542 Wisconsin Academy of Sciences, Arts, and Letters. 
In the process of cooling the thermometer lost per minute about 
1/18 of the difference between its temperature and that of the 
surrounding water. Its cooling, therefore, at temperatures near 
that of the water was very slow. If observations are taken on both 
sides of the temperature of the water as indicated above, no cor¬ 
rection is needed for loss or gain from this source. 
The record of a single series of observations is given (table 2) 
in order to show the kind of results which may be expected from 
the black-bulb thermometer. 
TABLE 2 
Observations with black-bulb thermometer in vacuo Lake Mtendota, September 5, 1912 
Hour 
Depth, cm. 
Temperature 
of Water 
Rise of 
Temperature 
Seconds 
Rise per 
Minute 
11:37... 
50 
22.8° 
22.6°-22.8° 
16.5° 
0.74° 
22.8 -23.0 
16.0 
11:25. 
100 
22.8 
22.6 -22.8 
32.5 
0.36 
22.8 -23.0 
32.5 
11:50... 
150 
22.2 
22.0 -22.2 
50.5 
0.20 
22.2 -22.4 
59.0 
11:55... 
200 
22.2 
22.0 -22.2 
120.8 
0.10 
These observations show that 27 per cent, of the radiation pres¬ 
ent at a depth of 50 cm. was transmitted to the depth of 150 cm., 
and that 28 per cent, of that at 100 cm. was found at 200 cm. 
The transmission measured by the pyrlimnometer between 11:00 
and 11:15 a. m. was 26 per cent, between 50 cm. and 150 cm., 
and 30 per cent, in the 100-200 cm. interval. 
B. The Electrical Pyrlimnometer (Plate XXXIX, figures 3, 4; 
Plate XI, figure 5; text figures 4, 5) 
The term pyrlimnometer has been recently devised as the name 
for an instrument which this Survey has used since 1912. It is 
essentially an arrangement of electrical thermal couples for meas¬ 
uring the intensity of solar radiation below the surface of a lake, 
and the name given above is intended to indicate these facts. 
The Receiver. The instrument was designed for this Survey 
by Professor C. E. Mendenhall, of the department of physics of 
the University of Wisconsin, and was constructed by Mr. J. P. 
Foerst, mechanician of the same department. The radiation of 
