GENERAL PRINCIPLES 461 



record of the gradations in plankton distribution from considerable depths to the 

 surface. Instead of each inch or two of the record obtained representing a horizontal 

 traverse of a mile or so of sea as at present designed, they could be made to represent a 

 vertical scale of say 10 m. to the inch. Such hauls, repeated at different times of the 

 day and at the same time on different days under different degrees of light intensity, 

 pH, oxygen, phytoplankton, etc., should yield valuable data regarding vertical migration 

 and the possible factors which govern it. Again, the vertical distribution and migration 

 could be studied by several machines being towed horizontally and simultaneously at 

 different levels. If such a series of recorders were towed continuously for a number of 

 hours in a small circle, say of 1 mile diameter, the changes in the plankton at different 

 levels could be recorded with the passage of time. If towed at approximately 6 knots, 

 the completed circles, and so later determined fractions of the total obtained, would 

 yield information as to the plankton at each level over approximately the same area for 

 consecutive half-hour intervals. The recorder would be giving a continuous record in 

 a time instead of in a space dimension. In relation to its use in a time dimension see also 

 the section on possible developments in the machine design (p. 501). 



GENERAL PRINCIPLES EMPLOYED 



Before passing to a detailed description of the machine it may be convenient to 

 describe briefly the general principles employed. Whilst the details of structure have 

 from time to time been modified, the general principles have remained the same. The 

 body of the machine (cylindrical in the first, but of rectangular section in the later 

 models) has tapered ends giving it a general streamlined or torpedo-shaped form. At 

 the front end is a small opening leading to a wider tunnel through which the water 

 passes to an exit at the back as the machine is towed through the sea. A long banding of 

 silk gauze is arranged to wind slowly across the flow of water in the tunnel. It winds off 

 a spool placed below the tunnel, which it enters through a narrow slit; it is wider than 

 the section of the tunnel, and its edges pass up in narrow recesses at the sides. It leaves 

 the tunnel by a slit and at the same time is joined by another gauze banding winding off 

 a spool placed above the tunnel. In passing across the water tunnel the gauze is sup- 

 ported behind by a grid-work of fine metal rollers. The tunnel is rectangular in section, 

 and at the point where the gauze is situated the area of the cross-section is much greater 

 than that of the opening in front, so that the flow of water entering the machine is 

 slowed down and is approximately equal to the filtering capacity of the gauze. The 

 gauze sieves out the plankton, which remains pressed against it by the flow of water 

 until it is joined by the second gauze banding at its exit from the tunnel. The two 

 bandings sandwich the plankton between them and are drawn between two driving 

 rollers which grip the edges of the bandings tightly but do not press on the middle 

 section of the gauze and thus do not damage the imprisoned plankton. From the driving 

 rollers the two bandings pass together on to a storage spool. 



The driving rollers are worked through a series of gears from a propeller which is 

 situated on the outside of the machine, so that it is turned by the passing water as the 



