ancisco. He had a large oil tank and wanted a simple way
of telling at a glance how full it was. One of his workmen
suggested that he attach a long piece of glass tubing to the
side of the tank, connecting it with an extra faucet near the
bottom of the tank. A second workman said, "No, that won't do.
Your tank holds ever so much more than the tube would hold, so
the oil in the tank would force the oil up over the top of the
tube, even when the tank was not full." Who was right?

[Illustration: FIG. 3. When the tank is full, will the oil overflow
the top of the tube?]

SECTION 3. _The sea of compressed air in which we live: Air pressure._

Does a balloon explode if it goes high in the air?

What is suction?

Why does soda water run up a straw when you draw on the straw?

Why will evaporated milk not flow freely out of a can in which
there is only one hole?

Why does water gurgle when you pour it out of a bottle?

We are living in a sea of compressed air. Every square inch of our
bodies has about 15 pounds of pressure against it. The only reason
we are not crushed is that there is as strong pressure inside of our
bodies pushing out as there is outside pushing in. There is compressed
air in the blood and all through the body. If you were to lie down
on the ground and have all the air pumped out from under you, the air
above would crush you as flat as a pancake. You might as well let a
dozen big farm horses trample on you, or let a huge elephant roll over
you, as let the air press down on you if there were no air underneath
and inside your body to resist the pressure from above. It is hard to
believe that the air and liquids in our bodies are pressing out with
a force great enough to resist this crushing weight of air. But if you
were suddenly to go up above the earth's atmosphere, or if you were to
stay down here and go into a room from which the air were to be pumped
all at once, your body would explode like a torpedo.

When you suck the air out of a bottle, the surrounding air pressure
forces the bottle against your tongue; if the bottle is a small one,
it will stick there. And the pressure of the air and blood in your
tongue will force your tongue down into the neck of the bottle from
which part of the air has been taken.

In the same way, when you force the air out of a rubber suction cap,
such as is used to fasten reading lamps to the head of a bed, the