becomes weaker, and then the air-atoms spring farther apart, and
the air becomes thinner. Suppose that the lines in this diagram
represent layers of air. Near the earth we have to represent
them as lying closely together, but as they recede from the earth
they are also farther apart.

But the chief reason why the air is thicker or denser nearer the
earth, is because the upper layers press it down. If you have a
heap of papers lying one on the top of the other, you know that
those at the bottom of the heap will be more closely pressed
together than those above, and just the same is the case with the
atoms of the air. Only there is this difference, if the papers
have lain for some time, when you take the top ones off, the
under ones remain close together. But it is not so with the air,
because air is elastic, and the atoms are always trying to fly
apart, so that directly you take away the pressure they spring up
again as far as they can.



Week 8

I have here an ordinary pop-gun. If I push the cork in very
tight, and then force the piston slowly inwards, I can compress
the air a good deal. Now I am forcing the atoms nearer and
nearer together, but at last they rebel so strongly against being
more crowded that the cork cannot resist their pressure. Out it
flies, and the atoms spread themselves out comfortably again in
the air all around them. Now, just as I pressed the air together
in the pop-gun, so the atmosphere high up above the earth presses