The instrument used to measure air pressure is called a barometer. A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. It works by balancing the mercury in the glass tube against the outside air pressure, just like a set of scales. As air pressure increases, it pushes more of the mercury up into the tube, and as air pressure decreases, more of the mercury drains from the tube. There are two basic types of barometers: the mercury barometer and the aneroid barometer. The mercury barometer uses mercury that rises and falls in response to air pressure changes, while the aneroid barometer uses cells that expand and contract as air pressure changes. The aneroid barometer is often used in portable devices, while the mercury barometer is used in fixed installations.