SOME PLANKTON STUDIES IN THE GREAT LAKES. 137 



reckoned as a part of the plankton. The constitution of the plankton varies, since 

 occasionally forms which belong on the shore or bottom wander into the free water and 

 become a part of the plankton. 



In order to measure the quantity of plankton contained in a lake, or in any por- 

 tion of a lake, it is necessary to strain a certain volume of the water through a fine 

 net and to weigh or measure the material thus obtained. The volume of the water 

 taken should be a representative one; that is, it should consist of water taken at all 

 depths. In other words, it is necessary to strain a vertical column of water extend- 

 ing from the bottom to the surface. The only practical way of doing this seems to be 

 to draw a fine net through the water vertically from the bottom to the surface. Such 

 a net strains a column of water of known height and with a base equal in area to the 

 net opening. It collects at a single haul all the plankton under an area of lake sur- 

 face equal to the area of the net opening, and by measuring the plankton thus taken 

 under a small area it is possible to calculate the total plankton contained in the lake, 

 or under any desired area of the lake. 



The net used for this purpose is shown in the accompanying plate, fig. 1. The 

 upper part of the net. consists of a truncated canvas cone (en) supported on an iron 

 framework. This cone is about 40 cm. high; the smaller end has a diameter of 40 cm. 

 and the larger end a diameter of 00 cm. It is impervious to water and serves two 

 purposes. When the net is let to the bottom it prevents the mud which may be upon 

 the bottom from getting into the net, and when the net is being drawn up it prevents 

 its contents from spilling over the edge, a thing which might otherwise happen with 

 a boat pitching in a heavy sea. 



From the iron ring which supports the broader end of the cone depends the net 

 proper (nt). The net is a cone with a slant height of about 100 cm. It consists of No. 

 20 silk bolting cloth, a very strong fabric, which contains many very small openings 

 of uniform size. This cloth or gauze has the further advantage of not undergoing 

 changes in water and of not yielding any lint to contaminate the plankton. The out- 

 side of the gauze net is protected by a twine net of inch mesh, which serves to take up 

 the strain on the gauze net when it is being drawn through the water. 



The lower end of the gauze net does not run to a point, but is truncated and 

 attached to a flat metal ring. To this ring there is attached a bucket (bJc), which is 

 shown separated from the net in fig. 2, A. In order that the weight of the bucket; 

 may not be borne by the net, six stout cords (crd) run from the upper net ring to the 

 lower net ring, and are made of such length that they support the weight of the bucket. 

 The bucket is essentially a metal cylinder about 6 cm. in diameter inside and cm. 

 deep. It is supported on three legs. At the top it is arranged to be attached to the 

 bottom ring of the net by means of three binding screws. The sides of the bucket 

 are cut away as much as possible, so as to leave only six narrow strips of metal, and 

 the windows thus formed are filled with gauze like that of the net. The bottom of 

 the bucket is conical and has at its middle an outlet tube, closed by an accurately 

 fitted plug, which may be removed from the inside. This plug is shown separately at 

 fig. 2, B. 



The whole net is suspended by means of three cords from a support consisting of 

 three radiating arms. At the junction of the three arms is a strong iron ring, from 

 which runs a rope, by means of which the net is drawn up. The rope is graduated in 



