Barometer

Evangelista Torricelli proved we live at the bottom of an ocean of air. In 1643, working in Florence as Galileo's successor, he filled a meter-long glass tube with mercury, sealed one end, and inverted it into a basin of the liquid metal. The mercury column fell until it stood 76 centimeters above the basin's surface, leaving a vacuum at the sealed end—the first permanent vacuum created by human hands.

This was not an accident but a designed experiment. Miners pumping water from deep shafts had noticed that suction pumps could not lift water higher than about 10 meters. Galileo had speculated about nature's limited "abhorrence of vacuum." Torricelli hypothesized that air had weight and that atmospheric pressure supported both the water in pumps and the mercury in his tube. Mercury, being 13 times denser than water, should stand at 1/13 the height—approximately 76 centimeters.

The experiment worked because the adjacent possible had aligned. Glassblowing had advanced to produce tubes of sufficient length and quality. Mercury was available in purified form. The conceptual framework of air pressure, building on work by Galileo and others, suggested the experiment's design. Torricelli assembled these elements into proof.

Blaise Pascal extended the demonstration by predicting that mercury height should decrease at elevation. His brother-in-law carried a barometer up the Puy de Dôme mountain in 1648, confirming a drop of about 8 centimeters. Air pressure decreased with altitude exactly as the theory predicted.

The barometer transformed meteorology from speculation to measurement. Day-to-day variations in mercury height correlated with weather changes—falling pressure preceded storms. This was the first instrument for weather prediction based on physical principles rather than folk wisdom.

Beyond meteorology, Torricelli's vacuum sparked philosophical and scientific debates. Could space be truly empty? What filled the space above the mercury? The barometer became simultaneously a practical instrument and a philosophical puzzle, its implications extending far beyond weather forecasting into the nature of matter and space.