A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more downward force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the downward force placed on the reservoir.The first barometer of this type was devised by evangelista torricelli, a student of galileo galilei, in 1643. Torricelli had set out to create a perfect vacuum, and an instrument to measure air pressure. He succeeded in creating a vacuum in the top of a tube of mercury. Torricelli also noticed that the level of the fluid in the tube changed slightly each day and concluded that this was due to the changing pressure in the atmosphere.
