air
pressure outside holds the suction cap tightly to the object against
which you first pressed it, making it stick there.

We can easily experiment with air pressure because we can get almost
entirely rid of it in places and can then watch what happens. A place
from which the air is practically all pumped out is called a _vacuum_.
Here are some interesting experiments that will show what air pressure
does:

[Illustration: FIG. 4. When the point is knocked off the electric
lamp, the water is forced into the vacuum.]

EXPERIMENT 2. Hold a burned-out electric lamp in a basin of
water, break its point off, and see what happens.

All the common electric lamps (less than 70 watts) are made with
vacuums inside. The reason for this is that the fine wire would burn
up if there were any air in the lamps. When you knock the point off
the globe, it leaves a space into which the water can be pushed. Since
the air is pressing hard on the surface of the water except in the
one place where the vacuum in the lamp globe is, the water is forced
violently into this empty space.

It really is a good deal like the way mud comes up between your toes
when you are barefoot. Your foot is pressing on the mud all around
except in the spaces between your toes, and so the mud is forced up
into these spaces. The air pressure on the water is like your foot
on the mud, and the space in the lamp globe is like the space between
your toes. Since wherever there is air it is pressing hard, the only
space into which it can force water or anything else is into a place
from which all the air has been removed, like the inside of the lamp
globe.

The reason that the water does not run out of the globe is this:
the hole is too small to let the air squeeze up past the water, and
therefore no air can take the place of the water that might otherwise
run out. In order to flow out, then, the water would have to leave an
empty space or vacuum behind it, and the air pressure would not allow
this.

WHY WATER GURGLES WHEN IT POURS OUT OF A BOTTLE. You have often
noticed that when you pour water out of a bottle it gurgles and gulps
instead of flowing out evenly. The reason for this is that when
a little water gets out and leaves an empty space behind, the air
pushing against the water starts to force it back up; but since the
mouth of the bottle is fairly wide, the air itself squeezes past the
water and bubbles up to the top.

EXPERIMENT 3. Put a