Torricelli
Why does water boil at 100°C?
That time, that place
Torricelli was Galileo's last student, and the problem in front of him came from the Grand Duke's gardeners: however hard a pump worked, water would not rise past about ten meters.
The explanation of the day was that nature abhors a vacuum, and so the water follows. Then why stop at ten meters? The answer offered — that nature's abhorrence has a limit — was thin.
Why this question
Torricelli reversed the direction. Perhaps nothing pulls from above; perhaps something pushes from below. The weight of all the air stacked over our heads presses on the water's surface, and that push drives it up.
If so, there is a way to check. A much heavier liquid should stop at a much lower height. Mercury is about fourteen times heavier than water, so it should stop at roughly a fourteenth of ten meters.
What was found
He filled a glass tube with mercury, stopped it, and stood it upside down in a dish of mercury. The mercury sank a little and stopped — at about seventy-six centimeters. Exactly the predicted height.
Above it, in the sealed tube, sat an empty space. A vacuum. The thing that could not exist had been made inside a glass tube.
Watched over several days, the seventy-six centimeters varied slightly, meaning the weight of the air pressing down changes. The barometer was born. He wrote in a letter that we live at the bottom of a sea of air.
Nature abhors a vacuum, and so the water follows
A mercury column halting at exactly the predicted height, and the emptiness above it
The device soon became an instrument for predicting weather