The following seems to be the most satisfactory explanation of the matter: It is a well known fact that the air has the power to absorb and hold, in an invisible form, a certain amount of moisture. The quantity which it can contain depends upon its temperature. If the air is cooled, it parts with a portion; thus if the grass radiates its heat, dew is deposited upon it; if it is very cold, the frost covers it with sparkling crystals. It is thought that when cooling from any cause takes place in higher altitudes, the atmospheric moisture changes from the invisible to the visible form, and assumes the physical condition of spheroids or vesicles, minute bubbles of water in point of fact, each bubble filled with air. These bubbles, heated by the sun’s rays, would become lighter than the medium in which they float, for the same reason that soap bubbles float while they are warm. In this condition they are drifted along by currents until they reach a colder stratum of air, when they are condensed and fall as rain. If cooled sufficiently, snow would be formed. Cloud forms are four in number, cirrus, cumulus, nimbus, and stratus, all of which may sometimes be seen at once, in the sky of a summer’s day. At times they float above the loftiest mountains. Gay-Lussac,[15] rising in his balloon to an elevation of 21,600 feet, perceived clouds drifting far above him. [Illustration: AIR CURRENTS—SHOWING THE PRINCIPLE ON WHICH MINES ARE OFTEN VENTILATED.] The work performed by the atmosphere in supplying water to the soil is worthy of profound attention. Close observation shows that it varies in amount, year by year, much less than one would suppose. For example, the average yearly rain-fall in western New York, for the last thirteen years, has been thirty-six inches. Contrary to the general impression, the record shows a slight yearly increase in the amount. While this is true, the quantity of water carried down by our rivers is constantly diminishing. All must have observed the lessening size of our streams. Many a mill has ceased to run from lack of its former supply of water. This has resulted from the destruction of forests, and clearing of land, which have greatly increased evaporation of moisture from the soil. So grave a matter has this become, that it is attracting the attention of governments, because of its relation to agriculture and the navigation of rivers. One can have but little idea, unless he carefully calculates it, of the inestimable blessings conferred by the atmosphere upon man, in furnishing to the soil its supply of water. MR. THOUGHTFUL THANKFUL,