"Yes. This ball is hollow, and was filled with a chemical. It was
dropped on the roof, and, after a certain time, the plug in the tube was
eaten through, the chemicals ran out, set the roof ablaze, and, dripping
down inside spread the choking odors that nearly prevented you from
getting out your aeroplane."

"Are you sure of this?" asked the young inventor.

"Positive. I read about these bombs recently. A German invented them to
be used in attacking a besieged city in case of war."

"But how did this one get on my shed roof?" asked Tom.

"It was dropped there by the mysterious airship!" exclaimed the odd man.
"That was why the aeroplane moved about over your place. Those in it
hoped that the fire would not break out until you were all asleep, and
that the shed and the Humming-Bird would be destroyed before you came to
the rescue. Some of your enemies are still after you, Tom."

"And it was Andy Foger, I'll wager!" he cried. "He was in that aircraft!
Oh, I'll have a long score to settle with him!"

"Of course you can't be sure it was he," said Mr. Damon, "but I wouldn't
be a bit surprised but what it was. Andy is capable of such a thing. He
wanted to prevent you from taking part in the race."

"Well, he sha'n't!" cried Tom, and then he thought of his invalid
father. They made a further examination of the shed, and discovered
another empty bomb. Then Tom recalled having seen something drop from
the mysterious aeroplane as it passed over the shed.