“Well, it proved the accuracy of your aim, at any rate,” observed Lieutenant Marbury.  “The bomb dropping device of your aerial warship is perfect—­I can testify to that.”

“And I’ll have the guns fixed soon, so there will be no danger of a recoil, too,” added Tom Swift, with a determined look on his face.

“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, which happened to be when we took our first flight, the acid ate through whatever it was contained in, and then attacked the wood of the propeller blade.  It weakened the wood so that the force used in whirling it around broke it.”