CATCHING POWER 497 



from mile b, but that the proportions of the water of b and of c filtered along a whole 

 line such as w will be equal to the proportions of the line that cross respectively the areas 

 b and c shown in the figure. 



THE CATCHING POWER OF THE MACHINE 

 The full opening at the front, \\ in. square, could only be used with a much wider 

 mesh gauze than the 60 threads to the inch at present employed as the standard mesh. 

 The water tunnel enlarges to a cross-section of 8 sq. in. at the region of the gauze so 

 that with a front opening of 1 in. sq. the speed of the water at the gauze is reduced to 

 one-eighth of its speed at entry ; thus at speeds of 12 knots it will be reduced to i\ knots 

 and at 8 knots to 1 knot. With the f in. sq. opening, used as a standard in the North 

 Sea survey, the speed of the water will be reduced at the gauze to one-fourteenth, so 

 that at 12 knots it is reduced to less than 1 knot. 



Assuming that all the water entering the tunnel flows through the gauze, then for 

 every mile towed the following quantities of water would be filtered for the three 



openings: , . ,, , 



r ° IJ in. sq. : 66 cu. ft. or 1-87 cu. m. 



1 in. sq. : 42-2 cu. ft. or 1-19 cu. m. 



I in. sq. : 2375 cu. ft. or 0-67 cu. m. 



Actually the quantity filtered is unlikely to reach the theoretical maximum if the 

 larger openings are used with the 60-mesh silk. 



An idea of the catching power of the machine may be gained from the following 



table which expresses the numbers taken per 1 mile tow of a selected number of 



organisms when the f in. sq. opening has been used. The figures are maximum numbers 



taken from the survey at present in progress in the southern North Sea : 



Diatoms Rlrizosolenia styliformis 60,000 



Biddulphia sinensis 54,000 



Dinoflagellates Ceratium 200,000 



Chaetognatha Sagitta 457 



Cladocera Podon 503 



Evadne !633 



Copepoda Calanus finmarchicus 168 1 



Pseudocalanus elongatus 14s 



Temora longicornis 1040 



Centropages 390 



Acartia 500 



Amphipoda Proto pedata 47 



Pteropoda Limacina 980 



Echinoderm larvae 3 2 4° 



Fish eggs 7 



Post-larval fish 20 



The above figures illustrate its capacity ; a typical fluctuation in numbers is shown 

 in the specimen graph in Fig. 21. It is a feature of the method that smaller samples 

 are collected than by the use of the ordinary tow-net, but large enough ones to show 

 the important differences in the abundance of the plankton. More often than not the 

 tow-net yields samples, the contents of which can only be estimated in the laboratory by 

 1 The numbers of Calanus in the southern North Sea are very low compared with more northern waters. 



D.XI g 



