experiment will by no means convey to your mind the whole literal truth of
this matter. It is wonderful how it accumulates when you come to larger
volumes. This bulk of air [a cubic foot] weighs 1-1/5 ounce. What do you
think of the contents of that box above there, which I have had made for
the purpose? The air which is within that box weighs one pound--a full
pound; and I have calculated the weight of the air in this room,--you
would hardly imagine it, but it is above a ton. So rapidly do the weights
rise up, and so important is the presence of the atmosphere, and of the
oxygen and the nitrogen in it, and the use it performs in conveying things
to and fro from place to place, and carrying bad vapours to places where
they will do good instead of harm.

Having given you that little illustration with respect to the weight of
the air, let me shew you certain consequences of it. You have a right to
them, because you would not understand so much without it. Do you remember
this kind of experiment? Have you ever seen it? Suppose I take a pump
somewhat similar to the one I had a little while ago to force air into the
bottle, and suppose I place it in such a manner that by certain
arrangements I can apply my hand to it: my hand moves about in the air so
easily that it seems to feel nothing, and I can hardly get velocity enough
by any motion of my own in the atmosphere to make sure that there is much
resistance to it.

[Illustration: Fig. 27.]

But, when I put my hand here [on the air-pump receiver, which was
afterwards exhausted], you see what happens. Why is my hand fastened to
this place, and why am I able to pull this pump about? And see! how is it
that I can hardly get my hand away? Why is this? It is the weight of the
air--the weight of the air that is above. I have another experiment here,