A mercury barometer

A barometer is an instrument used to measure air pressure. The barometer measures air pressure in various kinds of units including hectopascals (hPa) and millibars.

An aneroid barometer

There are various types of barometers such as the water barometer, aneroid barometer, and the mercury barometer. The mercury barometer, the earliest barometer, was created by an Italian mathematician named Evangelista Torricelli in 1643.