The man who weighed the air
Before anyone could measure air pressure, someone first had to prove that air weighs something. That took a glass tube, some mercury, and an argument over whether a vacuum can exist.
Air has no colour, no taste, and for thousands of years, no weight. Not because no one had thought about it, but because no one could show it. It was mercury that settled the matter.
Take a glass tube, closed at one end, fill it completely with mercury, and turn it upside down into a basin of more mercury, without letting a single bubble of air in. That's the experiment the Italian scientist Evangelista Torricelli carried out in the 1600s, and it's still the basic principle behind every mercury barometer built since.
The mercury that wouldn't fall all the way
What you'd expect was for the mercury to run out of the tube and down into the basin until the tube was empty. That's not what happened. The column fell a little way — and then stopped, leaving an empty space at the top of the tube. Something was holding up the rest of the liquid.
That something was the air outside, pressing down on the surface of the mercury in the basin. If air could push a column of heavy metal upwards inside a tube, air itself had to have weight — the weight of the whole atmosphere stacked on top of the open basin.
It was an entirely new kind of proof. No one had weighed air on a scale — you can't — but here stood a column of mercury, literally held up by it, that could be read off with a ruler. The weight of air was suddenly something you could point to.
That's how high the pressure of the atmosphere at sea level holds up an ordinary column of mercury. It's the same quantity modern barometers are still measured against — just expressed in hectopascals instead of millimetres of mercury.
WMO, Guide to Meteorological InstrumentsThe vacuum that wasn't supposed to exist
The empty space at the top of the tube raised another question scholars of the time had argued over for centuries: can a vacuum even exist? The prevailing view was that nature abhorred a vacuum — that something would always rush in to fill it. Torricelli's tube looked set to disprove that claim with nothing more than a glass tube and a bowl of mercury.
The dispute went on for decades, across Europe, with researchers building on the experiment, repeating it with other liquids and other tube lengths, and trying to settle whether the space at the top was truly empty, or whether it held some invisible "spirit of air".
The bottom of an ocean of air
The picture the experiment left behind is the part that has held up best: we don't live in a vacuum with a bit of air floating around in it. We live at the bottom of an ocean of air, many kilometres deep, and what a barometer measures is simply the weight of the air standing above the instrument right now.
The higher up you go, the less air you have above you, and the less it weighs. It's the same physics, whether it's read off a column of mercury in the 1600s or a spring-loaded can two hundred years later.
And because the weight of air is neither fixed nor irrelevant — it changes with the weather, hour by hour — the tube of mercury quickly became more than proof of a point. It became the first instrument that could tell you something about the weather before you could feel it yourself.
This article is about the history of physics, not about health. Aneroid measures air pressure on the same basic principle, but the app is not medical equipment and doesn't predict or treat anything.
The mercury tube stood for two hundred years as the only way to measure it. Then came an empty metal can that didn't need a gram of liquid — that's a story you can read on its own.