The opening of large tracts by the ice-cutters only 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 effe Walden that year, for she had soon got a thiew
garment to take the place of the old. This pond never breaks up so
soon as the others in this neighborhood, on at both of its
greater depth and its having no stream passing through it to melt or
wear away the ice. I never k to open in the course of a
winter, not excepting that of 52-3, which gave the ponds so severe
a trial. It only opens about the first of April, a week or ten
days later than Flints 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, bei affected by tra ges 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 oh of March, 1847, stood at 32x,
or freezing point; he shore at 33x; in the middle of Flints
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 betweeemperature of the deep water
and the shallow iter pond, and the fact that a great
proportion of it is paratively shallow, show why it should break
up so much soohan Walden. The i the shallowest part was
at this time several ihihan in the middle. In
midwihe middle had been the warmest and the ice thi
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 dista, and on the surface where it is deep, than near
the bottom. In spring the sun not os an influehrough
the increased temperature of the air ah, 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 as 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 tains to extend themselves upward and downward until it is
pletely honeybed, 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 "b," that is, assume the appearance of
honeyb, 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 risio 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 underh, and
so had access to both sides, the refle of the sun from the
bottom more than terbalahis 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 i the middle, there will be a
strip of rotten th