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It was traditionally thought (especially by the Aristotelians) that the air did not have weight - that is, that the miles of air above us don't weigh down on the air at our level. Even Galileo Galilei had accepted the weightlessness of air as a simple truth. Evangelista Torricelli, an Italian mathematician and scientist, questioned that assumption, and proposed that the air had weight, and that it was the weight of the air (not the attractive force of the vacuum as many thought) which held (or rather, pushed) up the column of water. He thought that the level the water stayed at (thirty-four feet) was reflective of the force of the air's weight pushing on it (specifically, pushing on the water in the basin and thus limiting how much water can fall from the tube into it). In other words, he viewed the barometer as a balance, an instrument for measurement (as opposed to merely being an instrument to create a vacuum), and because he was the first to view it this way, he is traditionally considered the inventor of the barometer (in the sense in which we use the term now).
The idea of using a heavier liquid then water was probably not Torricelli's own and Galileo (of whom Torricelli was briefly a student) himself may have suggested that other substances (including wine and mercury) would stop at a higher or lower level than water did. Regardless of who came up with the idea, Torricelli was the first to seriously take it up as an experiment.
Early in 1644, a friend of Torricelli carried out Torricelli's experiment, and it was seen that the mercury (which was known to be fourteen times heavier than water) stopped flowing into the basin when the level in the tube reached a height fourteen times smaller than that at which water stopped. Thus, Torricelli's ideas that the weight of the air from the atmosphere caused the liquid to stop falling seemed to be confirmed.
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. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".
At first, barometers used a column of water in a tube 34 ft (10.4 m) long. This inconvenient water column was soon replaced by Torricelli with mercury, which is denser than water and requires a tube about 3 ft (0.9 m) long. The basic mercurial barometer consists of a glass tube, sealed at one end and filled with pure mercury. After that it is inverted in a small cup of mercury called the cistern. The mercury in the tube sinks slightly, creating above it a vacuum (the Torricellian vacuum). Atmospheric pressure on the surface of the mercury in the cistern supports the column in the tube, which varies in height with variations in atmospheric pressure and hence with changes in elevation, generally decreasing with increases in height above sea level.
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches: the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type can usually measure atmospheric pressures in the range between 28 and 31 inches of mercury.
A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activity.
Make your own barometer
Make Your Own Barometer - Franklin Institute
The Mercurial Barometer: Measuring Pressure - NASA
Make a Barometer - WorldNow
Make Your own Barometer! - StarrySkies.com
Make Your own Weather Station and Barometer - PBS
History of the Barometer - Strange Loops
Torricelli's Letter to Michelangelo Ricci concerning the Barometer - Carmen Giunta
Robert Fitzroy - Julian Rubin
Brief History of the Barometer - barometer.ws
Evangelista Torricelli - MacTutor
Evangelista Torricelli - Catholic Encyclopedia