462 DISCOVERY REPORTS 



machine is towed along. Thus the gauze bandings are wound on in direct relation to the 

 distance travelled by the machine through the water ; that which winds across the water 

 tunnel and sieves out the plankton is ruled at equal intervals by transverse lines into a 

 series of numbered sections. The space between these lines equals the distance from the 

 bottom to the top of the water tunnel at the line of traverse. The pitch of the propeller 

 blades may be adjusted, so that the speed at which the gauze winds on may be varied ; 

 thus one section may be made to pass across the water tunnel for approximately every 

 i, 2 or more miles of sea traversed, as desired. In practice the distance travelled by the 

 recorder is read from the ship's log, so that when the number of sections exposed 

 during the run is known the distance that each section represents can be easily found. 

 The gears between the propeller shaft and the driving rollers bring about a reduction 

 in rotation of approximately 6000 : 1. Each section is 2 in. across in the present 

 machine (6 in. across in the first machine), so that for every mile by sea traversed 2 in. 

 or less of gauze are wound on. 1 The storage spool has a capacity for a 250-section 

 banding. 



The storage spool, on which the two bandings with their imprisoned plankton are 

 wound, is turned by a friction drive from the driving rollers. The drive to this spool 

 is not a direct one for two reasons. Firstly, if direct, the speed of winding would increase 

 as the diameter of the rolled up gauze increased and so would no longer bear a direct 

 ratio to the passing water outside ; secondly, if the storage spool alone wound on the 

 gauze (instead of its being driven by the driving rollers) the roll would be wound so 

 tightly that the plankton would be rubbed and pressed into an unrecognizable paste. 

 By the device of the slipping friction drive the storage spool just takes up the slack on 

 the gauze bandings as they are passed through from the driving rollers and winds them 

 on without undue pressure. 



The function of the second gauze banding, which joins the first after it leaves the 

 water tunnel, is to imprison the plankton and prevent it being smeared from one section 

 of the roll to another as it is wound up. In the first machine it also served another 

 purpose : it wound off a spool in a tank containing strong formaldehyde, so that when 

 it met the other fabric it was still soaked in formalin and killed and preserved the 

 plankton. In the later models the storage spool itself is placed in a formalin tank, which 

 the two bandings enter by a narrow slit kept all but closed by a flap of soft rubber. 

 There is a slight but continuous loss of formalin through the slit when the machine is 

 in the water, but this loss is made good by slow diffusion through small perforations at 

 the bottom of the tank from an auxiliary tank containing formaldehyde. This arrange- 

 ment will maintain a sufficient strength of formalin in the main tank during a 300-mile 

 run. 



The spools may be taken out and changed at the end of a run just as are the spools of 

 a roll-film camera. The storage spool when taken out may be kept in formalin for any 

 length of time before being worked up. In working up it is placed on a special micro- 



1 By fitting a larger propeller or altering the gear ratios one could adopt a larger scale of working, i.e. 

 many inches to the mile. 



