When water is cooled, it diminishes in bulk like other bodies; but at a particular temperature it deviates from the general law of contraction, and by doing so, becomes, perhaps, the most striking example of providential design that is to be met with in inorganic nature. Cold water is specifically heavier than warm water, in consequence of the contraction it has undergone, and hence will sink in it, as water would sink in oil. Now, if we consider the surface of a lake exposed to the cooling action of a wintry wind, the water which is first cooled becomes heavier, and, sinking to the bottom, is replaced by the warmer water, which floats up to the top; there is thus a current established of cold water descending and of warmer water rising up. This continues until all the water in the lake has been cooled down to the temperature at which its specific gravity is greatest, which is about 40 degrees, or about eight degrees above the point at which it begins to freeze. The action of the cold wind continuing, the water at the surface is still further cooled; but now, in place of contracting, it expands--instead of becoming heavier, it becomes lighter, and remains floating upon the surface. It is then still further cooled, and finally its temperature being reduced to 32 degrees, it freezes, and a layer of ice is formed on the surface of the lake. This ice, and the cold water next it, are impermeable to heat: it actually serves as a blanket to the water at 40 degrees which is below, preventing the escape of the heat, and retaining it at that temperature, sufficient for the purposes to which it is subservient; for at the temperature of 40 degrees, the life and enjoyments of all the various tribes of animals and vegetables which reside permanently under the surface of the water are perfectly secured, at least for a very considerable time; the water holding dissolved a quantity of oxygen for the animal respiration, and the vegetables living on the carbonic acid which is formed by the respiration of the fish. On the approach of spring, the warmer air, and the rays of the more elevated sun, act directly on the surface of the ice, and each portion of water formed by melting, becoming heavier, sinks, so as to expose the ice itself to the source of heat. Thus the ice is rapidly dissolved, and after a few days the lake throws off its wintry aspect altogether.