A mercury barometer is a scientific instrument used to measure atmospheric pressure. It consists of a glass tube filled with mercury, which is a dense liquid metal. The tube is inverted into a basin of mercury, and the height of the mercury column in the tube changes with variations in air pressure. Higher pressure pushes the mercury up, while lower pressure allows it to drop. Mercury barometers are essential for weather forecasting and studying meteorology. They provide accurate readings of atmospheric pressure, which can indicate changes in weather patterns. Despite their effectiveness, they are less commonly used today due to the availability of digital barometers.