[Gk. atmos, vapor , and sphaira, sphere.] The name of the great body of gaseous substance which surrounds the earth. The atmosphere consists essentially of two gases, oxygen and nitrogen, with a small quantity of carbonic acid and some aqueous vapor, with minute proportions 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 pressure and temperature. There is reason to believe that its extreme height may be not less than 500 miles, this result having been found by observations on the flight of meteorites. Air being a fluid, exerts pressure equally in ' all directions. This pressure, 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 pressure of the atmosphere is about 30 inches. (See Barometer.)