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
ask you to turn that tumbler upside-down, so that the water should not
fall out, and yet not be kept in by your hand, but merely by using the
pressure of the atmosphere. Could you do that? Take a wine-glass, either
quite full or half-full of water, and put a flat card on the top, turn it
upside-down, and then see what becomes of the card and of the water. The