A Emo sphere

[Gk. atmos, vapor and sphaira, sphere.] The name of the great body of gaseous substance which surrounds the earth. The atmo¬ sphere consists e entially of two gases, oxygen and nitrogen, with a small quantity of carbonic acid and some aqueous vapor, with minute pro¬ portions of a few other substances. The vapor of water is of much importance in relation to the weather of any locality. The atmosphere being gaseous, obeys the same laws as gases under varying conditions of pre ure and temperature. There is reason to believe that its extreme height may be not le than 500 miles, this result having been found by observations on the flight of mete¬ orites. Air being a fluid, exerts pre ure equally in all directions. This pre ure, or the weight of the atmosphere, at sea-level, is equal, to 15 pounds on every square inch of surface. It has been found that the average height of a column of mercury which will balance the pre ure of the atmosphere is about 30 inches. (See Barometer.)