18 Air Pressure
Torricelli filled a six-foot glass tube with liquid mercury and shoved a cork into the
open end. Then he inverted the tube and submerged the corked end in a tub of liquid mercury before he pulled out the stopper. As he expected, mercury flowed out of the tube and
into the tub. But not all of the mercury ran out.
Torricelli measured the height of the remaining mercury column—30 inches, as expected. Still, Torricelli suspected that the mystery’s true answer had something to do with
the vacuum he had created above his column of mercury.
The next day, with wind and a cold rain lashing at the windows, Torricelli repeated his
experiment, planning to study the vacuum above the mercury. However, on this day the
mercury column only rose to a height of 29 inches.
Torricelli was perplexed. He had expected the mercury to rise to the same height as
yesterday. What was different? Rain beat on the windows as Torricelli pondered this new
wrinkle.
What was different was the atmosphere, the weather. Torricelli’s mind latched onto a
revolutionary new idea. Air, itself, had weight. The real answer to the suction pump mystery lay not in the weight of the liquid, nor in the vacuum above it, but in the weight of the atmosphere pushing down around it.
Torricelli realized that the weight of the air in the atmosphere pushed down on the mercury in the tub. That pressure forced mercury up into the tube. The weight of the mercury in
the tube had to be exactly equal to the weight of the atmosphere pushing down on the mercury in the tub.
When the weight of the atmosphere changed, it would push down either a little bit
more or a little bit less on the mercury in the tub and drive the column of mercury in the tube
either a little higher or a little lower. Changing weather must change the weight of the atmosphere.
Torricelli had discovered atmospheric pressure and a way to measure and study it.
Fun Facts: Home barometers rarely drop more than 0.5 inch of mercury
as the weather changes from fair to stormy. The greatest pressure drop
ever recorded was 2.963 inches of mercury, measured inside a South Dakota tornado in June 2003.
More to Explore
Asimov, Isaac. Asimov’s Chronology of Science and Discovery. New York: Harper &
Row, 1989.
Clark, Donald. Encyclopedia of Great Inventors and Discoveries. London: Marshall
Cavendish Books, 1991.
Haven, Kendall. Marvels of Science. Englewood, CO: Libraries Unlimited, 1994.
Macus, Rebecca. Galileo and Experimental Science. New York: Franklin Watts, 1991.
Middleton, W. E. The History of the Barometer. New Brunswick, NJ: Johns Hopkins
University Press, 2003.