In Evangelista Torricelli’s 1643 barometer experiment, what force supported the column of mercury?

Evangelista Torricelli’s 1643 barometer experiment showed that atmospheric pressure supports a column of mercury.

Torricelli filled a long glass tube with mercury, closed one end, inverted it into a dish of mercury, and observed that the liquid did not drain completely. A vacuum formed above the mercury column, while air pressing on the mercury in the dish supported the column in the tube.

The column’s height was about 760 millimetres at sea level under standard conditions. This experiment challenged the older idea that nature universally resisted a vacuum. Torricelli argued instead that air has weight and exerts pressure.

The height varies with weather and altitude, which is why mercury barometers can indicate atmospheric changes. The unit torr is named after Torricelli and is defined as one millimetre of mercury. The experiment did not measure gravity directly; gravity pulled the mercury downward, while atmospheric pressure balanced it.