510 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



of water Avill be filtered, and different volumes of catch 

 will be obtained. Thus, if the quantity and constitution 

 of the plankton were exaetly alike at two stations, bui I he 

 net was hauled up with a greater speed at one than at the 

 other, the volume, and moreover the constitution, of the 

 two catches would be different, even though the distance 

 through which the net was hauled was the same at both 

 places. Hence the necessity of some recording machine. 



It is further important, in quantitative work, that 

 the net should descend and ascend vertically. If this is 

 not the case, two errors may enter into the work : — (1) The 

 net will be towed more or less in a horizontal direction 

 and not give a true picture of the plankton in a column 

 of water. (2) The depth recorded by the amount of rope 

 paid out will not give the true depth of the net. 



In order to obviate this oblique descent, which occurs 

 when there is any considerable current in the water, the 

 net .should be weighted with a heavy leaden weight slung 

 under the filtering bucket. The net provided with the 

 closing apparatus does not need such a heavy weight as 

 the ordinary Apstein open net, since it is naturally some- 

 what heavy. Both, however, should be weighted, other- 

 wise even in a region with no current their descent would 

 be too slow. It is always safer to sink the nets as rapidly 

 as possible, particularly if there be much current. 



Herdman (17) refers to the " Nansen " net and its 

 easy method of working, due in part to its lightness. If, 

 however, the Nansen net is to be used as a satisfactory 

 vertical net, it must be weighted until it is relatively as 

 heavy as the other plankton nets. Furthermore, it may 

 be slightly inaccurate for quantitative work, since it lacks 

 the conical mouth-piece, and, in addition, no formula has 

 been calculated to determine the volume of water it filters. 

 In order to determine the true depth of the net, when 



