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?"