"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 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?"