Atmospheric Pressure

Aristotle, whose teachings sometimes otherwise inhibited the advancement of science, was right on target in his belief that the atmosphere surrounding the Earth had weight. Moreover, Aristotle stated 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 considered 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 hold 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 20 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 the atmosphere above it. Robert Boyle duplicated Torricelli's experiment as well. In 1660, Boyce 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 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. The normal barometric pressure at sea level, equal to one atmosphere (1 atm) of pressure equals 14.7 psi (pounds per square inch), or 760 mm (29.92 inches) of mercury.

See also Air masses and fronts; Weather forecasting