General Waller was much interested in going about Tom's shop, and
expressed his surprise at the many inventions he saw. While ordnance
matters, big guns and high explosives were his hobby, nevertheless the
airships were a source of wonder to him.

"How do you do it, Tom?" he asked.

"Oh, by keeping at it," was the modest answer. "Then my good friends
here--Ned and Mr. Damon--help me."

"Bless my check book!" exclaimed the odd gentleman. "It is very little
help I give, Tom."

General Waller soon took his departure, promising to call again, to see
Tom's test if one were held. He also repeated his determination to set
the Secret Service men at work to discover the mysterious German.

"I can't imagine who would want to injure you or me, Tom Swift," he
said.

"Do you think they wanted to injure you, General?" asked Mr. Damon.

"It would seem so," remarked Ned. "That man doped Tom's powder, hoping
to make it so powerful that it would blow up everything. Then he sends
word to the General to be present. If there had been a blow-up he would
have gone with it."

"Bless my gaiters, yes!" exclaimed Mr. Damon.

"Well, we'll see if we can ferret him out!" spoke the officer as he
took his leave.

Tom, Ned and the others talked the matter over at some length.

"I wonder if we could trace that man who rode away on the motor-cycle?"
said Ned.

"We'll try," decided Tom, energetically, and in the electric runabout,
that had once performed such a service to his father's bank, the young
inventor and his chum were soon traversing the road taken by the spy.
They got some traces of him--that is, several persons had seen him
pass--but that was all. So they had to record one failure at least.

"I wonder if the General himself could have sent that letter?" mused
Ned, as they returned home.

"What! To himself?" cried Tom, in amazement.

"He might have," went on Ned, coolly. "You see, Tom, he admits that he
was jealous of you. Now what is there to prevent him from hiring
someone to dope your powder, and then, to divert suspicion from
himself, faking up a letter and inviting himself to the blowout."

"But if he did that--which I don't believe--why would he come when
there was danger, in case his trick worked, of the whole place being
blown to kingdom come."

"Ah, but you notice he didn't arrive until after danger of an explosion
had passed," commented Ned.

"Oh, pshaw!" cried Tom. "I don't take any stock in that theory."