Torricelli was the first to define clearly the fundamental concept of “atmospheric pressure,” confirming that air has weight. He invented the mercury barometer to measure this atmospheric pressure and to demonstrate that nature does not abhor a vacuum. His work supported Galileo’s idea that Earth could move through space without losing its atmosphere, and he also noted the day-to-day variation in the height of mercury in a column, thus initiating the scientific study of meteorology.
Evangelista Torricelli studied the work of Galileo
The invention of the mercury barometer followed a question raised by Galileo, who was curious why the grand duke’s pump makers could raise water by suction to a height of only about 34 feet. Ironically, Galileo had appealed to the scholastic idea that “nature abhors a vacuum” to suggest that the “horror” extends to only about 34 feet. It occurred to Torricelli that Galileo’s concept of gravity—applied to air—was the true explanation, suggesting that the weight of the air raises the water being pumped. He proposed that humans live at the bottom of a “sea of air,” which extends up some 50 miles. To test this idea, he sealed one end of a two-cubit (each cubit equals about 20 inches) glass tube (called the Torricelli tube) and filled it with mercury. When he inverted the tube with the open end in a bowl of mercury, the column did not empty completely, but instead fell to a height of about 30 inches. Torricelli maintained that this 30-inch column of mercury, weighing about the same as 34 feet of water in a column of the same diameter, is held up by the weight of the air.
Torricelli expected this result because he knew that mercury is 13.6 times as heavy as water and, thus, 34 feet of water divided by 13.6 matches the 30 inches of mercury needed to counterbalance the weight of the air. Since a 30-inch column of mercury with one square inch of cross-sectional area weighs about 15 pounds, the air pressure is about 15 pounds per square inch, or 2,100 pounds per square foot at sea level. Torricelli also observed that the height of the mercury column varied from day to day because of changes in atmospheric pressure. These ideas later became important in the development of meteorology and of the steam engine.
Torricelli also maintained that the space above the mercury column is a vacuum, contrary to the scholastic opinion of the day, which held to Aristotle’s argument that a void is logically impossible. The “Torricellian vacuum” was the first sustained vacuum. Demonstrating the two concepts, that a vacuum can exist and that the sea of air is held to Earth by gravity, was critical to support the new idea that Earth moves through the vacuum of space.
The first description of the mercury barometer was in a letter that Torricelli wrote on June 11, 1644, to his friend Michelangelo Ricci in Rome, a fellow student of Castelli. (Torricelli’s letters on atmospheric pressure are translated into English in a volume of his Collected Works
Unfortunately, Torricelli died of typhoid fever on October 25, 1647, at the young age of thirty-nine. He is honored in low-pressure research by the unit for pressure called the torr, equivalent to the pressure of one millimeter of mercury. Standard atmospheric pressure is defined as 760 torrs (76 cm of mercury).
Torricelli’s ideas on atmospheric pressure and the vacuum were quickly confirmed and extended by other experimenters. In France, Blaise Pascal
In a famous experiment, Pascal refuted the idea that the mercury column is held up by vapor at the top of the column, thereby preventing a vacuum. He repeated Torricelli’s experiment with red wine in a 14-meter tube. If the gap at the top of the column indeed were made of vapor instead of being a vacuum, then the volatile wine should fall lower than water; but if it were a vacuum, the lower-density wine should fall less than water to balance the weight of the air, as was observed. Pascal is honored by the International System of Units and the meter-kilogram-second (MKS) system for pressure called the Pascal.
The ideas of “air pressure” and “the vacuum” led to the invention of the air pump in 1650 by the German engineer Otto von Guericke,
At Oxford University in England, Robert Boyle
In 1662, Boyle found the pressure-volume law now known by his name: He showed that the volume of a gas is inversely proportional to the applied pressure. The same law was discovered independently several years later by the French physicist Edmé Mariotte,
The invention of the mercury barometer by Evangelista Torricelli introduced the concept of “air pressure” and demonstrated the existence of a vacuum. His ideas solved one of the problems raised by the Copernican theory and Galileo’s emphasis on a moving Earth: If Earth is in motion, it must carry its “sea of air” with it. Gravity acting on the air and producing air pressure holds the air in its place around the earth, and the surrounding space must be a vacuum if the atmosphere is not to be stripped away.
The barometer and the concept of “air pressure” led to many other important discoveries and inventions. It opened up the scientific field of meteorology in the development of the weather barometer, and it led to the work of Blaise Pascal and the concept of the “altimeter” to measure altitude above sea level. It also led to the invention of the air pump and to partial-vacuum experiments in both physics and physiology. The air pump in turn led to the invention of the steam engine, in which a vacuum can be produced by condensing steam in a cylinder and the resulting air pressure used to drive a piston, as first suggested by Robert Hooke.