In mercury barometer, when mercury level falls inside glass tube then there leaves a blank space. This space is called torricellian vaccum. It is invented by the scientist toricelli.
Torricellian vacuum (Physics), a vacuum produced by filling with a fluid, as mercury, a tube hermetically closed at one end, and, after immersing the other end in a vessel of the same fluid, allowing the inclosed fluid to descend till it is counterbalanced by the pressure of the atmosphere, as in the barometer.The vacuum formed when a long tube, closed at one end and filled with mercury, is inverted into a mercury reservoir so that the open end of the tube is below the surface of the mercury. ... It is named after Evangelista Torricelli, who was responsible for the design of the mercury barometer.
Evangelista Torricelli, (born Oct. 15, 1608, Faenza, Romagna—died Oct. 25, 1647, Florence), Italian physicist and mathematician who invented the barometer and whose work in geometry aided in the eventual development of integral calculus.A barometer is essentially a balance. The weight of the atmosphere is balanced by the weight of a much shorter mercury column. You can't use an ordinary pan balance to weight the atmosphere (because air is pushing down on both sides).
Air pressure pushes down on the surface of the mercury, making some rise up the tube. The greater the air pressure, the higher the mercury rises. ... We use mercury in barometers because it's more convenient than using water.Torricelli explained this by assuming that mercury drains from the glass tube until the force of the column of mercury pushing down on the inside of the tube exactly balances the force of the atmosphere pushing down on the surface of the liquid outside the tube.
Torricelli experiment did not done on moon. Because torricelli experiment when air is present. There is no air in the moon.so,the mercury level will doesnot change.The torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.Torricelli law states that the speed of flow of fluid from an orifice is equal to the speed that it would attain if falling freely for a distance equal to the height of the free surface of the liquid above the orifice.