Evangelista Torricelli (1608 – 1647)
“We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight.”
Air has weight and exerts pressure and we can measure it
NARRATOR: By now, it had been established that a vacuum existed above the piston in the water pump. Still, the question of why it failed to lift water above 32 feet remained unanswered. Yet it was this drive to create a vacuum that finally led to the answer. An answer which changed our understanding of air forever. So, to continue our story, I would like to invite Mr. Torricelli, a young mathematician, secretary, and assistant to Galileo.
TORRICELLI: Thanks!
NARRATOR: You’re welcome. How are you?
TORRICELLI: Fine, what about you?
NARRATOR: Good too! So can you tell us about your journey with this problem?
TORRICELLI: Sure! Well, I joined Galileo in his last days, and yet in those few months that I spent with him, I learned a lot. Especially about the pump problem and his explanation.
NARRATOR: And how was it working with him?
TORRICELLI: It was okay. But it was the worst time of his life. He was on house arrest, lost his daughter, and even lost sight. Yet, I must admit that it was motivating to see him in high spirits.
NARRATOR: Oh! And what was your opinion about the pump problem?
TORRICELLI: Well, I was not convinced by his explanation.
NARRATOR: Have you told him about this?
TORRICELLI: No.
NARRATOR: Oh! Why?
TORRICELLI: I did not have the time.
NARRATOR: So, when did you start working on this?
TORRICELLI: It happened after his death. I was friends with Magiotti and so got to know about their experiment. It was fascinating, especially the height of the water column remaining in the tube.
NARRATOR: What was so special about it?
TORRICELLI: I don’t think you noticed that their estimated water level was the same as the height observed by Baliani.
NARRATOR: Oh yes! You are right.
TORRICELLI: But why? Why this consistency? So I decided to investigate it and repeat their experiment. I was sure that it would lead me to find an answer to this problem.
NARRATOR: Hmm.
TORRICELLI: Then, one day while working on this, I remembered Descartes’s work, where he theorized that air could have weight and speculated that it might be possible to construct a device to measure it. I realized that I should look for evidence outside the tube. It then became clear to me that the water in the tube was not running out because of the weight of the air above it. Further, I realized that Berti and Magiotti’s experimental apparatus was that device.
NARRATOR: So this is how you solved the pump problem.
TORRICELLI: Well, at least this was my thought, and I wanted to confirm it.
NARRATOR: And how did you confirm it?
TORRICELLI: For this, all I wanted to do was to build my apparatus.
NARRATOR: But you could have used Magi’s device.
TORRICELLI: I could have, but there was a problem.
NARRATOR: Problem?
TORRICELLI: Yes. I had noisy and gossipy neighbors. And a two-story barometer erected in my courtyard could have been viewed as the devil’s work. In those days, scientists were considered heretics. They were either locked in dungeons or burned.
NARRATOR: Oh…
TORRICELLI: Unfortunately, my laboratory’s ceilings were too low to accommodate a 34-foot barometer. So I needed a replacement of water. Then my friend Magi suggested that I use something denser that would permit me to reduce the size of the apparatus. Subsequently, I thought of Mercury, and it fit the bill. I shared this with my close friend Vincenzo Viviani. At one time, he also served as an assistant to Galileo. It was he who, in 1644, carried out my experiment.
NARRATOR: Oh! But why only Mercury!
TORRICELLI: Well, because it was about 14 times denser than water. And this allowed the 34-foot long lead pipe, replaced with a 32-inch glass tube. Simple mathematics,
34 feet = (34 ft. x 12) in. /14 = 29 in (approx.)
Torricelli’s tube was later named Barometer by Boyle
NARRATOR: Oh yes!
TORRICELLI: Thus, I repeated their experiment with mercury in a 32-inch tube. And once the mercury reached its level, I was able to see the gap at the sealed top. Most importantly, the whole apparatus fits easily on a small desk.
NARRATOR: Hmm.
TORRICELLI: As always, the important stuff was invisible at first. And then the unseen and impalpable realm of atmospheric pressure became visible. I realized we were surrounded by air with weight exerting pressure on us at all times and in all places.
NARRATOR: Yeah, I know about it. It was simple.
TORRICELLI: Yeah, you know this because we found it.
NARRATOR: Hahaha… That’s true. So does this put the controversy to rest? And were you able to convince the Aristotelians?
TORRICELLI: No, the obstacles remained. While I denied that the vacuum’s attractive force held the liquid up, I still believed that the space above the liquid was indeed a vacuum. The Aristotelian arguments against this conclusion stood unanswered. They denied the existence of a vacuum and that air had weight. They had their explanations for the space above the new liquid.
NARRATOR: And now what was their explanation?
TORRICELLI: Well, they suggested that mercury, like water, gave off the vapors that were stronger than that of water and so pushed the column of liquid down further. Added to this, the original Aristotelian arguments related to the transmission of light and sound against the space in the apparatus being a vacuum were also unanswered.
NARRATOR: It is a bit confusing as both of your arguments (yours and those of Aristotelians) sound convincing.
TORRICELLI: Okay. But I was convinced that it was because of the weight of the air. And not only was it visible but also measurable. I started documenting the daily variations in the mercury column. I observed changes in the atmosphere, which sometimes became heavier and at other times, lighter and thinner.
NARRATOR: Oh! But how did you guess it to be heavier and lighter?
TORRICELLI: Well, because of the changing height of the mercury column. When it was heavier, the level of the column got increased while at other times it got reduced.
NARRATOR: Hmm.
TORRICELLI: I even noticed that it responded to the change in temperature. Further, it also responded in advance to variations in the weather. Lowered when the weather turned poor and elevated when it became pleasant.
NARRATOR: So you mean to say that this apparatus could predict the weather.
TORRICELLI: I was not sure. But I had a hunch that somehow they were related.
NARRATOR: Oh! So it was the weight of air rather than a vacuum that balanced the water in the tube.
TORRICELLI: Yes. And people started calling this apparatus Torricelli Tube.
NARRATOR: Great!
The Torricelli tube underwent many changes over the next few years. The dispute surrounding the weight of air continued. It was Blaise Pascal, who proved that it was the weight of the air that prevented the falling of water.