The Dredge. 
425 
provide an air escape, in the form of a short brass tube at¬ 
tached to the center of the cover. To this can be tied a cloth 
tube 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 liand 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 (A"), 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 top, 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 
