Evangelista Torricelli Built the First Barometer to Settle an Argument About Nothing

For roughly two thousand years, going back to Aristotle, a substantial portion of Western natural philosophy held that a true vacuum — a space containing absolutely nothing, not even air — could not exist in nature. The idea was summarized in the phrase "nature abhors a vacuum," treated less as an open question than as a settled principle. In 1643, an Italian physicist named Evangelista Torricelli built a simple glass tube experiment that produced, by most physicists' later interpretation, exactly that supposedly impossible thing — and in the process invented the barometer, the instrument still used today to measure atmospheric pressure and forecast weather.

A tube, a dish, and an unplanned emptiness

Torricelli's experiment was mechanically simple, even if its implications were not. He filled a glass tube, sealed at one end, entirely with mercury, then inverted it into a shallow dish also containing mercury, keeping the open end submerged so no air could enter. Some of the mercury in the tube flowed down into the dish, but not all of it — a column of mercury remained standing in the tube, consistently settling at a height of about 76 centimeters, roughly 30 inches, leaving an apparently empty space at the sealed top of the tube where the mercury had retreated from.

Torricelli's genuinely novel interpretation was that the mercury column wasn't being held up by anything pulling from above, the intuitive assumption of the time, but was being pushed up from below, by the weight of the surrounding atmosphere pressing down on the mercury's surface in the open dish. The height of the mercury column in the tube, in his interpretation, was a direct, physical measurement of how much that surrounding air weighed and pressed at that moment — the first working barometer, and simultaneously a demonstration that air, something people did not intuitively think of as having substantial weight, was in fact heavy enough to visibly support a column of liquid metal.

An argument that needed a mountain to finish

Torricelli's interpretation was compelling but not universally accepted immediately, and one of the most decisive follow-up tests came a few years later from the French polymath Blaise Pascal, who reasoned that if atmospheric pressure really was responsible for holding up the mercury column, then the column's height should measurably decrease at higher altitude, where there is simply less air overhead pressing down. In 1648, at Pascal's request, his brother-in-law carried a barometer up the Puy de Dôme, a mountain in central France, taking readings at different elevations along the way. The mercury column dropped steadily and measurably as the instrument climbed higher, exactly as Pascal's atmospheric-pressure explanation predicted, providing strong, direct experimental confirmation of Torricelli's theory and dealing a serious blow to the older vacuum-denying framework.

What the space at the top was actually doing

The empty-looking space at the sealed top of Torricelli's tube — now commonly called a Torricellian vacuum — became a genuinely important object of scientific study in its own right, since it appeared to be about as close to a true, matter-free vacuum as seventeenth-century experimenters could produce and directly observe. It was used in subsequent decades by various natural philosophers investigating the behavior of light, sound, and small objects in the near-absence of air, contributing meaningfully to the broader scientific project of understanding what a vacuum actually was and how ordinary physical processes behaved differently inside one.

From laboratory curiosity to weather instrument

It took a while for the barometer to move from a physics demonstration into a practical, everyday instrument, but the connection between atmospheric pressure and weather was recognized and refined over the following century and beyond: falling atmospheric pressure is broadly associated with approaching storms and unsettled weather, while rising pressure tends to indicate clearing, more stable conditions. By the eighteenth and nineteenth centuries, barometers had become a standard household and maritime instrument, mounted in ships' cabins and, later, in countless ordinary homes as a genuinely useful, low-tech forecasting tool — an object still recognizable today as the classic circular dial barometer, hung on a wall, needle pointing somewhere between "stormy" and "fair."

The instrument that outlived its own liquid

Mercury barometers remained the standard, most accurate design for centuries, valued for mercury's high density, which keeps the instrument's overall length manageable — a water-based barometer, using the same physical principle, would require a tube more than 30 feet tall to register the same range of pressure, since water is so much less dense than mercury. But mercury's toxicity became an increasing practical and regulatory concern over the twentieth century, and modern barometers, including most weather-station instruments today, generally use an aneroid design instead — a sealed, flexible metal chamber that physically expands and contracts in response to changing air pressure, avoiding liquid mercury entirely while measuring the same underlying atmospheric phenomenon Torricelli first isolated in a glass tube in 1643, settling, almost as a side effect, a two-thousand-year argument about whether nothingness itself was allowed to exist.

Torricelli's basic mercury-column design also quietly solved a related, everyday measurement problem: it gave scientists their first reliable standard unit for expressing pressure at all, well before formal metric units existed. Pressure readings expressed in "millimeters of mercury" trace directly back to this original tube-and-dish experiment, and the unit remained in serious scientific and medical use for centuries afterward -- blood pressure is still commonly reported in millimeters of mercury today, a direct linguistic fossil of a seventeenth-century argument about whether empty space could exist. Torricelli himself did not live to see how far his instrument would travel; he died in 1647, just a few years after the experiment, at only 39 years old, having spent his short career mostly as an assistant and eventual successor to Galileo in Florence. It is a fairly unusual scientific legacy: a single afternoon's tabletop experiment with mercury and glass, aimed at nothing more ambitious than settling an old philosophical dispute, quietly became the working basis for weather forecasting, medical diagnostics, and basic atmospheric science for the next three and a half centuries.