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. When the atmospheric pressure changes, it causes the mercury level in the tube to rise or fall, indicating the pressure level. Mercury barometers are often used in meteorology to help predict weather changes. Higher pressure usually indicates fair weather, while lower pressure can signal storms. Despite their accuracy, they are less common today due to the toxicity of mercury and the availability of safer alternatives like aneroid barometers.