Posted by Richard B. Primack
On
August 22, 1860, Henry David Thoreau measured the temperature at the bottom of
Walden Pond, one hundred feet below the surface. In his journal, he describes
lowering a bottle of water on a line and leaving it there for thirty minutes.
Hauling the bottle up, he used a thermometer to measure a water temperature of
53 degrees, which was 22 degrees lower than the surface. This range of
temperatures surprised him:
What various temperatures,
then, the fishes of this pond can enjoy! They require no other refrigeration
than their deeps afford. They can in a few minutes sink to winter or rise to
summer. How much this varied temperature must have to do with the distribution
of the fishes in it. The few trout must oftenest go down below in summer.
On August 23 of this year I
repeated Thoreau’s measurement. My son Jasper, our friend Ariel, and I paddled
a canoe out to the deepest spot in the middle of Walden, using Thoreau’s 1846
map as a guide. The air temperature in the Boston metropolitan area has risen
by about 4 to 5 degrees since Thoreau’s time, and the ice on Walden is now
melting two weeks earlier in the early spring. Would the temperature on the
bottom of Walden still be 53 degrees, or is it now warmer due to climate change?
At the middle of Walden, we
lowered three bottles into the water on a weighted line: a hot one at 109 degrees, a cold one at 55
degrees, and one filled with lake water at 78 degrees. Would they all be the
same temperature after 30 minutes on the bottom? The weighted line took the
bottles down to a depth of exactly 100 feet.
Lowering bottles into Walden Pond on a weighted
line. Photo by Ariel Chua.
After 30 minutes, we hauled up
the bottles. The lake water bottle and the cold water bottle each measured 50
degrees, a bit colder than Thoreau’s measurement of 53 degrees. The hot water
bottle measured 52 degrees; was it hotter because it started hotter? A re-measurement of the lake water bottle showed
54 degrees; it had gained 4 degrees in 4 minutes from the surrounding 76-degree
air. In our case, we were using a fast
electronic thermometer that took only seconds for a reading, whereas Thoreau was
using a slow mercury thermometer that would have taken several minutes. So our
first slightly colder temperature reading was probably just due to our ability
to measure the temperature more quickly than Thoreau had before the water had
warmed up by a few degrees.
After 153 years, the bottom of
Walden Pond is still a cold place for fishes, and has changed little since the
time of Thoreau. The great depth of Walden Pond has apparently insulated the bottom
from the changes above, at least for now.
