barometer

Barometer comes from Greek báros (weight, pressure) and métron (measure). The word was coined by Robert Boyle in 1665 to describe the instrument Evangelista Torricelli had invented in 1643. Torricelli, a student of Galileo, filled a glass tube with mercury, inverted it into a dish of mercury, and watched the column drop to a height of about 760 millimeters. The space above the mercury was a vacuum. The column was held up by the weight of the atmosphere pressing on the mercury in the dish.

The experiment was not about weather. It was about a philosophical question: does air have weight? Aristotle had said no — air was naturally 'light' and rose upward. Torricelli proved that air was heavy enough to support a column of mercury 760 millimeters tall. Blaise Pascal extended the experiment in 1648, sending his brother-in-law up the Puy de Dôme mountain with a mercury tube. The column dropped as the altitude increased, confirming that air pressure decreased with height.

The connection between barometric pressure and weather was noticed almost immediately. When the mercury fell, storms often followed. When it rose, fair weather came. By the 1700s, the barometer was standard equipment in ships, harbors, and wealthy households. Robert FitzRoy, captain of the HMS Beagle (and Darwin's captain), developed the first weather forecasting service in the 1860s using a network of barometers across Britain.

The mercury barometer has been largely replaced by aneroid barometers (using a sealed metal cell) and digital pressure sensors. Mercury barometers are now restricted in many countries due to mercury toxicity. But the unit of barometric pressure — millimeters of mercury (mmHg) — still references Torricelli's original column. A dead instrument's measurement lives on in the units.