Note: Much of this post was generated by AI. Some portions were edited and small sections were written by the author.

Setting the Stage

In the 17th century, science was developing the foundation of theory, observation, and experiment that remains today. The Earth’s magnetic field was newly discovered. In astronomy, the Copernican revolution was underway as the newly invented telescope enabled important observational and theoretical breakthroughs. In mathematics, the discovery of logarithms made calculations substantially faster, while the development of analytic geometry introduced the framework for graphing using Cartesian coordinates.

Among these groundbreaking discoveries, Evangelista Torricelli and Galileo Galilei made significant strides in understanding pressure and fluid dynamics, while Otto von Guericke invented the vacuum pump, which was improved by Robert Boyle and Robert Hooke, paving the way for future developments. This article traces the story of the study of air pressure by these giants of science.

Galileo Galilei: Observing the Power of Air

Galileo (1564–1642) is often hailed as the father of modern science, renowned for his revolutionary work in physics, astronomy, and mathematics. Although his contributions to fluid dynamics are less celebrated, Galileo’s curiosity about air pressure laid the foundation for future explorations in this field.

In the early 1600s, Galileo observed a curious phenomenon while studying water pumps. Italian miners had been using pumps to raise water from deep pits, but they found that no matter how much suction was applied, water could only be lifted about 10.3 meters (about 33.8 feet). Galileo recognized that the suction applied a force that was lifting the water up the tube, but the limitation of 10.3 meters for the height of this applied force puzzled Galileo, as classical physics provided no clear explanation.

Galileo hypothesized that a vacuum could not exist (a widely held Aristotelian view) and speculated that “nature abhors a vacuum.” Although incorrect, this idea reflected his growing interest in air and atmospheric pressure. Galileo did not solve the mystery of why water could only rise to a certain height, but he did pass on his questions to his protégé, Evangelista Torricelli.

Berti and Torricelli: Water and Mercury Barometer Experiments

Italian physicist Gasparo Berti (c. 1600–1643) was intrigued by Galileo’s work on suction and vacuums. Berti built a lead tube with a slightly taller maximum height than water was shown to reach, filled it with water, and sealed both ends. One end was submerged in a pool of water, and then the bottom of the tube (which remained underwater) was unsealed. Some of the water flowed out into the pool, but much of it remained in the tube. Yet again, the tube was filled to a height of about 10.3 m above the surface of the pool. But what was keeping the water in the tube, and what was filling the evacuated space above the water in the tube?

Evangelista Torricelli (1608–1647) was not only Galileo’s student but also his eventual successor as philosopher and mathematician to the Grand Duke of Tuscany and a professor of mathematics at the Academy of Florence. Inspired by Galileo’s work, Torricelli took up the challenge of understanding why water pumps had a height limitation. He shifted his focus from water to mercury, which, being much denser, offered unique insights.

In 1643, Torricelli conducted his own experiment, but with the liquid metal mercury instead of water. Mercury is much denser that water, so it required a much shorter tube. He filled a glass tube with mercury, sealed it, and inverted it into a basin of mercury, where it was unsealed. Similarly to Berti’s experiment, the mercury in the tube fell to a height of about 76 centimeters (about 30 inches), leaving a void above it in the tube. The key to Torricelli’s experiment was in his interpretation of the findings.

First, he attributed the mercury’s filling most of the tube to atmospheric pressure. This was the first confirmation that the atmosphere produced a force on objects and surfaces below, with the mercury in the tube rising as the pressure increased and falling as the pressure decreased. Second, he proposed that the void at the top of the tube was a vacuum — challenging the Aristotelian (and Galileian) belief that a vacuum could not exist. And finally, his mercury barometer would become a tool employed by meteorologists to this day.

Torricelli’s work laid the groundwork for the study of fluid dynamics, especially in understanding the movement of liquids and gases. This was further expanded by his statement of Torricelli’s Law, which describes the speed of efflux of a fluid under gravity.

French mathematician, inventor, and scientist Blaise Pascal (1623-1662) theorized that pressure decreases with altitude. To test this theory, Pierre Petit (1617-1687) carried a barometer to the top of a small mountain in France. Because the rate of pressure decrease in the lower atmosphere is roughly 1 hPa per 10 m, the mountain didn’t have to be very tall to confirm Pascal’s theory. The many implications of Torricelli’s findings marked the beginning of meteorology as a quantitative science.

Guericke’s Invention of the Vacuum Pump

While the mercury barometer was a useful device for measuring atmospheric pressure, it was passive tool. There was a need for a device with which experimenters could test the behavior of pressure and, in doing so, to expand the knowledge of how pressure relates to other variables.

The vacuum pump was invented in 1650 by German scientist Otto von Guericke. Guericke was inspired by the work of Galileo and Torricelli, particularly Torricelli’s barometer experiment, which had demonstrated the existence of a vacuum. Guericke’s pump was the first device capable of creating and maintaining a partial vacuum, which he used to conduct groundbreaking experiments on the nature of air pressure and vacuums.

One of Guericke’s most famous experiments was the Magdeburg hemispheres demonstration in 1654, where he joined two large hemispheres, evacuated the air between them with his pump, and showed that teams of horses could not pull them apart due to the pressure exerted on the hemispheres by the air. Guericke proved that substances were not pulled by a vacuum but were pushed by the pressure of the surrounding fluids. This demonstration provided powerful evidence of atmospheric pressure and the vacuum’s integrity, significantly advancing the understanding of air and vacuums in the 17th century.

How Guericke’s Vacuum Pump Worked

Guericke’s pump consisted of a large cylinder and a piston that could be moved up and down. The basic process worked as follows. First, Guericke sealed the cylinder with a tightly fitting piston to trap air inside. By pulling the piston out of the cylinder, the volume inside the cylinder expanded, lowering the pressure in the sealed space and reducing the amount of air. A one-way valve at the cylinder’s bottom prevented outside air from re-entering. The seal wasn’t perfect, but by repeatedly pulling the piston up and down, Guericke could draw more air out each time, gradually reducing the air pressure in the cylinder. It took a great amount of effort involving several strokes to create the partial vacuum.

Flaws with Guericke’s vacuum pump

Although Guericke’s vacuum pump was groundbreaking for its time, it had several flaws. The leather and other seals used in the pump often leaked, allowing air to seep back into the chamber. This made it hard to maintain the vacuum over time or achieve consistent results. The pump used a single-cylinder mechanism, which limited the volume of air that could be removed with each cycle. More advanced pumps later used multiple cylinders or continuous pumping systems, which increased the speed and effectiveness of air removal.

The pump relied on one-way valves to prevent air from flowing back in, but the valves of the time were primitive and often unreliable. They would sometimes fail, causing the vacuum to break and reducing the effectiveness of the pump. Finally, Guericke’s pump had no way of accurately measuring the pressure inside the chamber, so he could only approximate the strength of the vacuum. This made it difficult to quantify the vacuum and limited the pump’s use in precise scientific experiments.

Hooke and Boyle and the vacuum pump

Inspired by Guericke’s vacuum pump, which they had never actually seen, Robert Boyle and his assistant, the mechanically-inclined Robert Hooke, designed and built a smaller, more efficient vacuum pump that could provide a stronger vacuum with more precision in its operation. They carried out experiments with fire, sound, and living organisms. The were also able to make exact measurements of pressure that contributed to Boyle’s greatest legacy: Boyle’s Law, which states the inverse relationship between pressure and volume at constant temperature. Their work paved the way for new developments in engine design and air pressure systems.

Conclusion

The 17th century marked tremendous advancements in the understanding of pressure. This progress was spearheaded by key developments in physics, mechanics, and experimentation by notable scientists such as Galileo, Torricelli, Boyle, and Hooke. In addition to laying the foundation for meteorology as a quantitative science, their pioneering work influenced engineering and related fields.