, although the
veteran airman, John says, can tell a good deal about what to expect
from the nature of the earth or clouds below him."

"That's true," averred John. "The closer you are to the earth the more
you will feel the 'bumps,' as we call them. They are a whole lot like
the waves of the ocean, only invisible, and there will be one straight
over every protuberance or depression of size in the surface of the
earth. Mountains, hills, houses, lakes, valleys, rivers, forests, all
cause bumps or holes in the air up above them. At one thousand feet
they are pretty bad. At ten thousand feet they are scarcely
noticeable. That's why most pilots prefer to fly high whenever they
can."

"What causes the air to act in this way over such configurations?"
propounded the publisher.

John looked helpless, and smiled. "You've got me there," he admitted.
"I haven't had the opportunity to study aerostatics the same as Paul
here. He can probably tell us."

"I'm not through my course yet," reminded his brother, "but I think I
can answer that. The air surrounding the earth is a great belt forty
or more miles through and is of an even thickness. As our globe sweeps
through it, the lower stratum of air naturally sinks down into the
valleys and like depressions. This action pulls down the upper
stretches of air, thus creating what are termed 'air-pockets' or
'air-holes.' Very dangerous they are, too."

"That is plain enough," declared Bob. "Now, dad, let Paul go on
explaining this 'automatic pilot.'"

"If the aviator is enshrouded in fog or tries to sail through a heavy
bank of clouds, he is quite likely to lose all sense of direction,"
continued Paul. "He will not know whether he is banking or traveling
on an even keel. Sometimes pilots have come out of a low cloud to find
themselves dangerously close to the earth and in an awkward position,
perhaps in a steep bank, a side-slip, or even in the terrifying
nose-dive, and they have not had time to right themselves before
crashing to earth. So you see that before flying can become reasonably
safe, some way must be found of keeping the machine automatically on a
level keel.

"To operate this stabilizer of ours all the pilot will have to do is to
guide the rudder with his feet. The automatic pilot works the elevator
and the ailerons. It takes care of 'bumps' and 'holes' and sees that
the machine banks at just the right angle on the turns. This makes the
operation of