286 
Birge—The Crustacea of the Plankton. 
most constant factors, and that it may be quite as accurately^ 
determined as any other. 
TEMPERATURES. 
Figs. 1-5. 
The following account of the temperatures of the lake is nok 
intended as a complete discussion of the subject. My tempera¬ 
ture observations were made at first with the aim of securing' 
approximate results in order to determine the biological rela¬ 
tions of temperature. The methods employed until July, 1896, 
while accurate enough for these purposes, are not sufficiently 
accurate for other ends. I have therefore refrained from print¬ 
ing the observations of temperature, and discuss chiefly the 
temperature diagrams, which give the result of my observa¬ 
tions by weekly or rather, quarter-monthly averages. 
A. Methods. 
Surface temperature observations were taken from the begin¬ 
ning of my study, and temperatures from all depths after Octo¬ 
ber 1st, 1894. A w 7 ater bottle and thermometer were the instru¬ 
ments employed until July 27th, 1896, after which date a ther¬ 
mophone was used. The latter instrument has proved extremely 
useful and accurate. A full description of the instrument may 
be found in Science, Vol. II. of 1895, page 639. As constructed 
for my work, the instrument ranges from minus 5 to plus 30> 
degrees C., each degree being graduated into fifths. There is 
no difficulty in reading the instrument to less than 0.1 degree^ 
C., and its readings are exceedingly accurate, agreeing exactly 
with those of a standard thermometer with which it has been* 
constantly compared. Observations can be made very rapidly, 
the time of a single reading varying from one to one and a half 
minutes, according to the amount of change of temperature- 
from the last reading. 
Tha temperature bottle contained about 1^- litres and had a 
small neck. It was lowered to the desired depth; allowed to re¬ 
main from one to three minutes for the glass to acquire the tem¬ 
perature of the water; was then uncorked by a sudden jerk on the- 
