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. Changes in atmospheric pressure cause the mercury level in the tube to rise or fall, providing a measurement of the pressure in millimeters or inches. This device was invented by Evangelista Torricelli in the 17th century and is based on the principle that atmospheric pressure can support a column of liquid. Mercury barometers are often used in meteorology to help predict weather changes, as variations in pressure can indicate different weather patterns.