be of the pusher type, and will revolve in
dense, compressed air, almost like water, and that will do away with
high speed motors, with all their complications, and make traveling in
the clouds as simple as taking out a little one-cylinder motor boat.
How's that, Tom Swift? How's that for an idea?"

To Mr. Damon's disappointment, Tom was not enthusiastic. The young
inventor gazed at his eccentric friend, and then said slowly:

"Well, that's all right in theory, but how is it going to work out in
practice?"

"That's what I came to see you about, Tom," was the reply. "Bless my
tall hat! but that's just why I hurried over here. I wanted to tell you
when I saw you going off on a trip with Miss Nestor. That's my big
idea--Damon's Whizzer--propellers revolving in compressed air like
water. Isn't that great?"

"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 liquid air,
and I could shatter the steel with a hammer as easily as a sheet of
ice. The cold of liquid air is beyond belief.

"Attempts have been made to make motors run with liquid air, but t