over it, and pump the air out of the jar. What makes the
balloon expand? What is in it? Why could it not expand before
you pumped the air out from around it?

A toy balloon expands for the same reason when it goes high in the
air. Up there the air pressure is not so strong outside the balloon,
and so the gas inside makes the balloon expand until it bursts.

[Illustration: FIG. 8. A siphon. The air pushes the water over the
side of the pan.]

EXPERIMENT 9. Lay a rubber tube flat in the bottom of a pan of
water, so that the tube will be filled with water. Let one end
stay under water, but pinch the other end tightly shut with
your thumb and finger and lift it out of the pan. Lower this
closed end into a sink or empty pan that is lower than the
pan of water. Now stop pinching the tube shut. This device is
called a _siphon_ (Fig. 8).

EXPERIMENT 10. Put the mouth of a small syringe, or better, of
a glass model lift pump, under water. Draw the handle up. Does
the water follow the plunger up, stand still, or go down in
the pump?

When you pull up the plunger, you leave an empty space; you shove the
air out of the pump or syringe ahead of the plunger. The air outside,
pressing on the water, forces it up into this empty space from which
the air has been pushed. But air pressure cannot force water up even
into a perfect vacuum farther than about 33 feet. If your glass pump
were, say, 40 feet long, the water would follow the plunger up for a
little over 30 feet, but nothing could suck it higher; for by the time
it reaches that height it is pushing down with its own weight as hard
as the air is pressing on the water below. No suction pump, or siphon,
however perfect, will ever lift water more than about 33 feet, and
it will do well if it draws water up 28 or 30 feet. This is because a
perfect vacuum cannot be made. There is always some water vapor formed
by the water evaporating a little, and there is always a small amount
of air that has been dissolved in water, both of which partly fill the
space above the water and press down a little on the water within the
pump.

[Illustration: FIG. 9. A glass model suction pump.]

If you had a straw over 33 feet long, and if some one held a glass of
lemonade for you down near the sidewalk while you leaned over from
the roof of a three-story building with your long straw, you could
not possibly drink the lemonade. The air pressure w