CAUSES OF FAILURE 499 



convenient depths apart, or the introduction of a device which, by moving the diving 

 planes by a cam action from the driving mechanism, would make the machine take a 

 regular undulating path through the sea. This latter very interesting suggestion was 

 made to me by Mr J. O. Borley, O.B.E., and is discussed in the later section on possible 

 developments in machine design ; it appears to be a perfectly feasible adaptation which 

 should remove what appears to be the most serious limitation to the method. 



Another disadvantage of the method lies in the damage that may be done to specimens 

 in the process of winding when they may often be crushed and broken, rendering 

 identification a longer and more tiring process than the examination of ordinary tow- 

 net samples. For this reason the instrument is not suitable for general survey work in 

 an area where the plankton is not well known; a good knowledge of the commoner 

 forms should first be obtained by the use of ordinary tow-net methods. It is hoped that 

 this difficulty too will largely be overcome by improvements in the mechanism (see 

 p. 500). It should be noted that the plankton is not crushed between the driving rollers 

 which do not press together except at their ends, where they tightly grip only the edges 

 of the gauze outside the plankton zone. 



SOME CAUSES OF MECHANICAL FAILURE 



At times from one cause or another the mechanism fails, causing a stoppage of the 

 record. It may be convenient to refer briefly to four possible causes of failure, so that 

 other workers using the machine may know what to look out for. 



(1) The brakes on the supply spools may have been adjusted too tightly, so that they 

 jam and the gauze fails to wind off. 



(2) Pieces of grit may get caught in between either roller / or roller and part of 

 the frame (see Fig. 10), so that they no longer turn; this may prevent the gauze from 

 passing on. 



(3) The friction drive may become too slack so that it slips the whole time, with the 

 result that the gauze bandings which have passed through the driving rollers are not 

 taken up by the storage spool but curl over the driving rollers until eventually they may 

 get caught round the forward driving roller and be wound upon it till the mechanism jams. 



(4) Another stoppage is one which will only develop in an old machine and is due to 

 the wearing of the rubber ends of the forward driving roller against which the toothed 

 metal ends of the rear driving roller press. It becomes reduced in diameter, so that the 

 other roller no longer exerts sufficient pressure to drive on the gauze ; the blunt teeth 

 just slip on the gauze which remains stationary. Any sign of wear should be carefully 

 looked for and as soon as it becomes evident the ends of the rollers should be replaced 

 by new ones. 



The present degree of efficiency of the mechanism may be judged from the following 

 figures relating to the North Sea recorder survey. During the year ending 30 November 

 1935 forty-seven recorder runs were made across the North Sea only two of which were 

 failures due to mechanical breakdown, thus showing a 957 per cent efficiency. The total 

 sea mileage recorded successfully during the period was 7827. The previous year was 



6-2 



