Torricelli's barometer
Torricelli's apparatus is extremely simple — just a glass tube closed at one end and filled with mercury.
How the apparatus is built
A glass tube about a metre long and closed at one end is filled with mercury, then held vertically upside down in a vessel of mercury, taking care that no air gets inside it.
Figure to be added
The observation
Some of the mercury in the tube falls, leaving an empty space at the top of it. The mercury column remaining in the tube is about 76 cm high.
Why all the mercury does not run out
Torricelli understood that all the mercury in the tube does not flow into the vessel because atmospheric pressure is pressing down on the exposed mercury surface outside the tube.
But atmospheric pressure is only sufficient to maintain a mercury column about 76 cm high. That is why the mercury column inside the tube falls to 76 cm and stops.
The space above it is a vacuum
Air cannot get into the empty space left above the mercury column — that end of the tube is closed, and the mercury did not fall by letting any air in. So that space must be a vacuum.
What this gives us
The height of the mercury column is a measure of atmospheric pressure. So this instrument, built to measure atmospheric pressure using mercury, is called a mercury barometer.