Two ways a gas presses
Just as solids and liquids do, gases too give rise to pressure. But there are two different ways a gas can produce one.
1 — from the weight of a column
In just the way that the weight of a liquid column produces a pressure, so does the weight of a column of gas. Atmospheric pressure arises in this way.
2 — from compression
A compressed gas also produces a pressure, through its attempt to expand. The pressure in a balloon or a tyre is of this kind.
What is the difference?
The first kind depends on weight — and therefore on depth as well. The second does not depend on weight at all, but on how much gas has been packed into a limited volume.
Either way the result is the same
However it arises, what you get is a pressure — and it behaves exactly like a liquid pressure. That is why a gas pressure can be measured with a column of liquid.
On the next page we confirm with a simple activity that a compressed gas does produce a pressure.