If you follow the weather faithfully, you know that meteorologists spend lots of time talking about air pressure. Those H's and L's on weather maps are highs and lows of pressure. In general, highs bring fair weather, while cloudy, stormy weather tends to accompany lows.

Of all the weather elements, pressure is probably the most mysterious, perhaps because our senses aren't well tuned to it. We can tell the difference between hot and cold. We know when it's humid. We can feel the wind moving past our bodies. But air pressure … Yes, some people can sense pressure changes, but in general, we need specialized instruments called barometers to actually measure pressure.

The barometer was invented in 1643 by Evangelista Torricelli, a student of Galileo. On tomorrow night's Franklin Fact, I'll show you what he invented, and the kinds of barometers in popular use today.