Trillions and trillions of air molecules are bombarding you and me right now. Barometers are instruments that measure the force, or pressure, exerted by these molecules.

The earliest barometers consisted of a glass tube about 3 feet long, sealed at one end. The tube is filled with mercury, and the open end immersed in a container of mercury. The force of the air on the mercury in the container supports a column of mercury about 30 inches high in the tube. This is a mercury barometer. We get the common unit of pressure, "inches", from the height of the column.

Most modern barometers measure air pressure using something akin to a sealed miniature "balloon". When pressure changes, the balloon compresses or expands, moving a needle over a scale. This is an "aneroid" barometer. Even though aneroid barometers don't use mercury, the pressure scale is usually still in inches.