1880.] 
61 
[Nichols. 
The difference between the summer temperature for the two 
years is, however, not so great. The bottom water was appreci- 
ably warmed in 1879, but the monthly mean temperature during 
the summer, observed as before in Boston and reported in Fah- 
renheit degrees, were slightly higher in 1878, thus, 
• 
1878. 
1879. 
June . 
• < 
> • 
64.2 
64.2 
July . 
• • 
• 
72.7 
69.9 
August 
• « 
» • 
68.1 
67.7 
Mean for three months 
68.3 
67.3 
As to the manner of freezing . — The observations on these 
ponds show that the water reaches a uniform, or all but uniform, 
temperature from top to bottom in the early part of December 
and that at that time, or shortly after, the temperature is that of 
the greatest density of water (about 4° C. 1 ) ; thereafter the cool- 
ing goes on, the ice forms, and, as soon as the ice becomes strong 
enough to be safely walked upon, the temperature is found to be 
very much below 4° C., even at the bottom of the pond. I must 
confess that I was surprised at the results of the observations on 
Fresh Pond on January 2, 1879, on finding that the temperature 
at the bottom was as low as 1°. 0 C. 
At that time, I had not succeeded in finding the published record 
of any observations made through the ice in ponds or lakes, but 
afterwards the observations of Buchanan on Lochs Lomond and 
Linlithgow, 2 and those of Forel on the Lakes of Morat and Zurich 3 
were published, and I have since found that the fact was noted 
many years ago by Zadock Thompson 4 , who made observations 
through the ice on Lake Champlain, and gives the following as a 
1 The temperature of the greatest density of pure water is generally spoken of as 4° C., 
but is likely to be a few hundredths of a degree lower. The presence of saline matter, 
as is well known, lowers the temperature of maximum density, but in the case of our 
soft pond waters the difference is probably not very great. 
2 Nature, xix(1879), p. 412. 
3 Comptes rendus, Feb. 16, 1880. 
4 Appendix to Thompson’s Natural History of Vermont, 1853. Also Amer. Journ. 
Sci., (2) xii (1851), p. 22. 
