Weighing Air
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Hi, I'm Dave Thurlow and today on The Weather Notebook the accidental origin of the single most important weather forecasting instrument. The barometer.

 Evangelista Torricelli
The year was 1643, and in Renaissance Italy, Evangelista Torricelli, a distinguished philosopher and mathematician, was attempting to answer a question that had long perplexed natural philosophers: Does air have weight?

Torricelli's experiment was this: he filled a glass tube with mercury and submerged the open end of the tube in a basin of mercury. Noting that the mercury in the tube did not flow out into the basin but rather came to rest at a level partway up the tube, Torricelli correctly reasoned that air exerts weight, or pressure, on the mercury in the basin which in turn pushes up the mercury in the tube and holds it in place.

He also observed that the column of mercury did not rest at a fixed level but rather moved slightly up and down the tube, from which Torricelli deduced a second crucial fact: that air pressure or weight changes. And thus it was that the apparatus of a science became enshrined as the barometer. Eventually, a link was established between the rise and fall of barometric pressure and changes in the weather, and the barometer became the basis of the science of meteorology.

Now, it's possible to fashion a barometer out of any liquid including water - but, since water is 13 times lighter than mercury, a water barometer would have to be inconveniently large. A column of mercury, however, under normal atmospheric conditions, rises to about 30 inches or 760 millimeters: a handy height for an indispensable instrument of meteorology.

Thanks to today's contributing writer David Laskin. The Weather Notebook is funded by Subaru, the beauty of all wheel drive.

Related Links

Barometers explained

Torricelli the mathematician
Other contributions of Torricelli.

Letter to Michelangelo Ricci concerning the Barometer
(1644) Collected Works Vol. III (1919) [from William Francis Magie, A Source Book in Physics (New York: McGraw-Hill, 1935) (translator?)]