It has been condensed so much that 100 cubic feet of air was reduced to the bulk of only one cubic foot.
What makes the air so dense as it is at the surface of the earth?
The pressure of the air above the surface.
Does the air then become less dense the higher we ascend?
Yes; because there is less air above to press upon it.
Does the air become more dense the deeper we go in the earth?
It does; because there is more air above to press upon it.
How can we understand this?
In a large heap of wool, the hairs of the wool at the bottom of the heap are pressed closely together, but they are farther and farther apart the nearer they are to the top of the heap.
How high does the air extend above the earth?