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, yet you will see,
when we exhaust it by-and-by, no power of any two of you will be able to
pull them apart. Every square inch of surface that is contained in the
area of that vessel sustains fifteen pounds by weight, or nearly so, when
the air is taken out; and you may try your strength presently in seeing
whether you can overcome that pressure of the atmosphere.
Here is another very pretty thing the boys sucker, only refined by the
philosopher. We young ones have a perfect right to take toys, and make
them into philosophy, inasmuch as now-a-days we are turning philosophy
into toys. Here is a sucker, only it is made of india-rubber: if I clap it
upon the table, you see at once it holds. Why does it hold? I can slip it
about, and yet if I try to pull it up, it seems as if it would pull the
table with it I can easily make it slip about from place to place; but
only when I bring it to the edge of the table can I get it off. It is only
kept down by the pressure of the atmosphere above. We have a couple of
them; and if you take these two and press them together, you will see how
firmly they stick. And, indeed, we may use them as they are proposed to be
used, to stick against windows, or against walls, where they will adhere
for an evening, and serve to hang anything on that you want. I think,
however, that you boys ought to be shewn experiments that you can make at
home; and so here is a very pretty experiment in illustration of the
pressure of the atmosphere. Here is a tumbler of water. Suppose I were to
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