A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. It is commonly used by meteorologists to predict weather phenomena, as pressure tendency can forecast short-term changes in the weather. There are two main types of barometers: mercury and aneroid. Mercury barometers use a vertical glass tube containing mercury which rises or falls to determine atmospheric pressure. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Aneroid barometers, on the other hand, use a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule is prevented from collapsing by a strong spring. The wall of the capsule deflects with changes in atmospheric pressure, and this deflection is coupled mechanically to an indicating needle. Barometers are also used to determine altitude since atmospheric pressure changes will occur as you go further, closer, or below sea level.