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. 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.