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 32°,or freezing point;near the shore at 33°;in the middle of Flint's Pond,the same day,at 32°;at a dozen rods from the shore,in shallow water,under ice a foot thick,at 36°.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 a 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.
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 all alive and covered with papill.The largest pond is as sensitive to atmospheric changes as the globule of mercury in its tube.