210 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



It consists of two vessels, one fitting in the other. The outer one is 

 of cylindrical shape, 20 centimeters deep, and measuring 30 centimeters 

 across. The upper edge protrudes 1 centimeter. 



Fig. 1. 



On the outside a tube is fastened, which widens into a funnel at the 

 top and serves for admitting the water ; its diameter is centimeters, 

 and just above the bottom of the vessel it communicates with the in- 

 terior by means of an opening 5 centimeters wide. Opposite the inflow- 

 opening the edge of the vessel is cut out 5 centimeters and has an outlet 



Fig. 2. 



8 centimeters broad and centimeters long, which is slightly inclined. 

 Immediately above the opening of the tube described above, and 5 

 centimeters above the bottom of the vessel, a projecting edge, 1 cen- 



