NATURE ADRIFT 



overspill at the edges and this results in inaccurate register in a small meter. 

 Expense is quite an important factor as the loss of a net, which can often 

 happen in a heavy sea, can mean the loss of the meter also. Meters are there- 

 fore not used if only the proportions of the various organisms or a very 

 rough estimation of numbers is all that is required. 



To take the net below the surface a weight is needed and, although a 

 couple of pounds is adequate for a small net behind a rowing boat, it re- 

 requires an 80 lb. lead to take a i -metre net down at 2 knots, using a 

 wire: depth ratio of about 2 to i. The accuracy of the depth reached may be 

 important and small variations in the speed of the ship, caused by wind or tide, 

 not easy to correct by adjusting engine speed, can make big differences to 

 the wire : depth ratio, histcad of a weight, it is therefore better (but again more 

 expensive) to use a depressor (Fig. 3; ib). This is like an underwater kite 

 and is attached to a line in such a way that the water pressure forces it down, 

 just as wind pressure forces the conventional aerial kite upwards. The 

 greater the speed the greater the downward pull and, within a resonable 

 range of speeds, the net stays at about the same level. For accurate depth 

 location, a depth-recording flow-meter is used (Plate II). The water pressure, 

 increasing with depth, works a pressure gauge calibrated in depth. In its 

 simplest form it will merely register the maximum depth reached, but 

 linked with a revolving drum worked by the flow part of the meter it can be 

 made to give a continuous record of the depth of the path of the net. This 

 very useful instrument costs the best part of ;;/^ioo! 



A simpler depth indicator, but one recording only the maximum depth, 

 is the Kelvin Tube. This is a length of glass tubing closed at one end and 

 coated inside with a water soluble stain. As the depth increases the air inside 

 the tube is more and more compressed, and water enters at the open end of 

 the tube and dissolves the stain. On recovery it is easy to sec just how far 

 the water has penetrated and the tube can be compared with a gauge 

 calibrated to give the depth in fathoms or metres. As glass tubes are delicate 

 these Kelvin tubes are usually sent down protecteci by an outer brass case. 



Now let us return again to our basic net (Fig. 3 ; i). It is towed by three 

 or four bridles which keep the ring correctly opposed to the water-flow and 

 these in turn are attached to a single towing warp. For sampling at different 

 depths at the same time several nets can be attached to the same warp if the 

 weight or depressor at the end is big enough. This arrangement of bridles 

 means that the water just in front of the net is disturbed and those creatures 

 capable of a quick dart away can see or feel the approach of the net in time 

 to dodge it. One way of avoiding this when using a single net is to fix a 

 paravane to the lower edge of the ring and attach the towing warp only 

 to upper edge (Fig. 3; 2). Nevertheless, fast moving creatures with good 



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