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Glass tubes at the time were fragile and hard to come by. They often broke when filled with a kilogram of mercury. But with the help of a skilled assistant the experiment was done. The mercury in the tube fell and stabilized at a level about 76 centimeters above the level in the dish. Torricelli surmised correctly that the mercury rose in the tube because of the weight of the atmosphere pressing down on the mercury in the dish, and that the space above the mercury column was a vacuum.
It was the first time that a vacuum had been created in the laboratory, and understood as such. The concept of a vacuum had been contentious since antiquity. Both Plato and Aristotle thought the existence of a vacuum to be impossible, against Nature.
Cranmer was convicted of heresy in , and was burned at the stake the following year. Mindful of the contention around the idea of vacuum, he did not make his experiment public at first, but disclosed it only in letters to a friend, Michelangelo Ricci.
In October, , the French scientist Marin Mersenne visited Torricelli, who repeated the experiment for him and gave him copies of the letters to Ricci. Pascal immediately understood the meaning of the experiment, and repeated it in He believed that the atmospheric pressure should decrease with altitude, and engaged a relative and some friends to carry a barometer up a mountain in the south of France.
They found the anticipated decrease of pressure with altitude, laying the foundation for the science of meteorology. Pascal understood the pressure to be equal to the weight of the atmosphere per unit area. He combined this with the surface area of the earth and calculated the total mass of the atmosphere.
Minor improvements were later made to increase the precision of the readings, but the basic design remained unchanged. In meteorological stations around the world it served as the reference standard for measuring atmospheric pressure for more than three centuries, perhaps a record time for an instrument to be used with the same design. In two men hiked up the Puy de Dome, a small grass-covered extinct volcano in central France with a summit elevation of meters.
They carried with them a meter-tall glass column filled with mercury. The tall column of mercury was sitting in a pool of mercury at the base. The men were Blaise Pascal and Florin Perier.
Their unusual contraction - the glass column of mercury - was the first barometer, a device for measuring air pressure. Today, a barometer can be carried in one hand.
But this 17th Century barometer, invented by Italian scientist and mathematician Evangelista Toricelli, was huge and heavy. Mercury is more than ten times heavier than water. Carrying this instrument must have made the hike a challenge. Why did they bring such an instrument? They wanted to know how air pressure changed with altitude in the atmosphere. The barometer would allow them to measure this.
When air pressure was higher, the mercury in the pool would fill the space in the column, raising the level within the glass. When air pressure was lower, the mercury in the pool would spread out, extracting some of the mercury from the column and lowering the level in the glass.
Pascal and Perier measured the height of mercury in the column of glass at three elevations on the mountain. They found that at higher elevations on the side of the Puy de Dome the mercury level in the glass column was lower than it was at lower elevations.
They concluded that the differences in height of the mercury were caused by differences in the weight of air above. As they went up the mountain there was less atmosphere above them, and thus less weight of air to compress the mercury. Based on this finding, Pascal concluded that a vacuum exists above the atmosphere.
In , Horace Benedict de Saussure climbed to the summit of Mont Blanc in order to explore the atmosphere. Just a year before, this rugged peak in the Alps had been successfully climbed for the first time. Saussure made it to the top carrying a barometer and thermometer with him and making measurements along the way.
By this time, smaller and more portable instruments were available than the unwieldy barometer Pascal and Perier carried with them in Saussure was an Earth scientist as well as a mountaineer. It was a challenging climb. Even through he had to cut the time he spent on the mountain a bit short because he was weakened by altitude sickness, the measurements Saussure made were very valuable. They demonstrated that air temperature decreases with height in the atmosphere by about 0.
With that information, Hermann von Helmholtz and others deduced that about 30 km high in the atmosphere the temperature would be C F. At that temperature, no heat remains. It is known as absolute zero. Absolute zero is thought of as the lowest temperature possible. It has never been found even with modern technologies, although 21st Century scientists have come very close.
In the 18th Century when Helmholtz was doing his calculations, temperature that cold was off the charts. It would be about 60 years before Lord Kelvin invented a temperature scale that used absolute zero as its zero value.
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