"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.
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