The Italian scientist Evangelista Torricelli (1608-1647) measured the pressure exerted by the atmosphere at sea level.

Let's see how he did it.

Sketch of the scientist Evangelista Torricelli.

Torricelli was working for Galileo Galilei when Galileo told him about an old problem.

Pumps couldn't extract water from wells if water was deeper than 10 metres. This problem had been known since the time of the ancient Greeks. But nobody really knew why.

Galileo believed it was because of the weight of the column of water. 10 metres of water weighed too much, he thought.

Now, Torricelli had an idea...

Torricelli thought of an experiment to try and find out why water couldn't be elevated higher than 10 metres.

For his experiment, he used mercury. Since it was 13 times more dense than water, he thought he could obtain results using a shorter column of 1 metre.

Torricelli used a tube with one end open and the other one closed. He filled it with mercury. He also filled a plate with mercury and held the tube so that the open side of it was immersed in the mercury in the bucket.

Whatever the length of the tube, the mercury inside it only reached a height of 76 cm above the level of the liquid in the plate.

Torricelli concluded that the pressure exerted by the atmosphere is the same as that exerted by a column of mercury 76 cm high (or 760 mm). He called this amount of pressure 1 atmosphere (1 atm).