Torricelli Measures Atmospheric Pressure
Evangelista Torricelli was a prominent 17th-century Italian scientist known for his groundbreaking work on atmospheric pressure and the invention of the mercury barometer. His exploration began as he studied under Galileo, where he became intrigued by the limitations of water pumps and their inability to draw water beyond 34 feet due to air pressure. By sealing one end of a glass tube filled with mercury and inverting it, Torricelli demonstrated that a column of mercury approximately 30 inches tall remains suspended due to the weight of the surrounding air, a revolutionary idea that challenged existing beliefs about vacuums.
Torricelli's findings established that air has weight and can exert pressure, leading to the concept of a "sea of air" enveloping the Earth. His work laid the foundation for modern meteorology, as he observed variations in mercury height corresponding to changes in atmospheric pressure. This understanding was further validated by subsequent scientists, such as Blaise Pascal, who conducted experiments using the barometer to measure altitude and predict weather changes. Additionally, Torricelli's invention was vital in the development of technologies like the air pump and the steam engine, which harnessed the principles of air pressure and vacuums for practical applications. His legacy is honored in the unit of pressure known as the torr, critical in various scientific fields today.
Torricelli Measures Atmospheric Pressure
Locale Florence, Tuscany (now in Italy)
Date 1643
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.
Key Figures
Evangelista Torricelli (1608-1647), Italian mathematician and physicist who invented the mercury barometerGalileo (1564-1642), Italian physicist and astronomer who discovered the laws of falling bodies and projectilesBenedetto Castelli (1577/1578-1643), student of Galileo and teacher of TorricelliFerdinand II de’ Medici (1610-1670), grand duke of Tuscany, r. 1621-1670Blaise Pascal (1623-1662), French philosopher, mathematician, and physicist who supported and developed Torricelli’s air pressure conceptsOtto von Guericke (1602-1686), German engineer who invented the air pumpRobert Boyle (1627-1691), Irish physicist who experimented with reduced pressuresRobert Hooke (1635-1703), English experimenter and associate of BoyleEdmé Mariotte (1620-1684), French Catholic priest and scientist who coined the word “barometer”
Summary of Event
Evangelista Torricelli studied the work of Galileo in Rome as a student of Benedetto Castelli, Galileo’s favorite student who recommended him to Galileo. Torricelli invented the mercury barometer two years after serving as Galileo’s secretary and assistant for the last three months of his long life. After Galileo died, Grand DukeFerdinand II de’ Medici of Tuscany appointed Torricelli to succeed Galileo as court mathematician at the Florentine Academy.
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 , 1919.) Later in 1644, he published in Florence his Opera geometrica (geometric works), which included original geometric theorems on the cycloid, his studies on projectile motion, and his work on fluids that led to his equation, known as Torricelli’s law, to determine the speed of fluid flow from an opening in a vessel.
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 recognized that if air has weight, it should diminish with altitude. In 1646, he engaged his brother-in-law to climb the Puy-de-Dôme with a mercury barometer, finding that the mercury level dropped from its sea-level value of 76 centimeters to about 70 centimeters at a height of about one mile. In addition to inventing this altimeter concept, he suggested using the barometer to predict weather after noting that stormy conditions were usually preceded by falling air pressure.
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, the burgomaster of Magdeburg. He showed that a close-fitting piston in an evacuated cylinder could not be removed by the effort of twenty men. In 1654, he gave a public demonstration of the power of a vacuum by evacuating two large metal hemispheres fitted together along a greased flange. Air pressure held these “Magdeburg hemispheres” together so tightly that even a team of sixteen horses could not pull them apart.
At Oxford University in England, Robert Boyle engaged Robert Hooke in 1657 to build an improved version of the air pump, and together they began to experiment with reduced air pressures. They showed that a ringing bell produced no sound in a vacuum and that a feather and lead ball fall at the same rate in an evacuated jar. In his first scientific work, New Experiments Physico-Mechanicall, Touching the Spring of the Air and Its Effects (pb. 1660), Boyle described his experiments in both physics and the physiology of respiration. By gradually exhausting the air in a jar containing a mouse and a candle, he observed the resulting expiration of the candle at about the same time as the mouse.
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, who coined the word “barometer” in his Discours de la nature de l’air (pb. 1676; discourse on the nature of air).
Significance
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.