it were not that there are gases and fluids inside our bodies
which press outwards and balance the weight so that we do not
feel it at all.

This is why Mr. Glaisher's veins swelled and he grew giddy in
thin air. The gases and fluids inside his body were pressing
outwards as much as when he was below, but the air outside did
not press so heavily, and so all the natural condition of his
body was disturbed.

I hope we now realize how heavily the air presses down upon our
earth, but it is equally necessary to understand how, being
elastic, it also presses upwards; and we can prove this by a
simple experiment. I fill this tumbler with water, and keeping a
piece of card firmly pressed against it, I turn the whole upside-
down. When I now take my hand away you would naturally expect
the card to fall, and the water to be spilt. But no! the card
remains as if glued to the tumbler, kept there entirely by the
air pressing upwards against it. (The engraver has drawn the
tumbler only half full of water. The experiment will succeed
quite as well in this way if the tumbler be turned over quickly,
so that part of the air escapes between the tumbler and the card,
and therefore the space above the water is occupied by air less
dense than that outside.)

And now we are almost prepared to understand how we can weigh the
invisible air. One more experiment first. I have here what is
called a U tube, because it is shaped like a large U. I pour
some water in it till it is about half full, and you will notice
that the water stands at the same height in both arms of the