Barometer

— An instrument for measuring the pre ure of the atmosphere. The mercurial barometer is a gla tube 33 to 34 inches high, sealed at the top, filled with pure mercury and inverted in an open cup of mercury. A graduated scale on the instrument permits observations of the fluctuations in the height of the mercurial column, which is highest when the atmosphere is dry, weighing more. then than when saturated with aqueous vapor, which is lighter than air. The height of barometric measurement is about 30 inches.