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,
which I think will explain to you more about it. When the air is pumped
from underneath the bladder which is stretched over this glass, you will
see the effect in another shape: the top is quite flat at present, but I
will make a very little motion with the pump, and now look at it--see how
it has gone down, see how it is bent in. You will see the bladder go in
more and more, until at last I expect it will be driven in and broken by
the force of the atmosphere pressing upon it.

[Illustration: Fig. 28.]

[The bladder at last broke with a loud report.] Now, that was done
entirely by the weight of the air pressing on it, and you can easily
understand how that is. The particles that are piled up in the atmosphere
stand upon each other, as these five cubes do. You can easily conceive
that four of these five cubes are resting upon the bottom one, and if I
take that away, the others will all sink down. So it is with the
atmosphere: the air that is above is sustained by the air that is beneath;
and when the air is pumped away from beneath them, the change occurs which
you saw when I placed my hand on the air-pump, and which you saw in the
case of the bladder, and which you shall see better here. I have tied over
this jar a piece of sheet india-rubber, and I am now about to take away
the air from the inside of the jar; and if you will watch the
india-rubber--which acts as a partition between the air below and the air
above--you will see, when I pump, how the pressure shews itself. See where
it is going to--I can actually put my hand into the jar; and yet this
result is only caused by the great and powerful action of the air above.
How beautifully it shews this curious circumstance!

Here is something that you can have a pull at, when I have finished
to-day. It is a little apparatus of two hollow brass hemispheres, closely
fitted together, and having connected with it a pipe and a cock, through
which we can exhaust the air from the inside; and although the two halves
are so easily taken apart, while the air is left within