Of all terrestrial substances water has the greatest specific heat. It cools more slowly than any other. Its specific heat is four times greater than that of air. When the temperature of a kilogram of water falls one degree, it raises the temperature of four kilograms of air one degree. But water is seven hundred and seventy times heavier than air, so that if we compare two equal volumes of water and air, we find that a cubic meter of water, in losing one degree of temperature, raises the temperature of seven hundred and seventy times four, or 3080 cubic meters of air by the same amount. This is the explanation of the influence of the sea in modifying the climate of continents. The heat of summer is stored in the ocean and is slowly given out in winter. This explains why islands and seashores have no extremes of climate. The heat of summer is tempered by the breezes, and the cold of winter is alleviated by the heat stored in the water. The invisible vapor of water, distributed through the atmosphere, exercises the greatest possible influence on temperature. In quantity this vapor seems almost negligible, since oxygen and nitrogen alone form ninety-nine and one-half per cent. of the air we breathe; and the remaining one-half of one per cent. contains, besides the vapor of water, carbonic acid, ammonia and other substances. There is scarcely more than a quarter of one per cent. of aqueous vapor. If we consider the constituent atoms of the atmosphere, the physicist tells us that for two hundred atoms of oxygen and nitrogen there is scarcely one of water-vapor; but this one atom has eighty times more absorptive energy than the two hundred others. The radiant heat of the sun, after traversing the atmosphere, warms the surface of the earth. The heat waves reflected from the warmed earth are not lost in space. The aqueous vapor atoms, acting like a barrier, turn them back and preserve them for our benefit. This is one of the most brilliant and the most fruitful discoveries of modern physics. The oxygen and nitrogen molecules of dry air do not oppose the radiation of heat; but, as we have just said, one molecule of water-vapor possesses eighty times the absorptive energy of the other two hundred molecules of dry air, and consequently such a molecule is sixteen thousand times more efficacious in so far as the conservation of heat is concerned. So that it is the vapor of water and not the air, properly speaking, which regulates the conditions of life upon the earth.