Barometer Invented

In 1643, the Italian physicist and mathematician Evangelista Torricelli built the first mercury barometer, an instrument that showed the atmosphere has weight and exerts pressure. The experiment, usually dated to 1643 (some accounts place it in 1643–44), also produced the first sustained artificial vacuum in a laboratory. It overturned a long-held assumption about how nature behaved and opened the way for modern studies of air, pressure, and weather.

For centuries, European thinkers had followed the Aristotelian view that "nature abhors a vacuum," meaning that empty space could not exist and that liquids rose in pumps because nature rushed to fill any void. Practical experience complicated this idea. Well-diggers and fountain engineers in Tuscany found that suction pumps could not lift water higher than roughly ten meters, which raised the question of why a supposedly universal principle had such a limit. Galileo had pondered the problem in his later years, and Torricelli, who served as his assistant and secretary in Florence shortly before Galileo's death in 1642, inherited the puzzle.

Torricelli reasoned that if air had weight, the limit on pumps might reflect the balance between the weight of the water column and the weight of the air pressing on the water's surface. Water would make a very tall and awkward test, so he used mercury, which is about fourteen times denser. The experiment itself was carried out with his colleague Vincenzo Viviani, who is the main source for early accounts of it. A glass tube, sealed at one end, was filled with mercury and inverted into a dish of the same liquid. The mercury sank partway down the tube and stopped at a height of a bit less than a meter, leaving an apparently empty space above it.

Torricelli interpreted the column as being held up by the pressure of the surrounding air, and he noticed that its height varied slightly from day to day, which he linked to changing atmospheric conditions. This made the device both a scientific demonstration and a practical instrument. The word "barometer" came into use somewhat later.

News of the experiment spread across Europe through correspondence, notably reaching the French circle around Marin Mersenne. Blaise Pascal repeated and extended the work, and in 1648 his brother-in-law Florin Périer carried a barometer up the Puy de Dôme, finding that the mercury stood lower at altitude, which strongly supported the air-pressure explanation. Robert Boyle's later work on gases drew on the same tradition.

The barometer became a standard tool in meteorology, aviation, and altitude measurement. Its legacy is also commemorated in the "torr," a unit of pressure named in Torricelli's honor.