Torricelli's Barometer (1643): The Discovery of Air Pressure

Torricelli’s barometer was the mercury-filled glass tube with which the Italian mathematician Evangelista Torricelli showed in Florence in 1643 that air has weight. The column of mercury settled at about 76 centimetres, leaving an empty space above it. The experiment created the first lasting vacuum, overturned an ancient belief that nature abhors a vacuum, and gave science its first instrument for measuring air pressure.

Background: why pumps could not lift water higher

Well diggers and engineers had long known that a suction pump could not raise water more than about 10 metres. Galileo mentioned this limit in his Two New Sciences of 1638, but he thought it reflected the limited strength of a column of water, which broke under its own weight.

Most scholars still followed Aristotle in holding that a true vacuum was impossible. The idea, later called horror vacui, explained suction as nature rushing to fill any gap. Around 1640–1643 Gasparo Berti in Rome raised a tall water-filled pipe against the side of a house and saw the water fall to about the same 10-metre height, leaving a space above. What filled that space was hotly argued.

The experiment

Evangelista Torricelli, from Faenza in Romagna, became Galileo’s assistant at Arcetri near Florence in 1641 and succeeded him as mathematician to Grand Duke Ferdinando II de’ Medici after Galileo died in January 1642. He reasoned that mercury, about 14 times heavier than water, should stand about one-fourteenth as high.

In 1643 a glass tube about a metre long, sealed at one end, was filled with mercury, closed with a finger, turned upside down and opened under the surface of a dish of mercury. Vincenzo Viviani, Galileo’s last pupil, is usually credited with performing the first trial. The mercury fell until the column stood at about 76 centimetres, or roughly 29.9 inches, leaving an apparently empty space at the top.

Torricelli explained the result in a letter to his friend Michelangelo Ricci dated 11 June 1644. The mercury, he argued, was held up not by any horror of the vacuum but by the weight of the air pressing on the dish. He wrote that “we live submerged at the bottom of an ocean of the element air”. He also noticed that the height changed slightly from day to day, which he linked to changes in the air.

The vacuum debate and Pascal’s test

News of the experiment spread through the letters of the Paris friar Marin Mersenne. In France, Blaise Pascal repeated it in Rouen in 1646–1647 with tubes of water and wine many metres long, and published Expériences nouvelles touchant le vide in 1647. Some opponents, including René Descartes, denied that the space could be truly empty and proposed that a subtle matter filled it.

Pascal designed a decisive test. If air pressure held up the column, the mercury should stand lower on a mountain, where there is less air above. On 19 September 1648 his brother-in-law Florin Périer carried tubes from Clermont to the summit of the Puy de Dôme, about 1,465 metres high. The mercury stood roughly 8 centimetres lower at the top than in the town, and intermediate readings fell steadily on the way up. Pascal published the results the same year.

Consequences

The experiments showed that the atmosphere has weight and that its pressure varies with height and weather. They encouraged Otto von Guericke’s work on the vacuum, demonstrated with the Magdeburg hemispheres, and Robert Boyle’s studies with the air pump that led to Boyle’s law. The name “barometer” came into English in the 1660s, when Boyle used the word.

Torricelli did not live to see much of this. He died in Florence on 25 October 1647, aged 39, after a short illness. His other work included improvements to telescope lenses and mathematical results on areas and solids, including the infinitely long solid known as Gabriel’s Horn.

Myths and facts

Myth: Torricelli built the barometer as a weather instrument. His aim was to test the vacuum and the weight of air. Its use for weather forecasting developed later, once observers saw that the mercury tended to fall before storms.

Myth: the space above the mercury is a perfect vacuum. It contains a very small amount of mercury vapour, but for practical purposes it was the best vacuum anyone had yet made.

Timeline

• 1638: Galileo’s Two New Sciences notes that pumps cannot lift water above about 10 metres.
• c. 1640–1643: Gasparo Berti’s water-column experiment in Rome.
• 1642, January: Galileo dies; Torricelli becomes court mathematician in Florence.
• 1643: The mercury tube experiment is carried out in Florence.
• 1644, 11 June: Torricelli explains the experiment in a letter to Michelangelo Ricci.
• 1647: Pascal publishes his New Experiments Concerning the Vacuum.
• 1647, 25 October: Torricelli dies in Florence.
• 1648, 19 September: Florin Périer’s Puy de Dôme experiment.

Why it matters

Torricelli’s tube turned the air from an invisible nothing into a measurable substance with weight. It produced the first laboratory vacuum, helped defeat an Aristotelian doctrine through experiment, and gave rise to the barometer, still central to weather forecasting and the measurement of altitude.