LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSISSIPPI. 
355 
(Ward, 1900; Kofoid, 1897), and the author can only confirm the conclusions of 
these men. The inaccuracy of this method is so great that it must be abandoned 
entirely. More exact results are obtainable with the centrifuge method. A small 
hand centrifuge was used, 50 revolutions of its handle corresponding to 1,000 
revolutions of the test tubes. During the observations 50 revolutions of the 
handle were made in 1 minute. The movement was controlled by counting the 
revolutions and simultaneously observing the stop-watch hand. After 2 minutes 
of centrifuging the volume of plankton settling on the bottom of the test tubes 
was read; from this the volume of plankton per cubic meter of water was calculated. 
A complete quantitative study of the plankton was not undertaken. However, 
the number of Copepoda and Cladocera was counted because these two groups form 
the most important part of the food of plankton-eating fishes. In order to separate 
the Crustacea from other organisms, the plankton sample was filtered through 
No. 12 bolting silk and the remainder placed in a Petri dish 8 cm. in diameter 
with 1 cm. squares ruled on the bottom, a type of counting chamber used in quanti- 
tative bacteriological investigations. The uniform distribution of organisms 
was secured by shaking. If the number of Crustacea was less than 200 in a catch, 
all were counted; if greater, only 20 squares were examined and the average was 
taken, from which the number of organisms in a catch was calculated. The results 
of all the observations, including the number of Copepoda and Cladocera per 1 cm. 3 
of water, are given in Table 29 (pp. 422 to 433) , arranged in chronological order. 
The distances of stations from St. Paul are given in miles, the depths in meters. 
The distances were taken from the official publication of the Bureau of Lighthouses 
(Department of Commerce) “Light List, Upper Mississippi River and Tributaries, 
13th Lighthouse District, 1914.” 
STATIONS. 
Each point where observations were made is called a “station,” and the location 
of the stations is shown in Figure 3. The number of each station on the map 
corresponds to its number in the tables. The stations were chosen after a careful 
examination of the map of the Mississippi River and on the basis of the informa- 
tion obtained from local men familiar with the river. The principal purpose was 
to make observations at every point where a substantial change in the river condi- 
tions could be expected. Therefore, the samples of plankton were collected and 
the observations made above and below the principal tributaries, in the main chan- 
nel as well as in the sloughs and in the bayous. Usually in a section across the 
river or lake three stations were made, but if only one station was made it was in 
the main channel. The lake stations are shown in Figures 4 and 7. There being 
no intention to study the pollution of the river, no observations were made at cities 
where the river was polluted. More attention was paid to the lakes (Pepin and 
Keokuk), and therefore the number of stations on these lakes is considerably 
greater than on the river. In all, 171 stations were occupied, from which 673 
samples were obtained after pumping and filtering a total of 25,800 liters of water 
through bolting silk. 
