"What's next?" asked Mr. Damon, looking at his watch. "I really ought
to be home, Tom."

"We're going back now, and down. Are you sure you don't want me to drop
you in your own front yard, or even on your roof? I think I could
manage that."

"Bless my stovepipe, no, Tom! My wife would have hysterics. Just land
me at Shopton and I'll take a car home."

The damaged airship seemed little the worse for the test to which she
had been subjected, and made her way at good speed in the direction of
Tom's home. Several little experiments were tried on the way back. They
all worked well, and the only two problems Tom had to solve were the
taking care of the recoil from the guns and finding out why the
propeller had broken.

A safe landing was made, and the Mars once more put away in her hangar.
Mr. Damon departed for his home, and Lieutenant Marbury again took up
his residence in the Swift household.

"Well, Tom, how did it go?" asked his father.

"Not so very well. Too much recoil from the guns."

"I was afraid so. You had better drop this line of work, and go at
something else."

"No, Dad!" Tom cried. "I'm going to make this work. I never had
anything stump me yet, and I'm not going to begin now!"

"Well, that's a good spirit to show," said the aged inventor, with a
shake of his head, "but I don't believe you'll succeed, Tom."

"Yes I will, Dad! You just wait."

Tom decided to begin on the problem of the propeller first, as that
seemed more simple. He knew that the gun question would take longer.

"Just what are you trying to find out, Tom?" asked Ned, a few nights
later, when he found his chum looking at the broken parts of the
propeller.

"Trying to discover what made this blade break up and splinter that
way. It couldn't have been centrifugal force, for it wasn't strong
enough."

Tom was "poking" away amid splinters, and bits of broken wood, when he
suddenly uttered an exclamation, and held up something. "Look!" he
cried. "I believe I've found it."

"What?" asked Ned.

"The thing that weakened the propeller. Look at this, and smell!" He
held out a piece of wood toward Ned. The bank employee saw where a
half-round hole had been bored in what remained of the blade, and from
that hole came a peculiar odor.

"It's some kind of acid," ventured Ned.

"That's it!" cried Tom. "Someone bored a hole in the propeller, and put
in some sort of receptacle, or capsule, containing a corrosive acid. In
due time, whi