t speed stops it
begins to fall, or volplane, as we call it."

"Exactly. Now, instead of having to depend on the speed of the
aeroplane for this, why not depend on the speed of the propeller--in
other words, the whizzer?"

"Well, we do," said Tom, a bit puzzled as to what his friend was trying
to get at. "If the propeller didn't move the airship wouldn't
rise--that is, unless it's of the balloon type."

"What I mean," said Mr. Damon, "is to have an aeroplane that will move
in the air the same as a boat moves in the water. You don't have to get
the propeller of a boat racing around at the rate of a million
revolutions a minute, more or less, before your boat will travel, do
you? If the engine turns the screw, or propeller, just over say fifty
times a minute you would get some motion of the boat, wouldn't you?"

"Why, yes, some," admitted Tom.

"And what causes it?" asked Mr. Damon, anticipating a triumph.

"The resistance of the water to the blades of the screw, or propeller,"
answered Tom.

"Exactly! And it's the resistance of the air to the blades of an
airship propeller that sends the craft along, isn't it?"

"Yes. And because of the difference in density between air and water it
becomes necessary to revolve an aeroplane propeller many times faster
than a boat propeller. It's the density that makes the difference, Mr.
Damon. If air were as dense as water we could have comparatively
slow-moving motors and propellers and--"

"Ha! There you have it, Tom! And there is where my Whizzer--Wakefield
Damon's Whizzer--is going to revolutionize air travel!" cried the
eccentric man. "The difference in density! If air were as dense as
water the problem would be solved. And I have solved it! I'm going to
turn the trick, Tom! One more question. How can air be made as dense as
water, Tom Swift?"

"Why, by condensation or compression, I suppose," was the rather slow
answer. "You know they have condensed, or compressed, air until it is
liquid. I've done it myself, as an experiment."

"That's it, Tom! That's it!" cried Mr. Damon in delight. "Compressed
air will do the trick! Not compressed to a liquid, exactly, but almost
so. I'm going to revolve the propellers of my new airship in compressed
air, so dense that they will not have to have a speed of more than
seven hundred revolutions a minute. What's that compared to the three
to ten thousand revolutions of the propellers now used? The propellers
of Damon's Whizzer will