Torricelli hid his barometer experiments amid neighbourhood rumours of sorcery
When gossip spread through Evangelista Torricelli's Italian neighbourhood that he was dabbling in witchcraft, he decided to keep his experiments quiet to avoid arrest. Switching from water to mercury let him shrink a ten-metre tube to under a metre, and in doing so he proved that air has weight.
The puzzle began in 1630, when Giovanni Battista Baliani told Galileo that a siphon over a hill about 21 metres high refused to work; the water stalled around 10 metres up. Galileo blamed the limited strength of the vacuum, likening it to a cord that snaps under too much weight. Around 1639 to 1643 Gasparo Berti filled a long tube with water, stood it in a basin and unplugged the bottom. The column dropped and then held at about 10.3 metres, leaving an apparently empty space above it.
Torricelli, who had served as Galileo's assistant in the last three months of his life, turned the explanation around. Aristotelians, and even Galileo, assumed air was weightless. Torricelli argued that the weight of the atmosphere pressing on the basin was holding the column up. Mercury, roughly 14 times denser than water, needed a tube of only about 80 centimetres, and he found the atmosphere supported about 30 inches of it. He also noticed the height shifted slightly from day to day and wrote that we live at the bottom of an ocean of air.
Blaise Pascal took up the work in 1646. To test the Aristotelian claim that vapour filled the empty space, he publicly compared water with wine, supposedly the more vaporous liquid; the Aristotelians predicted the wine would stand lower, and it did not. Then he asked his brother-in-law Florin Perier to carry a barometer up the Puy de Dôme. In September 1648 Perier confirmed that the mercury sank as altitude rose.
The name comes from Greek words for weight and measure. Pressure is still quoted in millimetres of mercury, and one torr originally equalled one of them. A European Union directive in 2007 restricting mercury in new consumer instruments effectively ended production of new mercury barometers in Europe.
Source: Barometer
