Air Pressure
Year of Discovery: 1640
What Is It? Air (the atmosphere) has weight and presses down on us.
Who Discovered It? Evangelista Torricelli
Why Is This One of the 100 Greatest?
It is a simple, seemingly obvious notion: air has weight; the atmosphere presses down
on us with a real force. However, humans don’t feel that weight. You aren’t aware of it because it has always been part of your world. The same was true for early scientists, who
never thought to consider the weight of air and atmosphere.
Evangelista Torricelli’s discovery began the serious study of weather and the atmosphere. It launched our understanding of the atmosphere. This discovery helped lay the
foundation for Newton and others to develop an understanding of gravity.
This same revelation also led Torricelli to discover the concept of a vacuum and to invent the barometer—the most basic, fundamental instrument of weather study.
How Was It Discovered?
On a clear October day in 1640, Galileo conducted a suction-pump experiment at a
public well just off the market plaza in Florence, Italy. The famed Italian scientist lowered a
long tube into the well’s murky water. From the well, Galileo’s tube draped up over a
wooden cross-beam three meters above the well’s wall, and then down to a hand-powered
pump held by two assistants: Evangelista Torricelli, the 32-year-old the son of a wealthy
merchant and an aspiring scientist, and Giovanni Baliani, another Italian physicist.
Torricelli and Baliani pumped the pump’s wooden handlebar, slowly sucking air out
of Galileo’s tube, pulling water higher into the tube. They pumped until the tube flattened
like a run-over drinking straw. But no matter how hard they worked, water would not rise
more than 9.7 meters above the well’s water level. It was the same in every test.
Galileo proposed that—somehow—the weight of the water column made it collapse
back to that height.
In 1643, Torricelli returned to the suction pump mystery. If Galileo was correct, a
heavier liquid should reach the same critical weight and collapse at a lower height. Liquid
mercury weighted 13.5 times as much as water. Thus, a column of mercury should never
rise any higher than 1/13.5 the height of a water column, or about 30 inches.
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