er than it was. By what? By
the air that we have forced into it by the pump. There is not a greater
_bulk_ of air, but there is the same bulk of _heavier_ air, because we
have forced in air upon it. And that you may have a fair notion in your
mind as to how much this air measures, here is a jar full of water. We
will open that copper vessel into this jar, and let the air return to its
former state. All I have to do now is to screw them tightly together, and
to turn the taps, when there, you see, is the bulk of the twenty pumps of
air which I forced into the bottle; and to make sure that we have been
quite correct in what we have been doing, we will take the bottle again to
the balance, and, if it is now counterpoised by the original weight, we
shall be quite sure we have made our experiment correctly.

[Illustration: Fig. 26.]

It is balanced; so, you see, we can find out the weight of the extra
volumes of air forced in, in that way, and by that means we are able to
ascertain that a cubic foot of air weighs 1-1/5 ounce. But that small
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