METEOROLOGY, in its extended sense, embraces all physical causes which affect the state of the atmosphere or are affected by it. Hence it is connected with the phenomena of haat and cold, dew, rain, hail and snow, clouds, winds, aurora:, boreales and australes, or polar lights, haloes, parhelia, .to The sense in which Aristotle (MovetepoAe ytiri, i.) uses the term is still more extensive, comprehending, in addition to what are now called mete ,re, every affection (egiSor) common to the air and water, with the characters of the different parts of the earth, and their affection., as winds and earthquakes, and everything incident to s ich kinds of motion.
Our first inquiry shall be, what is the nature and what the probable extent of the terrestrial atmosphere! Essential as it is both to animal' and vegetable life, to the diatribut on of heat, and to various modifica tions of light, the knowledge of its nature and composition is eminently useful. The sir, though composed of several elastic fluids, obeys the same lawn to which they are individually subject, namely, its elasticity and density at • given temperature are proportional to the pressure which it sustains, and for every degree of the centigrade thermometer under a given pressure it expands very nearly of its volume at the temperature zero. Hence if its density be represented by 8 and its temperature by t (in centigrade degrees), its elastic force will be pro portional to (1 + a t). b (where a represents the decimal .00375) as well as the pressure sustained. Lastly, the pressure is equal to the weight of a vertical column of atmosphere, having the portion pressed on as a base, and extending upwards to its extreme limit.
According to Dalton's views, the various gases constituting the air are not chemically combined by the law of definite proportions, but only mechanically mixed, co-existing in the same space, and producing by the sum of their independent pressures the elevation of the mercury in the barometric tube. In modern chemical physics, they are said to be mingled by the principle of DIFFUSION, which is the form in which Daltun's conception is now held.
We have seen that heat increases the elastic power of air, and hence the equilibrium of a masa of air unequally heated is constantly dis turbed. The currents of warm and cold air charge places, the cold air moving to the warm region, and thence, when warmed, repeating the course of the previous warm air. Thus the atmosphere is a great agent in tending to equalise the mean temperatures of climates in various, latitudes. Besides, the atrial currents are vehicles for the transfer of clouds, for producing electrical discharges, for clearing away malaria, and are turned by the ingenuity of man to promote his industry and extend his knowledge of the globe which he inhabits.
The atmosphere, considered as a transparent medium, has also great effects on light by its refractive power, and the reflection of the aqueous masses it contains. 'knee arises twilight, which mitigates the transition of day to night, and from the duration of which it is easy for the astro nomer to compute that altitude of the atmosphere at which it CCIVICII to act sensibly on light, either from its total absence or extreme tenuity. This altitude is from forty to fifty tulles above the level of the sea. Again, by the refractive power of the atmosphere distant terrestrial objects are elevated to the view when the spherical curvature of the earth would otherwise have caused their concealment; various optical illusions, as the mirage, fats morgana, &c., are all easily explained from the same refractive power under peculiar circumstances of temperature. By this medium sound is conveyed and odours are, at least in part, disseminated ; the clouds which float in it soften the direct glare of the solar beams, and its aqueous particles, fluid or frozen, produce the beautiful phenomena of haloes), rainbows. fable suns, &c. Its greater specific gravity elevates the balloon, by means of which the nature of the upper strata of the air below the altitude of betwe n four and five miles, may be ascertained, and the barometric elevation and tempera ture obeerved, which furnish data for calculating the physical limits of the atmosphere.