00:00Aristotle famously said,
00:08nature fears of empty space,
00:11when he claimed that a true vacuum,
00:13a space devoid of matter,
00:15could not exist because the surrounding matter
00:17would immediately fill it.
00:19Fortunately, he turned out to be wrong.
00:22A vacuum is a key component of the barometer,
00:25an instrument for measuring air pressure.
00:27And because air pressure correlates to temperature,
00:30and rapid shifts in it can contribute to hurricanes,
00:33tornadoes, and other extreme weather events,
00:35a barometer is one of the most essential tools
00:38for weather forecasters and scientists alike.
00:41How does a barometer work, and how is it invented?
00:44Well, it took a while.
00:46Because the theory of Aristotle and other ancient philosophers
00:49regarding the impossibility of a vacuum
00:52seemed to hold true in everyday life,
00:54few seriously thought to question it for nearly 2,000 years,
00:58until necessity raised the issue.
01:01In the early 17th century,
01:03Italian miners faced a serious problem
01:05when they found that their pumps
01:07could not raise water more than 10.3 meters high.
01:10Some scientists of the time,
01:12including one Galileo Galilei,
01:15proposed that sucking air out of the pipe
01:17was what made water rise to replace the void,
01:19but that its force was limited,
01:22and could lift no more than 10.3 meters of water.
01:25However, the idea of a vacuum existing at all
01:28was still considered controversial,
01:30and the excitement over Galileo's unorthodox theory
01:33led Gasparo Berti to conduct a simple but brilliant experiment
01:38to demonstrate that it was possible.
01:40A long tube was filled with water
01:42and placed standing in a shallow pool with both ends plugged.
01:46The bottom end of the tube was then opened
01:48and water poured out into the basin
01:51until the level of the water remaining in the tube was 10.3 meters.
01:56With a gap remaining at the top
01:57and no air having entered the tube,
02:00Berti had succeeded in directly creating a stable vacuum.
02:04But even though the possibility of a vacuum had been demonstrated,
02:08not everyone was satisfied with Galileo's idea
02:11that this empty void was exerting some mysterious yet finite force on the water.
02:17Evangelista Torricelli, Galileo's young pupil and friend,
02:21decided to look at the problem from a different angle.
02:23Instead of focusing on the empty space inside the tube,
02:27he asked himself what else could be influencing the water.
02:30Because the only thing in contact with the water
02:32was the air surrounding the pool,
02:34he believed the pressure from this air
02:36could be the only thing preventing the water level in the tube
02:39from dropping further.
02:41He realized that the experiment was not only a tool to create a vacuum,
02:45but operated as a balance
02:47between the atmospheric pressure on the water outside the tube
02:50and the pressure from the water column inside the tube.
02:53The water level in the tube
02:55decreases until the two pressures are equal,
02:58which just happens to be when the water is at 10.3 meters.
03:01This idea was not easily accepted,
03:05as Galileo and others had traditionally thought
03:07that atmospheric air has no weight and exerts no pressure.
03:12Torricelli decided to repeat Berti's experiment with mercury instead of water.
03:17Because mercury was denser, it fell farther than the water,
03:20and the mercury column stood only about 76 centimeters tall.
03:23Not only did this allow Torricelli to make the instrument much more compact,
03:28it supported his idea that weight was the deciding factor.
03:32A variation on the experiment used two tubes,
03:35with one having a large bubble at the top.
03:37If Galileo's interpretation had been correct,
03:40the bigger vacuum in the second tube
03:42should have exerted more suction and lifted the mercury higher.
03:45But the level in both tubes was the same.
03:48The ultimate support for Torricelli's theory came via Blaise Pascal,
03:53who had such a mercury tube taken up a mountain
03:55and showed that the mercury level dropped
03:58as the atmospheric pressure decreased with altitude.
04:02Mercury barometers based on Torricelli's original model
04:05remained one of the most common ways to measure atmospheric pressure,
04:09until 2007,
04:10when restrictions on the use of mercury due to its toxicity
04:13led to them no longer being produced in Europe.
04:16Nevertheless, Torricelli's invention,
04:19born of the willingness to question long-accepted dogmas
04:22about vacuums and the weight of air,
04:24is an outstanding example of how thinking outside of the box,
04:28or the tube,
04:29can have a heavy impact.
04:30For more information,
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