Pressure (cont'd)

We have pressure on us at all times (not as much as the nail on the table in the example, of course). The pressure is caused by the atmosphere and it is much stronger than you may think. The average pressure of the atmosphere at sea level is about 14.7 pounds per square inch. This means that every square inch of your body is receiving around 14.7 pounds of pressure.

In the coming lessons we will discuss high pressure and low pressure. These concepts will refer to atmospheric pressure often. Atmospheric pressure is measured with a barometer and is sometimes referred to as barometric pressure.

Barometric pressure can be measured in several ways. The most common way is with a mercury barometer. As the pressure of the atmosphere pushes down on the liquid mercury at the bottom, the mercury inside the barometer rises. The height of the column of Mercury inside the barometer is measured to find the pressure. The measurement is then read in "inches of mercury".

The atmosphere is composed of many small particles. These particles are so small that we can not even see them, however they have mass and size. The combination of all of these particles composes the gas we call air.