174 J- N. Carruthers 



a strong pull by the drogues must urge the boats to close somewhat on each other, 

 whilst less than a really strong pull must leave the lines bowed to some extent. Never- 

 theless, the results were amply good enough to invest the application of the idea to 

 studies of the pelagic trawl with real promise. 



Certain full-scale experiments are envisaged thanks to expressions of interest and 

 the promise of facilities from the two British fisheries laboratories. Of an ample 

 assemblage of the compass-containing jelly bottles, half would have their " upper- 

 works " enamelled red for attachment on the port warp, whilst the other half would 

 have green floats and would go on the starboard warp. 



In a first investigation the average overall downslope of both warps will be worked 

 out from the mean of the jelly slopes, and the fishing depth of the net estimated 

 accordingly. From the compass readings of the bottles the course of the warps as 

 projected on the sea-floor will be derived and an estimate of the horizontal gape of 

 the net made. 



Apparently Dr. Scharfe holds the view that useful conclusions as to horizontal 

 net gape can even be made from noting only the separation angle of the warps behind 

 the slip hook. This he does by means of his special device the " Spreizmesser ". 



Since the investigating ship will have an echo-sounder the net tests could be done 

 in a chosen area with a flat bottom so that it could always be known that the foot-rope 

 would never be higher off" bottom than a distance " H ". If, when the net was shot, 

 a length of thin Kelvin wire of length about 1|H or 2H carrying an object like a smooth 

 cannon-ball at its free end were hung from the bottom of the net, then, if several jelly 

 bottles went down on this wire, one would learn on hauling what its slope had been 

 and so would know how high above bottom the foot-rope had ridden during the tow. 

 One would then have data enough to make an estimate of the vertical gape of the net. 

 In a subsequent investigation much better things will be attempted : 



A drogue of nearly neutral buoyancy will be towed inside the net from the midpoint 

 of a light line of not much greater length than the distance the net doors would be 

 apart if the net were stretched out tight on the quayside. An array of compass- 

 containing jelly bottles on each half of such a line will give a much better triangulating 

 situation with sound promise of an acceptable determination of horizontal gape. 



How the same sort of thing could be done to investigate vertical gape at different 

 points will readily suggest itself. 



For some of the determinations the jelly bottles would not need to contain 

 compasses. 



The need to weigh a few considerations has led to postponement for the time being 

 of the full scale net tests from a fisheries research vessel. For one thing there is the 

 need to hit upon a long-enough timing (congelation period) as already stated. Then, 

 although one could not possibly devise any other slope and direction indicators as 

 simple as the jelly bottles, it is a fact that difficulty attaches to reading the compass 

 directions precisely enough when (as so often must happen) the compasses are not 

 on an even keel but are awkwardly canted. 



Where slope only is needed, recourse will be had to a gelatine-filled perspex cylinder 

 containing a rolling clinometer furnished with a finger which travels over a protractor. 

 With this device (now being made), there will be no need for two liquids and no need 

 to measure jelly slopes with an external goniometer. 



At the time of writing this there is also being made (by my collaborator, Mr. A. J. 



