It may be used to tell the height or depth of any place.
Does the air become lighter as we ascend?
The air does become lighter as we ascend.
How does this affect the barometer?
As the air becomes lighter, the quicksilver in the barometer sinks lower.
At what height does the quicksilver stand at the level of the sea?
It stands at the height of thirty inches.
When we ascend a mountain 10,000 feet high, how much does the quicksilver sink?
It sinks ten inches, and, therefore, stands at the height of only twenty inches.
Does the same thing take place when we ascend in a balloon?