1920] Allen: Plankton of the San Joaquin River 19 



The Form op Net 



The first few collections in January were obtained with a tem- 

 porary net made of used mill silk. But a net, of number 25 new bolt- 

 ing silk, was put into use on January 15, 1913, and was used con- 

 tinuously to January 1, 1915. There were occasional changes of drain 

 cups. 



The net was constructed after a plan suggested by Professor C. A. 

 Kofoid. Outspread, its general form is that of a very broad and 

 short truncated cone, the base having a total circumference of 166.65 

 centimeters and the apex 17.59 centimeters. The slant length is 60 cen- 

 timeters. In order to avoid the awkwardness of such a shape, eight 

 equal folds are made lengthwise and their upper edges closed. The 

 inner points of the folds are then fastened directly to a brass ring of 

 proper size. The outer points of the folds and an unfolded part be- 

 tween each two is bound by butcher's linen to a slightly conical brass 

 plate with an opening in the middle which will pass just 10,000 cubic 

 centimeters of water for each meter hauled, i.e., 100 square centimeters 

 area. The convex surface of this subcorneal plate is kept outermost, 

 so that in hauling the water may all pass away from the opening, 

 except that which is in the column immediately before it. 



A small brass cylinder is fitted into the small end of the net and 

 fastened there by butcher's linen. Smooth grooves on the outer sur- 

 face of the cylinder, near each end, serve for securing it to the net as 

 well as for attaching the drain cup to the distal end. 



This drain cup is made of the same silk as the net. It is a very 

 simple pocket, about 5 centimeters deep, made of two semicircular 

 pieces sewed together by the circular edges. The straight edges thus 

 form the top of the cup, just large enough to slip over the end of the 

 brass cylinder. The size was not kept quite uniform but the filtering 

 surface of this cup averaged about 100 square centimeters in 1913. 

 A draw string of white tape is run around the margin of the cup and 

 fastened by a small hook and eye, such as is used by dressmakers. 

 The draw string makes it possible to slip the cup over the cylinder end 

 without tearing. The hook and eye, when properly adjusted, makes a 

 very secure fastening, easily and quickly opened or closed. 



The total net surface before shrinkage was 5,527.20 square centi- 

 meters, not including the drain cup. The total filtration opening as 

 calculated from micrometer measurements was 690.9 square centi- 

 meters. This gives a filtration outlet somewhat more than six times 



