Instruments devised for indicating the pressure and density of the air. They are made in various forms, of which the simplest consists of a straight glass tube closed at one end, filled with mercury and inverted with the open end in a tray of mercury. It is generally assumed that the average ordinary density of the air is when, at sealevel and at a temperature of zero, the column of mercury in the tube stands at 760 millimetres, or 2992 inches high. As the pressure or density of the atmosphere increases, the column rises, and as it diminishes the column falis. The normal pressure is about 147 pounds to the square inch. The space above the mercury, in the simplest form of a mercurial barometer, is practically vacuous, and is popularly known as the Torricellian vacuum. As the barometer measures the weight of the superincumbent air, it necessarily follows that the higher the altitude the lower the barometer indication. In chemical investigations, gases are weighed or measured subject to the atmospheric pressure, and vary directly in density and inversely in volume with the pressure; hence the necessity of recording the pressure and reducing the amount to a standard pressure, as also a standard temperature.
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