470 DISCOVERY REPORTS 



of formalin through small perforations in the bottom of the tank from an auxiliary 

 tank filled with cotton-wool and neat formalin (40 per cent formaldehyde solution 

 placed below it on the right-hand side (Fig. 16, qa). The strength of formalin in the main 

 tank will thus be maintained throughout a 24 hours run. 



The internal mechanism frame is in two parts which may be partially separated from 

 one another for the loading of the gauze bandings. It is divided along the line where 

 the gauze crosses the water tunnel, and the two parts pivot one on the other about the 

 driving rollers, but may be drawn a little way apart to allow the gauze bandings to be 

 passed between the rollers (see Fig. 15). On each side of the forward part are hinged 

 two long rods furnished at their extremities with wing nuts, which by engaging against 

 slotted lugs on the rear part draw the two parts tightly together. When so drawn together 

 the driving rollers tightly grip the gauze bandings as explained above, and a system of 

 plates (see Figs. 12, 13 and 15) prevents the escape of water from the bottom and sides 

 of the water tunnel. 



The graduated gauze banding which sieves out the plankton is ruled with transverse 

 lines 2 in. apart. Each section thus exposes an area 2 by 4 in. to the water stream and, 

 the gauze banding being 6 in. wide, there is a 1 in. margin on each side of the water 

 tunnel passing up in a shallow recess in the plates just mentioned. As already explained 

 (p. 461) the force of the water through the tunnel keeps the plankton pressed against the 

 gauze till it passes up through the exit slit to be joined by the second banding. The 

 driving rollers are so placed as to press the banding against the back of the slit so that the 

 plankton is not scraped off by its forward edge. The large exit behind ensures the 

 water passing through the gauze (the opening to the tunnel in front, if an inch square, 

 is only one-eighth 1 of the cross-section exposed to the gauze), and it cannot escape at the 

 sides; a little water may escape through the narrow slits through which the gauze 

 banding enters and leaves the tunnel, but this escape is negligible since the slits com- 

 municate with the closed space within the box body. 



LOADING THE MACHINE 



The internal mechanism frame is taken out for loading through the doors on the 

 left-hand side of the machine. After pulling out the locking rods qn and qo the formalin 

 storage tank is lifted out of the frame so that the auxiliary tank below may be filled with 

 cotton-wool and formaldehyde. The tank is then replaced in the frame with the lid 

 open. References may be conveniently made to Figs. 14-16. 



The next stage in preparing the instrument for use is to wind on to the two supply 

 spools g and h the gauze bandings, one plain and one graduated, the ends of which are 

 secured to the spools by spring clips ga and ha. Winding may be carried out by hand, but 

 this laborious and inefficient process can be avoided by using a special winding machine 

 (similar in principle to a cinematograph film winder) which allows a much more even 

 tension to be maintained on the bandings. The two spools are now secured in position 



1 Approximately one-fifth if the front opening is \\ in. square and approximately one-fourteenth if it is 

 J in. square. The latter opening is used as a standard in the present North Sea survey. 



