"I'm sorry to shatter your air castle," said Tom; "but for the
life of me I can't see how it will work. Of course, in theory, if
you could revolve a big-bladed propeller in very dense, or in
liquid, air, there would be more resistance than in the rarefied
atmosphere of the upper regions. And, if this could be done, I
grant you that you could use slower motors and smaller propeller
blades--more like those of a motor boat. But how are you going to
get the condensed air?"

"Make it!" said Mr. Damon promptly. "Air pumps are cheap. Just
carry one or two on board the aeroplane, and condense the air as
you go along. That's a small detail that can easily be worked
out. I leave that to you."

"I'd rather you wouldn't," said Tom. "That's the whole
difficulty--compressing your air. Wait! I'll explain it to you."

Then the young inventor went into details. He told of the
ponderous machinery needed to condense air to a form
approximating water, and spoke of the terrible pressure exerted
by the liquid atmosphere.

"Anything that you would gain by having a slow-speed motor and
smaller propeller blades, would be lost by the ponderous air-
condensing machinery you would need," Tom told Mr. Damon.
"Besides, if you could surround your propellers with a strata of
condensed air, it would create such terrible cold as to freeze
the propeller blades and make them as brittle as glass.

"Why, I have taken a heavy piece of metal, dipped it into