Barometer's measure atmospheric pressure. The most reliable barometers are the Mercury Barometers, or the newer digital friendly Aneroid Barometer.  The barometer works by balancing the weight of mercury in the glass tube against the atmospheric pressure just like a set of scales. If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises. If the weight of mercury is more than the atmospheric pressure, the mercury level falls.