## How great is atmospheric pressure?

Many later experiments by scientists have proved that at sea level the mercury always stands about 30 inches high in a barometer tube such as Torricelli used. It has been found, also, that a column of mercury 30 inches high and 1 inch square weighs 14.7 pounds.

Therefore the atmospheric pressure at sea level is about 15 pounds per square inch. Thus we say that a pressure of 15 pounds per square inch is 1 atmosphere of pressure. Air pressures are usually expressed in atmospheres.

## 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.

## What is altitude and pressure?

Air pressure varies at different altitudes, or heights above sea level. Experiments by Torricelli and other scientists have proved that the height of the column of mercury in a barometer becomes less if the barometer is carried up a mountain or toward the top of a high building.

At three miles above sea level the barometer column is somewhat less than 15 inches high.