Invented in 1644 by Evangelista Torricelli (1608–1647), a barometer is a device for measuring air pressure. Torricelli was a student (for a brief three months) of Galileo Galilei (1564–1642), and he was inspired by his mentor’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. After Galileo died, Torricelli continued to build on this observation. He theorized, correctly, that air had weight and, therefore, exerted pressure. He tested his theory by filling a dish with mercury (he used mercury because it was denser than water and therefore would require a much smaller amount to indicate pressure changes). He then took a four-foot-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. Some, but not all, of the mercury exited the tube; 30 inches (760 millimeters) remained. This meant that the remaining mercury in the tube stayed in the tube because air in the atmosphere was exerting pressure on the surface of the mercury in the dish. Not only did this experiment prove Torricelli’s theory that air had pressure, but he was also the first to create a vacuum (now called a Torricellian vacuum).

The word “barometer,” which means “weight measure,” was not coined until 1665 by Irish scientist and theologian Robert Boyle (1627–1691). Boyle came up with a new design for the barometer in which a U-shaped tube was used, eliminating the need for a mercury reservoir. English physicist Robert Hooke (1635–1703) made another improvement on the barometer by creating an easy-to-read dial display.