Nichols.].. 
72 
[October 6, 
1879, as a result of a week of warm weather. Here the heating 
up is more marked at the surface, but when there is little differ- 
ence of temperature at different depths, the temperature some- 
times changes to about an equal extent at all depths. Notice, for 
example, the observations in Mystic Pond, in March, 1880, 
especially in the early part of the month, when the water was 
cooling. As a rule, it seems that the diurnal variation below a 
depth of eighteen feet is very small. On one occasion, June 3, 
1879, my assistant, Mr. Macfarlane, spent the whole day on the 
pond. The results of the observations made are included in 
Table VI. 
Table VI. Observations on Mystic Pond. 
Hour of Day. 
Temperature at a depth from the surface of 
2 feet. 
18 feet. 
36 feet. 
54 feet. 
75 feet. 
8.15 
A.M. . 
20.1 
19.3 
9.2 
7.8 
6.9 
9.15 
it 
20.3 
19.3 
— 
— 
— 
10.15 
u 
20.4 
19.3 
— 
— 
— 
11.15 
u 
20.5 
19.3 
— 
— 
— 
12.15 
P.M. 
20.6 
19.3 
— 
— 
— 
1.15 
a 
20.8 
19.3 
9.2 
7.3 
6.9 
2.15 
a i . 
20.9 
19.0 
— 
— 
— 
3.15 
“ 2 . 
20.9 
19.0 
— 
— 
— 
4.15 
a 
20.9 
19.0 
— 
— 
— 
This table represents the results of a single day’s observation. 
The day was not a very warm one, — the mean daily temperature 
at Boston was only 14°, 3. Of course, in this matter of variation, 
different ponds would vary very much ; in a pond where there is 
very little difference of temperature from top to bottom we 
should expect the changes would be , felt to a greater extent. In 
fact I have, through the kindness of Mr. Fitzgerald, Superinten- 
dent of the Western Division of the Boston Water- works, the 
results of observations made by him on Chestnut Hill Reservoir 
during a day and night. The observations were made with an 
ordinary (i. <?., not self-registering) thermometer inclosed in a 
copper tube, and are recorded in Table VII. 
1 At this time the wind changed and the«boat swung to the opposite side of the buoy. 
2 At 6 feet the temperature was 20°. 9; at 12 feet, 20°. 4. 
