<P> A barometer is a scientific instrument used in meteorology to measure atmospheric pressure . Pressure tendency can forecast short term changes in the weather . Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems and frontal boundaries . </P> <P> Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes . An altimeter is intended to be transported from place to place matching the atmospheric pressure to the corresponding altitude, while a barometer is kept stationary and measures subtle pressure changes caused by weather . The main exception to this is ships at sea, which can use a barometer because their elevation does not change . </P> <P> Although Evangelista Torricelli is universally credited with inventing the barometer in 1643, historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a water barometer sometime between 1640 and 1643 . French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time . </P> <P> On July 27, 1630, Giovanni Battista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon, led over a hill about twenty - one meters high, failed to work . Galileo responded with an explanation of the phenomenon: he proposed that it was the power of a vacuum that held the water up, and at a certain height the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight . This was a restatement of the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resistenza del vacuo . </P>

Who invented the barometer what does it measure
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