Does air push or pull? – Torricelli’s Experiment

If you have learned to dive, you know that the water exerts’ pressure against your body. You know also that this pressure becomes greater, the deeper you dive. But nobody knew until the seventeenth century that the air exerts pressure against objects which it surrounds, in the same way that water does. This fact was learned as a result of digging a well.

In 1640 the duke of Tuscany dug a deep well and equipped it with a pump. To his disappointment, the pump would not lift the water to the top of the well, as had been expected. Nobody could understand why it would riot, because pumps had long been used for lifting water from wells. The pumps had been used, however, in wells in which the water was not many feet below the surface of the ground. In fact, in every case the water had been less than thirty feet from the top of the well.

The duke took his perplexing problem to the famous Italian scientist Galileo. Galileo was then seventy-six years old. Also, he was in such poor health that he could not undertake to find out why water could not be pumped from the duke’s well. He asked his pupil Torricelli to see if he could find out.

Torricelli’s experiment. Torricelli knew that the reason pumps had operated satisfactorily before was that the air pressure had pushed the water up in them. He thought the reason the duke’s pump had not been successful was that the air pressure was not great enough to push the water all the way up from such a deep well. He decided to find out how great the pressure of the atmosphere really is.

Since it would be difficult to experiment Vacuum with a column of water 30 feet high, Torricelli decided to use mercury. He reasoned that if the atmospheric pressure were able to hold up a column of water, it should also be able to hold up a column of mercury or any other liquid. He knew that, volume for volume, mercury is 13.6 times as heavy as water. He therefore decided that the atmospheric pressure ought to support a column of mercury about one thirteenth of 30 feet, or about 22 feet. To test this hypothesis) he performed an important experiment.

This experiment shows that the air pressure is great enough to hold up a column of mercury about 30 inches high.

Having performed this experiment, Torricelli had the answer to his problem. He knew that the atmospheric pressure would hold ‘up a column of water about 34 feet high, but no higher than that. Hence he concluded that the duke’s pump was not successful because the water in the well was more than 34 feet below the surface of the ground.