The Dredge. 425 



provide an air escape, in the form of a short brass tube at- 



t ached to the center of the cover. To this can be tied a cloth 



ube long enough to fall over and close the opening in the cover 



when the dredge is raised. In a row-boat the cover can be 



slipped by hand to allow the air to escape. 



Since the brass of the cover is so thin, it tends to sag in the 



middle of the front, although stiffened by the flange. It is 



well to fasten to the inside of the frame two or three short 



sloping guides (M), which will guide the front edge of the cover 



into its groove on the frame as it closes. 



The attachment of the bag is a circular piece of stout copper, 

 20 cm. in diameter on the inside (K), with a stout wire turned 

 into its lower edge on the outside. The net is held in place by 

 a collar of sheet copper 2 cm. wide, having at the ends eyes of 

 heavy wire, through which a bolt can be passed. The net is 

 slipped over the frame, the collar placed upon it, and drawn 

 tight by a screw bolt. This holds the net perfectly, without 

 the use of eyelets or similar arrangements, and the net can be 

 attached or removed in a minute. This collar is well shown in 

 PI. VII, fig. 1. 



The net. — The net is made of fine muslin known as "India 

 linen. " The opening of the meshes, while, of course, not per- 

 fectly regular, measures about 0.1 mm. It permits the passage 

 of water quite freely and is not affected by wetting nor does 

 the coefficient change with time to an appreciable degree. A 

 piece of stout muslin was sewed to the top of the net for at- 

 taching it to the frame. In the July observations the net was 

 attached directly to the square frame. Later the method de- 

 scribed above was substituted. There was no difficulty in wash- 

 ing the net clean when attached to the square frame, but 

 it was much more difficult to shape and fasten smoothly. 



The releasing apparatus (PI. VII, fig. 1) is a modified form of 

 one designed by Prof. C. Dwight Marsh, of Ripon College, Wis- 

 consin. It consists of an oblong frame of stout sheet brass 13 

 cm. long, 11 cm. wide, 2.5 cm. deep, divided by a horizontal 

 partition in the middle. In the center of the xop, bottom and 

 middle pieces is a hole one-fourth of an inch in diameter, 

 through which passes the dredge line. A fork of stout wire 



