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.
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