The known agents made use of in the economy of nature for the production of rain are changes of temperature and electricity. The latter principle is supposed to act most powerfully in the production of thunder-showers, in which case it is not unlikely that a portion of the air of the atmosphere is, by the passing of the lightning from one cloud to another, converted into water. The former seems to be the chief agent in the colder regions of the globe, where electricity is either more equal in its distribution, or not so active in its operations as in the warmer climates. From the beautiful theory of the late Dr. James Hutton, supported by the researches of professor Leslie, it appears, that “while the temperature advances uniformly in arithmetical progression, the dissolving power which this communicates to the air mounts with the accelerating rapidity of a geometrical series;” and this in such a ratio, that the “air has its dryness doubled at each rise of temperature answering to fifteen centesimal degrees,” or twenty-seven of Fahrenheit. Hence, “whatever be the actual condition of a mass of air, there must always exist some temperature at which it would become perfectly damp;” and hence whenever two streams of air saturated with moisture of different temperatures are mixed together, or brush against one another, in the form of different currents of wind, there must always be a quantity of moisture precipitated. For if two masses of air, of different temperatures, but equal in quantity, and both saturated with moisture, were mixed together, the resulting temperature would be nearly the mean of the two, but, at that temperature, the capacity of air for moisture being less than the quantity contained in the two commixed masses, the surplus must be deposited. Rain is by no means common in the Polar countries excepting in the months of July and August, and then only with southerly or westerly winds. During all seasons of the year, however, with strong gales blowing from a southern climate, rain is occasionally observed in situations near the edge of the ice; but snow or sleet are more common even under such circumstances; and in remote situations among ice, near the 80th parallel of latitude, rain seldom or never occurs. _Hail_ is a much more familiar meteor in temperate than in frigid climates. In the Greenland Sea, this aqueous concretion is very rarely seen; and if we define it as consisting of pellucid spheres of ice, generated in the atmosphere, it may be said to be unknown in very high latitudes. This fact is in favour of the electrical origin of hail, as it is well-known to be common in temperate climates, where the air is in a high state of electricity, and to be the frequent concomitant of thunder and lightning. The only substance resembling hail that is generated in the frigid zone consists of a white, porous, spherical concretion of light and snowy texture.