314 COEFFICIENT OF PLANKTON NETS [APP. V 



instreaming water) and least at the end of the net, furthest from the 

 instream, for immediately water enters leakage begins. The mean 

 pressure on the walls of the water can only be determined by 

 experiment and laborious calculation. 



Starting out from such considerations Hensen obtains a coefficient, 

 which he calls xj/, for each grade of the silk bolting cloth used in the 

 construction of the nets. In making a haul with a quantitative net 

 we have to consider (1) the velocity of haul — say 0-5 metre per second ; 

 (2) the area of filtering surface; and (3) the area of the mouth 

 opening. The velocity of haul can be made always (approximately) 

 the same ; if not a correction can be applied. Suppose that the area 

 of the mouth of the net is 0*1 sq. metre, then the volume of plankton 

 caught, or the number of organisms, must be multiplied by the 

 coefficient if/ in order to "approximate to" the number of organisms, 

 or the volume of plankton which would have been contained in a 

 column of water of 0*1 sq. metre in sectional area. 



The methods used by Hensen in order to find the mean pressure 

 on the walls of the net, and so the value of the coefficient i//, are to be 

 found in the Bestimmuyig des Planktons, and in the Methodik. If the 

 reader has sufficient mathematical attainments, and is able to master 

 the difficult German of these memoirs, he will, I think, find that 

 considerable reliance is to be placed on the value of these constants. Of 

 course the result of the reduction of the catch is to give only an 

 approximate value for the plankton contents of the unit column 

 of water. But when the net is carefully made and used (the haul 

 must really be a vertical one and not the oblique haul that may be 

 regarded as " vertical ") then the coefficient doubtless gives reliable 

 results. The limits of accuracy were investigated by Lohmann by 

 comparison of the calculated catch of a vertical net with the catch 

 made by the use of a pump and hose-pipe. 



It may be suggested that the value of i/^ varies with the continued 

 use of the net. But the nature of the fabric of the latter is such, 

 and the construction of the net is so planned, that the value of the 

 constant cannot vary much during the limited period that the 

 apparatus can withstand work at sea without disablement. 



