Chapter 17

SPRING

The opening of large tracts by the ice-cutters commonly causes a
pond to break up earlier; for the water, agitated by the wind, even
in cold weather, wears away the surrounding ice. But such was not
the effect on Walden that year, for she had soon got a thick new
garment to take the place of the old. This pond never breaks up so
soon as the others in this neighborhood, on account both of its
greater depth and its having no stream passing through it to melt or
wear away the ice. I never knew it to open in the course of a
winter, not excepting that of '52-3, which gave the ponds so severe
a trial. It commonly opens about the first of April, a week or ten
days later than Flint's Pond and Fair Haven, beginning to melt on
the north side and in the shallower parts where it began to freeze.
It indicates better than any water hereabouts the absolute progress
of the season, being least affected by transient changes of
temperature. A severe cold of a few days duration in March may very
much retard the opening of the former ponds, while the temperature
of Walden increases almost uninterruptedly. A thermometer thrust
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