A barometer is a device that measures air pressure. It was invented in 1644 by Evangelista Torricelli (1608–1647). As a student of Galileo Galilei (1564–1642) for a short time, Torricelli was inspired by Galileo’s observation that piston pumps can only lift water up 33 feet (about 10 meters), after which point it is impossible to pump the water any higher. Torricelli proposed that air had weight, and therefore, exerted pressure. He tested his theory by filling a dish with mercury, a liquid that is 13.6 times denser than water. (A liquid that is denser than water allowed him to use a smaller quantity and a glass tube, which was easier to manipulate). He then took a glass tube 4 feet (1.2 meters) long glass tube that was open on one end, filled it with mercury, and turned it upside down with the open end beneath the surface of the mercury in the dish. Some, but not all of the mercury, flowed from the tube into the dish; 30 inches (760 millimeters) remained. The only force that was able to support the mercury in the tube was the weight of the air exerting pressure on the mercury in the dish.

The word “barometer” which means “weight measure” (from the Greek báros, meaning “weight” and métron, meaning “meter”) was not coined until 1665 by Robert Boyle (1627–1691). Boyle also changed the design for the barometer by using a U-shaped tube, thus eliminating the need for a mercury reservoir. The English physicist Robert Hooke (1635–1703) further improved on the barometer by creating an easy-to-read dial display.