A mercury barometer is a tool that uses the element mercury to measure barometric (atmospheric) pressure to predict weather. Mercury is a metallic element that is liquid at room temperature, making it possible for the liquid to move within the barometer to signify the variations in pressure levels.
A mercury barometer uses a long tube of glass with one open end that is submerged in a vat of mercury. The other end of the tube is sealed. When atmospheric pressure is high, the mercury rises within the glass tube. Because mercury is highly toxic and dangerous to handle, this tool is not widely used. Instead, aneroid barometers, which do not use toxic materials, are more common.