The barometer is a scientific instrument that measures the pressure of the atmosphere.
Weather forecasters use barometers to deflect changes in air pressure.
Because atmos pheric pressure changes with distance above or below sea level, a barometer may also be used to measure altitude. The barometer was invented in 1644 by an Italian physicist named Evangelista Torricelli. His device consisted of a long glass tube filled with mercury and then inverted into a cup of mercury. The column of mercury in the tube fell until its top was about 30 inches above the surface of the mercury that was in the cup. The pressure of the air on the surface of the liquid in the cup held the mercury in the tube.