Evangelista Torricelli

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Earth's protective blanket of air is pretty thin -- only a few hundred miles, give or take. Beyond that is the vacuum of space -- a realm of near-nothingness. Yet until a few centuries ago, people thought the air extended forever, and that nature would not allow a vacuum to exist.

The man who proved otherwise was born on today's date in 1608.

Evangelista Torricelli had studied math and physics in Rome. He befriended Galileo Galilei, and became his secretary for the last few months of Galileo's life.

While he was with Galileo, Torricelli wondered why water could only be pumped so high. He reasoned that it might be because the air has weight, so it presses downward.

To test that, he filled a glass tube with mercury, then turned it upside down inside a basin. Some of the mercury spilled out, but a column about 30 inches tall remained. Torricelli realized that the pressure of the air kept the rest of the mercury inside the tube. He calculated that the weight of the mercury left in the tube equalled the weight of the air above the tube. Torricelli had invented the barometer -- a device that's used routinely today to measure changes in air pressure.

Torricelli calculated that the atmosphere must be about five miles deep. He was off because the air gets thinner as you go higher. But his basic conclusion was correct: at some point above Earth's surface, you leave the atmosphere behind -- and enter the vacuum of outer space.

Script by Damond Benningfield, Copyright 2008

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