Curious was the sight that met the gaze of Tom Swift and Mr. Wakefield Damon as they rounded the corner of the house and looked into the newly spaded garden. There stood the giant, Koku, holding aloft in the air, by one hand, the form of the struggling colored man, Eradicate Sampson. And Eradicate was vainly trying to get at his enemy and rival, but was prevented by the long- distance hold the giant had on him.

"Yo' let me go, now! Yo' let me go, big man cried Eradicate. "Ef yo' don't I'll bust yo' wide open, dat's whut I'll do! An' 'sides, I'll tell Massa Tom on yo', dat's whut I'll do!"

"Ho! You tell--I let you fall!" threatened Koku.

His threat was dire enough, for such was his size and strength that he held the colored man nearly nine feet from the ground, and a fall from that distance would seriously jar Eradicate, if it did nothing else. The colored man's eyes opened wide as he heard what Koku said, and then he cried:

"Let me down! Let me down, an' I won't say nuffin!"

"An' you let me scatter dirt?" asked Koku. for such was the giant's idea of working in the garden.

"Yes, yo' kin scatter de dirt seben ways from Sunday fo' all I keers!" conceded Eradicate. Then, as he was lowered to the ground, he and the giant turned and saw Mr. Damon and Tom approaching.

"What's wrong?" asked the young inventor.

"'Scuse me, Massa Tom," began Eradicate, "but didn't yo' tell me to spade de garden?"

"I guess I did," admitted Tom Swift.

"An' you tell me help--yes?" questioned Koku.

"Well, I thought it would be a little too much for you, Rad," said Tom, gently. "I thought perhaps you'd like help."

"Hu! Not him, anyhow!" declared the colored man in great disgust. "When I git so old dat I cain't spade a garden, den me an' Boomerang, we-all gwine to die, dat's all I got to say. I was a-spadin' my part ob de garden, Massa Tom, same laik Mr. Damon done tole me to, an' dish yeah big mess ob bones steps on my side ob de middle an--"

"Him too slow. Koku scatter dirt twice times so fast!" declared the giant, whose English was not much better than Eradicate's.

"Yes, I see," said Tom. "You are so strong, Koku, that you finished your part before Eradicate did. Well, it was good of you to want to help him."

At this the giant grinned at his rival.

"At the same time," went on Tom, winking an eye at Mr. Damon, "Eradicate knows a little more about garden work, on account of having done it so many years."

"Ha! Whut I tell yo', Giant!" boasted the colored man. It was his turn to smile.

"And so," went on Tom, judicially, "I guess I'll let Rad finish spading the garden, and you, Koku, can come and help me lift some heavy engine parts. Mr. Damon wants to explain something to me."

"Ha! Nothing what so heavy Koku not lift!" boasted the giant.

"Go on! Lift yo'se'f 'way from heah!" muttered Eradicate as he picked up his dropped spade. And then, with a smile of satisfaction, he fell to work in the mellow soil while Tom led Koku to one of the shops where he set him to lifting heavy motor parts about in order to get at a certain machine that was stored away in the back of one of the rooms.

"That will keep him busy," said the young inventor. "And now, Mr. Damon, I can listen to you. Do you really think you have a new idea in airships?"

"I really think so, Tom. My Whizzer is bound to revolutionize travel in the air. Let me tell you what I mean. Now cast your mind back. How many ways are now used to propel an airship or a dirigible balloon through the air? How many ways?"

"Two, as far as I know," said Tom. "At least there are only two that have proved to be practical."

"Exactly," said Mr. Damon. "One with the propeller, or propellers, in front, and that is the tractor type. The other has the propeller in the rear, and that is the pusher type. Both good as far as they go, but I have something better."

"What?" asked Tom with a smile.

"It's a Whizzer," said the eccentric man. "Bless my gold tooth! but that is the best name I can think of for it. And, really, the propeller I'm thinking of inventing does whiz around."

"But are you going to use a tractor or pusher type?" Tom wanted to know.

"It's a combination of both," answered Mr. Damon. "As it is now, Tom, you have to get an aeroplane in pretty speedy motion before it will rise from the ground, don't you?"

"Yes, of course. That's the principle on which an aeroplane rises and keeps aloft, by its speed in the air. As soon as that 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 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 they have not succeeded. To condense air and to carry it about so that propellers might revolve in it, would be out of the question."

"You think so, Tom?" asked Mr. Damon.

"I'm sure of it!"

"Oh, dear! That's too bad. Bless my overshoes, but I thought I had a new idea. Well, you ought to know. So Damon's Whizzer goes on the scrap heap before ever it's built. Well, we'll say no more about it. You ought to know best, Tom. I wasn't thinking of it so much for myself as for you. I thought you'd like some new idea to work on."

"Much obliged, Mr. Damon, but I have a new idea," said Tom.

"You have? What is it? Tell me--that is, if it isn't a secret," went on the eccentric man, as much delighted over Tom's new plan as he had been over his own Whizzer, doomed to failure so soon.

"It isn't a secret from you," said Tom. "I got the idea while I was riding with Mary. I wanted to talk to her--to tell her not to jump out when we had a little accident--but I had trouble making myself understood because of the noise of the motor."

"They do make a great racket," conceded Mr. Damon. "But I don't suppose anything can be done about it."

"I don't see why there can't!" exclaimed Tom. "And that's my new idea--to make a silent aircraft motor--perhaps silent propeller blades, though it's the motor that makes the most noise. And that's what I'm going to do--invent a silent aeroplane. Not because I want so much to talk when I take passengers up in the air, but I believe such a motor would be valuable, especially for scouting planes in war work. To go over the enemy's lines and not be heard would be valuable many times.

"And that's what I'm going to do--work on a silent motor for Uncle Sam. I've got the germ of an idea and now--"

"Excuse me," said a voice behind Mr. Damon and Tom, and, turning, the young inventor beheld the form of Mr. Peton Gale, president of the Universal Flying Machine Company.