Birge—A Second Report of Limnological Apparatus. 539 
TABLE 1 
Temperature of mud. Lake Mendota, 1910. Depth of water, 8 meters. “Surface” means 
temperature of water at surface of mud. 
Depth, cm. 
n, 12 
V, 28 
VII, 12 
Surface....... 
2.3° 
11.2° 
19.3° 
50........................ 
4.3 
10.7 
13.8 
100.......... 
5.8 
9.1 
11.8 
150... 
7.3 
8.4 
10.2 
200...... 
8.4 
8.0 
9.5 
250....... 
9.3 
8.3 
9.2 
300.... 
9.8 
8.5 
9.0 
850... 
10.2 
8.8 
8.9 
400... 
10.5 
9.2 
9.2 
450.... 
10.6 
9.6 
9.4 
500... 
10.6 
9.8 
9.0 
II. Apparatus for Determining Transmission and Absorption 
of Radiation by Lake Water 
Our attention was first called to this line of investigation by 
an experiment recorded by Forel (Le Leman 2: 294). He placed 
a thermometer with blackened bulb in a bottle, lowered it to the 
depth of 1 m. below the surface of the lake, and exposed it to the 
sun. He noted the rise of the temperature which came before 
the reading of the instrument remained stationary. It occurred 
to us that by using a black-bulb thermometer in vacuo and ex¬ 
posing it to the sun for known intervals of time and at different 
depths, some notion could be gained of the relative amount of 
heat absorbed by various strata of water. 
A. Black-Bulb Thermometer in Vacuo (Plate XXXIX, 
figures 1, 2) 
Experiments were first made in 1901. A black-bulb minimum 
alcohol thermometer in vacuo was employed. This instrument 
could be raised from small depths to the surface and read with 
no appreciable change of the readings during the process. It was 
attached to a frame so that it was nearly horizontal when low¬ 
ered into the water, and the bulb was protected by a sliding cover 
which could be operated by cords leading to the surface. 
