them, and at that moment they must have struck against the
atmosphere, and even travelled some way through it, to become
white-hot. In this way we have learnt that meteors burst into
light at least 100 miles above the surface of the earth, and so
the atmosphere must be more than 100 miles high.

Our next question is as to the weight of our aerial ocean. You
will easily understand that all this air weighing down upon the
earth must be very heavy, even though it grows lighter as it
ascends. The atmosphere does, in fact, weigh down upon land at
the level of the sea as much as if a 15-pound weight were put
upon every square inch of land. This little piece of linen
paper, which I am holding up, measures exactly a square inch, and
as it lies on the table, it is bearing a weight of 15 lbs. on its
surface. But how, then, comes it that I can lift it so easily?
Why am I not conscious of the weight?

To understand this you must give all your attention, for it is
important and at first not very easy to grasp. you must
remember, in the first place, that the air is heavy because it is
attracted to the earth, and in the second place, that since air
is elastic all the atoms of it are pushing upwards against this
gravitation. And so, at any point in air, as for instance the
place where the paper now is as I hold it up, I feel no pressure
because exactly as much as gravitation is pulling the air down,
so much elasticity is resisting and pushing it up. So the
pressure is equal upwards, downwards, and on all sides, and I can
move the paper with equal ease any way.

Even if I lay the paper on the table this is still true, because