An instrument for measuring the tension or elastic force
   of gases, steam, etc., constructed usually on the principle of allowing
   the gas to exert its elastic force in raising a column of mercury in an
   open tube, or in compressing a portion of air or other gas in a closed
   tube with mercury or other liquid intervening, or in bending a metallic
   or other spring so as to set in motion an index; a pressure gauge. See
   Pressure, and Illust. of Air pump.