10 PLANKTON OF WISCONSIN LAKES 



of 38,000 liters, depending upon the abundance of the organisms and 

 the size of the sample desired. In the great majority of instances, 

 however, the quantity was between 10,000 liters and 20,000 liters. The 

 apparatus shown in figure 1 was used in securing these large quantities 

 of water from various depths. Figure 1 shows the launch with the 

 apparatus in place ready for a run, except that the large plankton net 

 suspended in the bow of the launch is hung inside the large can during 

 the run. 



Figure 2 gives a more detailed view of the pumping outfit. It con- 

 sists of two small vane pumps each having a capacity of about 30 

 liters per minute at a speed of 300 revolutions. These pumps are 

 operated by a small gasoline engine, the kind used for the ordinary 

 milk separator. The engine and pumps are mounted on a substantial 

 metal base in order to hold them firm and rigid while in operation. 

 The engine is attached to the base with bolts so that it may be readily 

 removed for convenience in loading it into the launch. 



The water is obtained from different depths by means of two pieces 

 of hose, each 30 meters long and with an inside diameter of 2.5 centi- 

 meters. A calibrated line is attached to the intake end of each hose 

 which enables one to lower the hose to the depth from which water is 

 desired. The water delivered by the pumps is strained through the 

 large plankton net that is suspended inside the large can shown in 

 figures 1 and 2. The stream of water from the discharge hose is not 

 allowed to strike the straining part of the net directly because this 

 would result in many organisms being forced through the meshes. 

 When only a net catch is desired the overflow water is discharged over 

 the side of the launch through an outlet pipe that is attached near the 

 bottom of the can to facilitate the straining process. By means of a 

 pet cock attached to the can at about its mid-height samples of the 

 strained water from the various depths can be obtained for a study of 

 the organisms that escape through the meshes of the net. 



The straining cone of the plankton net is made of No. 20 * bolting 

 cloth; when thoroughly shrunken this silk gauze possesses more than 

 6,000 meshes per square centimeter, with the area of the openings 

 varying from 0.001 sq. mm. to 0.003 sq. mm. The straining cone of the 

 net is 30 centimeters in diameter at the upper end and it is 70 centi- 

 meters long, thus furnishing a large straining surface. 



* Lohmann states (Internat. Eevue, Bd. IV, 1911, p. 38) that a new system of 

 numbering was adopted by six Swiss manufacturers of bolting cloth in 1907, and 

 that the old No. 20 was changed to the new No. 25. Two American dealers had 

 not received notice of this change up to 1919, and a sample book of Sehindler's 

 "Genuine Swiss Silk" obtained in this year still shows the old system of 

 numbering. 



