"We have made many glass vessels... with tubes two cubits long. These were filled with mercury, the open end was closed with the finger, and the tubes were then inverted in a vessel where there was mercury"

Torricelli describes the essential steps of the first mercury barometer: a long glass tube is completely filled with mercury, the open end is temporarily sealed with a finger, and the tube is inverted into a basin of mercury. When the finger is removed, the mercury in the tube descends only partway, leaving a gap at the top. That persistent gap is the Torricellian vacuum, a space not filled by air or liquid, demonstrating that a void can exist without being instantly “filled” by nature.

The tube’s length, about two cubits (roughly a meter), is crucial. Because mercury is very dense, a column about 760 millimeters high exerts a weight per unit area equal to the pressure of the surrounding air at sea level. If the tube were shorter than this equilibrium height, no void would form. By choosing a tube longer than the expected column, the experiment ensures that mercury will descend until its weight balances atmospheric pressure, leaving empty space above. Using mercury has additional advantages: its high density keeps the apparatus compact compared with water, its low vapor pressure keeps the space near the top close to a true vacuum, and it doesn’t cling strongly to glass.

The experimental details reveal careful method. Closing the open end with a finger prevents air from entering as the tube is inverted; immersing the open end in a mercury reservoir allows the column to settle at an equilibrium height set by the weight of the air pressing on the reservoir’s surface. Torricelli’s reference to making “many” such vessels signals replication and confidence that the phenomenon is systematic, not a trick of glasswork or handling.

The implications are profound: air has weight and exerts pressure; pumps lift fluids not by suction but because external air pressure pushes them; and the height of the mercury column becomes a measure of atmospheric pressure, inaugurating barometry and a new, quantitative understanding of the atmosphere.