Some very simple devices for various oceanographical uses 173 



fastened immovably and diametrically across the top of the aluminium screw-on 

 closure cap. This was effected by riveting on from the underside. In the cap is a disc 

 of kerosene-resistant artificial rubber. On both sides of the disc the round wooden rod 

 has had thin but stiff metal bars driven through it to present prongs projecting equally 

 either side. Such a rod can be seen illustrated in another of the pictures (Fig. 5). 



With this convenient arrangement all that is necessary to effect secure attachment 

 is to hold the bottle-carrying rod against the rope^ wire or cable with one hand, and 

 to hook good quality office rubber bands on to the individual prongs in succession — 

 stretching them, winding them whilst in tension round rod and cable, and hooking 

 their other ends on to whatever prong offers to hold them tight. 



The last-mentioned picture (Fig. 5) shows a pair of compass-containing jelly bottles 

 which had also been used in the Burnham experiments. In this case the bottles are 

 wide-necked pyrex bottles which are a standard list item of laboratory furnishers. 

 With them, the attachment rods were secured on to the cork closure bungs by means 

 of slender washer-loaded bolts and small nuts. 



Various additional (and much more elegant) mountings for the compass-containing 

 jelly bottles were made by Mr. A. J. Woods of Messrs Kelvin & Hughes. He has been 

 associated with the writer throughout the experiments, and is making improved 

 devices (to be spoken of below) in the course of continuing collaboration. 



The excellent little aircraft ring compasses used in the experiments were made and 

 kindly loaned by Messrs. Kelvin & Hughes. For reasons of cost and space Mr. 

 Wood's additional mountings are not illustrated in this article, but it should be 

 mentioned that the two extra kinds which he constructed both made use of a pronged 

 attachment bar and rubber bind-on bands. One of them simulated the " Fair Wheel " 

 characteristics of the bracket-mounted rectangular inclinometer earlier described 

 (Fig. 2a and 2b). 



APPLICATION TO PELAGIC TRAWL INVESTIGATIONS 



Because there is a likelihood that commercial fishermen of future years will find an 

 ever-increasing need to operate on the stocks of non-bottom fish living in deep-water 

 oceanic areas, it is probable that the passage of time will invest pelagic trawling with 

 more and more importance. 



From inquiries made of fisheries research experts who concern themselves with the 

 modern one-ship pelagic trawl, need exists to know much more than is known at 



present about it. 



In the course of the experiments carried out in the Crouch estuary with jelly bottles, 

 a situation resembling " fishing in reverse " with such a net was staged. Under Mr. 

 Waugh's direction two boats were anchored a measured distance " X " apart across 

 the stream, and a line some " 2X " (and other multiples of " X ") long was streamed 

 from them— with one end secured at each boat and a large plankton net plus other 

 drogue objects attached to the mid-point of the line. 



In such circumstances, with a series of compass-containing jelly bottles secured on to 

 each half of the streamed line, it should be easily possible to determine the mean angular 

 divergence. By multiplying the total length of the line into the sine of half that angle, 

 one should get a value equ^al to the distance of separation of the two anchored boats. 



There were a few difficulties during the tests as can readily be imagined. For instance, 



