"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."
"Are you sure of that?" asked Ned.
"As sure as I am that I'm here! Now I know what caused the accident!"
"But who would play such a trick?" asked Ned. "We might all have been killed."
"Yes, I know we might," said Tom. "It must be the work of some of those foreign spies whose first plot we nipped in the bud. I must tell Marbury of this, but don't mention it to dad."