hrust into the middle of Walden on the 6th of March, 1847, stood at
32x, or freezing point; near the shore at 33x; in the middle of Flint's
Pond, the same day, at 32+x; at a dozen rods from the shore, in shallow
water, under ice a foot thick, at 36x. This difference of three and a
half degrees between the temperature of the deep water and the shallow
in the latter pond, and the fact that a great proportion of it is
comparatively shallow, show why it should break up so much sooner than
Walden. The ice in the shallowest part was at this time several inches
thinner than in the middle. In midwinter the middle had been the warmest
and the ice thinnest there. So, also, every one who has waded about the
shores of the pond in summer must have perceived how much warmer the
water is close to the shore, where only three or four inches deep, than
a little distance out, and on the surface where it is deep, than near
the bottom. In spring the sun not only exerts an influence through the
increased temperature of the air and earth, but its heat passes through
ice a foot or more thick, and is reflected from the bottom in shallow
water, and so also warms the water and melts the under side of the ice,
at the same time that it is melting it more directly above, making
it uneven, and causing the air bubbles which it contains to extend
themselves upward and downward until it is completely honeycombed, and
at last disappears suddenly in a single spring rain. Ice has its grain
as well as wood, and when a cake begins to rot or "comb," that is,
assume the appearance of honeycomb, whatever may be its position, the
air 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.