motor could be made to 

 draw a current of water through the gauze, so that changes in the composition of the 

 plankton at different levels could be measured throughout the day. The technical 

 difficulties of enclosing a motor of sufficient power in a watertight chamber in any but 

 shallow depths of water would be considerable, especially when the chamber would 

 have to be pierced by the drive for both the propeller and the winding mechanism. 

 But the possibility of such an adaptation should not be lost sight of. 



