To Walden to get its temperature.

August 24.

The air is only 66°; the water at top, 75°. What I had sunk to the bottom in the middle, a hundred feet deep by my line, left there half an hour, then pulled up and poured into a quart dipper, stood at 53°. 

It appears the bottom of Walden has, in fact, the temperature of a genuine and cold spring. It probably is of the same temperature with the average mean temperature of the earth, and, I suspect, the same all the year. This shows that springs need not come from a very great depth in order to be cold, and that Walden must be included among the springs.

As it has no outlet, Walden is a well, rather. It is not a superficial pond, not in the mere skin of the earth. It goes deeper. It reaches down to where the temperature of the earth is unchanging. 

What various temperatures the fishes of this pond can enjoy! They require no other refrigerator than their deeps afford. They can in a few moments sink to winter or rise to summer. The few trout must oftenest go down below in summer. How much this varied temperature must have to do with the distribution of the fishes in it!

H. D. Thoreau, Journal, August 24, 1860

What various temperatures the fishes of this pond can enjoy! See January 11, 1856 ("It is commonly said that fishes are long-lived on account of the equable temperature of their element. The temperature of the body of Walden may perhaps range from 85° - perhaps at bottom much less - down to 32°, or 53°, while that of the air ranges from 100° down to - 28°, or 128°, more than twice as much.")