A mercury barometer is a scientific instrument used to measure atmospheric pressure. It consists of a glass tube filled with mercury, which is inverted into a basin of mercury. As atmospheric pressure changes, it pushes the mercury up or down the tube, allowing for the measurement of pressure in millimeters or inches. This device was invented by Evangelista Torricelli in the 17th century and is based on the principle that the weight of the mercury column is balanced by the atmospheric pressure. Mercury barometers are often used in meteorology to help predict weather changes.