Chapter 17 - Page 2

cells are at right
angles with what was the water surface. Where there is a rock or a
log rising near to the surface the ice over it is much thinner, and
is frequently quite dissolved by this reflected heat; and I have
been told that in the experiment at Cambridge to freeze water in a
shallow wooden pond, though the cold air circulated underneath, and
so had access to both sides, the reflection of the sun from the
bottom more than counterbalanced this advantage. When a warm rain
in the middle of the winter melts off the snow-ice from Walden, and
leaves a hard dark or transparent ice on the middle, there will be a
strip of rotten though thicker white ice, a rod or more wide, about
the shores, created by this reflected heat. Also, as I have said,
the bubbles themselves within the ice operate as burning-glasses to
melt the ice beneath.
The phenomena of the year take place every day in a pond on a
small scale. Every morning, generally speaking, the shallow water
is being warmed more rapidly than the deep, though it may not be
made so warm after all, and every evening it is being cooled more
rapidly until the morning. The day is an epitome of the year. The
night is the winter, the morning and evening are the spring and
fall, and the noon is the summer. The cracking and booming of the
ice indicate a change of temperature. One pleasant morning after a
cold night, February 24th, 1850, having gone to Flint's Pond to
spend the day, I noticed with surprise, that when I struck the ice
with the head of my axe, it resounded like a gong for many rods
around, or as if I had struck on a tight drum-head. The pond began
to boom about an hour after sunrise, when it felt the influence of
the sun's rays slanted upon it from over the hills; it stretched
itself and yawned like a waking man with a gradually increasing
tumult, which was kept up three or four hours. It took a short
siesta at noon, and boomed once more toward night, as the sun was
withdrawing his influence. In the right stage of the weather a pond
fires its evening gun with great regularity. But in the middle of
the day, being full of cracks, and the air also being less elastic,
it had completely lost its resonance, and probably fishes and
muskrats could not then have been stunned by a blow on it. The
fishermen say that the "thundering of the pond" scares the fishes
and prevents their biting. The pond does not thunder every evening,
and I cannot tell surely when to expect its thundering; but though I
may perceive no difference in the weather, it does. Who would have
suspected so large and cold and thick-skinned a thing to be so
sensitive? Yet it has its law to which it thunders obedience when
it should as surely as the buds expand in the spring. The earth is