Atmospheric pressure is the weight that the atmosphere exerts upon us. The measured value of the atmospheric pressure fluctuates slightly in response to moving weather systems, and it drops as one ascends to higher elevations, since less air is overhead to exert a downward force. The mean atmospheric pressure at sea level is given in a number of different unit systems as 1013.25 millibars, 29.92 inches of mercury, and 14.7 lbs per square inch. The last unit makes plain the fact the pressure is defined as weight (force) per area.
Aristotle, whose teachings sometimes inhibited the advancement of science, was right on target in his belief that the atmosphere surrounding the Earth had weight. Moreover, he believed that as air density decreased, it would be possible for an object to move faster. However, he did not believe in the concept of a vacuum because the absence of an atmosphere meant an object could move infinitely fast, and since infinite speed was not possible, a vacuum that allowed infinite speed was not possible either. Galileo disputed some of Aristotle's contentions. In 1638 Galileo published a book in which he asserted a vacuum was possible. But Galileo did not believe that air had a weight that could exert a pressure, even though his own experiments showed clearly that air exerted a force on objects. This was perhaps because he discounted everything Aristotle said, even when he happened to be right. Consequently, the thermometer Galileo invented was inaccurate because it did not take the effect of air pressure into account. Otto von Guericke became interested in air pressure because of Galileo's comments on the subject. In a public demonstration in 1657, Guericke became the first to use an air pump and create a vacuum, thus ending the debate on whether one could exist.
In 1643, in Florence, Italy, Evangelista Torricelli furthered Guericke's work. Filling a narrow tube with mercury and upending it in a bowl of mercury, Torricelli found that only a portion of the tube emptied. He correctly surmised that the atmospheric pressure upon the mercury in the bowl kept the tube from draining completely, and the vacant area at the top of the tube was a vacuum. He noticed the height of the column of mercury fluctuated from day to day, indicating that the atmospheric pressure changed. The barometer, a device to measure the pressure of the atmosphere, was born, yet that name wouldn't exist for another twenty years.
Mathematician Blaise Pascal duplicated the experiments of Torricelli, and he expanded on them. In 1648, Pascal, who suffered from ill-health, had his brother-in-law make measurements of air pressure at various altitudes on a mountain. As expected, the higher the altitude, the less pressure registered on the barometer. Obviously, the weight of the air at the surface of the Earth was greater because it has to support all the atmosphere above it. Robert Boyle duplicated Torricelli's experiment as well. In 1660 he placed his mercury-filled tube in a container and removed the surrounding air, creating a vacuum. As the air was removed the column of mercury dropped. When completely evacuated, the mercury showed zero air pressure in the container. It was Boyle who coined the word "barometer" in 1665.
Today it is known that the weight of the Earth's atmosphere is more than five quadrillion tons. The weight of air pressing down on one 's shoulders is about one ton, but we aren't flattened because we are supported on all sides by an equal amount of pressure.
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