Birge—A Second Report of Limnological Apparatus. 549 
either in its original form as designed by Kemmerer or in that 
which we have used. 
IV. Ekman Dredge with Modified Release (Plate XL, 
figures 8, 9) 
This dredge was built according to the figures and description 
of Ekman 1 . It is used to obtain mud from the lake bottom. We 
have employed it in two sizes; in one of these the box was about 
15.3 cm. (6 in.) square, and in the other about 23 cm. (9 in.) 
Both worked equally well, but of course the smaller form is the 
more portable. The body and jaws of the dredge are made of 
stout sheet brass; the covers are made of very thin spring sheet 
brass so attached that they open readily as the dredge descends 
and permit the water to flow through it, but close when the dredge 
is drawn up. 
As is well known, the purpose of the Ekman dredge is to obtain 
a sample of mud from a known area of the bottom, so that the 
animals living in it may be determined. Thus information may 
be secured regarding the kind and number of the bottom popula¬ 
tion. The dredge is lowered until it rests on the mud; the jaws 
are then released, which bite into the mud and ordinarily secure 
enough to fill the entire dredge about two thirds full. It can be 
operated only on a soft bottom, since the springs are not strong 
enough to force the jaws into sand bottom, nor will the jaws close 
properly if sticks or stones or large animals like clams are pres¬ 
ent. But it is most successful and useful in exploring that great 
area of an ordinary lake bottom which is covered with lake mud 
and vegetable debris, and the Survey has made very extensive use 
of it in such studies. 
Ekman used a form of release held in place by friction and 
operated bj^ a vigorous pull on the line after the dredge had 
reached the bottom. As he states, great care had to be exercised 
to prevent the machine from “going off” prematurely. We there¬ 
fore adapted to the instrument a simple form of release like that 
described by Mr. Juday in his former paper. Its construction 
is clear from the figures, as also is the way in which the jaws are 
released by a brass messenger sent down the line. 
1 Ekman, S. Neue Apparate zur qualitative!! und quantitative!! Erfor- 
schung der Bodenfauna der Seen. Internat. Rev. Ges Hydrobiol. u. Hydrogr. 
a: 558. 1011; 7: 165.1915. 
