Torricellian vacuumTorricelli vacuum
In 1643 Italian physicist Evangelista Torricelli filled a tube with mercury to the top, then capped the top with his finger, then positioned the sealed top into the bowl of mercury, then slipped out his finger, then inverted the tube vertically upside down, a vacuum then formed at the top of the tube; he then measured the height of the column of mercury to be 72 centimeters. The vacuum has since come to be known as a Torricellian vacuum.
In science, Torricelli vacuum or Torricellian vacuum is the "vacuum space" or void created when a tube of mercury is inverted to a second dish of mercury, a volume of so-called empty space that results when the downward pull of the column of mercury pressing downward on the dish of mercury finds an equilibrium balance with the downward pull of the 62-mile high Karman line column of air also pressing down on the dish of mercury.

History
In 1643 or early 1644, Italian physicist Evangelista Torricelli, during his investigation into the pump problem, demonstrated the so-called "Torricellian vacuum", so to explain why hand operated water draining pumps stopped working past a depth of about 32 feet.

Mercury
The significance of the use of mercury in this experiment is the result of ingenuity on the part of Torricelli who sought a laboratory sized descaled reproduction of the pump problem and knew that the heaver density of mercury would give him experimentally measurable results that could be scaled up via ratio calculation to explain the water height situation.

Torricelli is quoted as having predicted, knowing the density of mercury (13.5 g/cm³) and water (1 g/cm³), based on his experimentally measured height of 72 centimeters of the column of mercury, that mine pumps would stop operating at 10 meters of water or about 32-33 feet.

Atmospheric pressure variations
Repeated daily observations of this vacuum showed that the size of the vacuum, and hence the height of column of mercury varied from day to day. This height variation phenomenon became the basis for the barometer.

Guericke
Word of Torricelli's vacuum soon spread and is said to have influenced German engineer Otto Guericke in his circa 1650 attempts to make a vacuum in a beer keg, which led to the development of the famous 1657 Magdeburg hemispheres.

Further reading
‚óŹ Boye, Martin H. (1855). A Treatise on Pneumatics: The Physics of Gases (Torricellian vacuum, pg. 39). E.C. & J. Biddle.

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