"You--you got my letter!" stammered Tom, holding out his hand for a missive which the General extended. "I--I don't exactly understand. My letter?"

"Yes, certainly," went on the officer. "It was very kind of you to remember me after--well, to be perfectly frank with you, I did resent, a little, your remarks about my unfortunate gun. But I see you are of a forgiving spirit."

"But I didn't write you any letter!" exclaimed Tom, feeling more and more puzzled.

"You did not? What is this?" and the General unfolded a paper. Tom glanced over it. Plainly it was a request for the General to be present at the test on that day, and it was signed with Tom Swift's name.

But as soon as the young inventor saw it, he knew that it was a forgery.

"I never sent that letter!" he exclaimed. "Look, it is not at all like my handwriting," and he took up some papers from a near- by table and quickly compared some of his writing with that in the letter. The difference was obvious.

"Then who did send it?" asked General Waller. "If someone has been playing a joke on me it will not be well for him!" and he drew himself up pompously.

"If a joke has been played--and it certainly seems so," spoke Tom, "I had no hand in it. And did you come all the way from Sandy Hook because of this letter?"

"No, I am visiting friends in Waterford," said the officer, naming the town where Mr. Damon lived. "My cousin is Mr. Pierce Watkins."

"Bless my doorbell!" exclaimed Mr. Damon, "I know him! He lives just around the corner from me. Bless my very thumb prints!"

General Waller stared at Mr. Damon in some amazement, and resumed:

"Owing to the unfortunate accident to my gun, and to some slight injuries I sustained, I found my health somewhat impaired. I obtained a furlough, and came to visit my cousin. The doctor recommended open air exercise, and so I brought with me my motor-cycle, as I am fond of that means of locomotion."

"I used to be," murmured Mr. Damon; "but I gave it up."

"After his machine climbed a tree," Tom explained, with a smile, remembering how he had originally met Mr. Damon, and bought the damaged machine from him, as told in the first volume of this series.

"So, when I got your letter," continued the General, "I naturally jumped on my machine and came over. Now I find that it is all a hoax."

"I am very sorry, I assure you," said Tom. "We did have a sort of test today; but it was a failure, owing to the fact that someone tampered with my powder. From what you tell me, I am inclined to the belief that the same person may have sent you that letter. Let me look at it again," he requested.

Carefully he scanned it.

"I should say that was written in a sort of German hand; would you not also?" he asked of Mr. Damon.

"I would, Tom."

"A German!" exclaimed General Waller.

At the mention of the word "German" Koku, the giant, who had entered the room, to be stared at in amazement by the officer, exclaimed:

"That he, Master! That he!"

"What do you mean?" inquired Tom.

"German man give me stuff for to put in your powder. I 'member now, he talk like Hans who make our garden here; and he say 'yah' just the same like. That man German sure."

"What does this mean?" inquired the officer.

Quickly Tom told of the visit of an unknown man who had prevailed on the simple-minded giant to "dope" Tom's new powder under the impression that he was doing his master a favor. Then the flight of the spy on a motor-cycle, just as the experiment failed, was related.

"We have a German gardener," went on Tom, "and Koku now recalls that our mysterious visitor had the same sort of speech. This ought to give us a clue."

"Let me see," murmured General Waller. "In the first place your test fails--you learn, then, that your powder has been tampered with--you see a man riding away in haste after having, in all likelihood, spied on your work--your giant servant recalls the visit of a mysterious man, and, when the word 'German' is pronounced in his hearing he recalls that his visitor was of that nationality. So far so good.

"I come to this vicinity for my health. That fact, as are all such regarding officers, was doubtless published in the Army and Navy Journal, so it might easily become known to almost anyone. I receive a letter which I think is from Tom Swift, asking me to attend the test. As the distance is short I go, only to find that the letter has been forged, presumably by a German.

"Question: Can the same German be the agent in both cases?"

"Bless my arithmetic! how concisely you put it!" exclaimed Mr. Damon.

"It is part of my training, I suppose," remarked the officer. "But it strikes me that if we find your German spy, Tom, we will find the man who played the joke on me. And if I do find him-- well, I think I shall know how to deal with him," and General Waller assumed his characteristic haughty attitude.

"I believe you are right, General," spoke Tom. "Though why any German would want to prevent my experiments, or even damage my property, and possibly injure my friends, I cannot understand."

"Nor can I," spoke the officer.

"I am sorry you have had your trouble for nothing," went on Tom. "And, if you are in this vicinity when I conduct my next test, I shall be glad to have you come. I will send word by Mr. Damon, and then there will be no chance of a mistake."

"Thank you, Tom, I shall be glad to come I do not know how long I shall remain in this vicinity. If I knew where to look for the German I would make a careful search. As it is, I shall turn this letter over to the United States Secret Service, and see what its agents can do. And, Tom, if you are annoyed again, let me know. You are a sort of rival, so to speak, but, after all, we are both working to serve Uncle Sam. I'll do my best to protect you."

"Thank you, sir," replied Tom. "On my part, I shall keep a good lookout. It will be a bold spy who gets near my shop after this. I'm going to put up my highly-charged protecting electric wires again. We were just talking about them when you came in. Would you like to look about here, General?"

"I would, indeed, Tom. Have you made your big gun yet?"

"No, but I am working on the plans. I want first to decide on the kind of explosive I am to use, so I can make my gun strong enough to stand it."

"A wise idea. I think there is where I made my mistake. I did not figure carefully enough on the strength of material. The internal pressure of the powder I used, as well as the muzzle velocity of my projectile, were both greater than they should have been. Take a lesson from my failure. But I am going to start on another gun soon, and--Tom Swift--I am going to try to beat you!"

"All right, General," answered Tom, genially. "May the best gun win!"

"Bless my powder box!" cried Mr. Damon. "That's the way to talk."

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."

"Well, maybe not," replied Ned. "But it's worth thinking about. I believe if General Waller could prevent you from inventing your big gun, he would."

The days that followed were busy ones for Tom. He worked on the powder problem from morning to night, scoring many failures and only a few successes. But he did not give up, and in the meanwhile drew tentative plans for the big gun.

One evening, after a hard day's work, he went to the library where his father was reading.

"Tom," said Mr. Swift, "do you remember that old fortune hunter, Alec Peterson, who wanted me to go into that opal mine scheme?"

"Yes, Dad. What about him? Has he found it?"

"No, he writes to say he reached the island safely, and has been working some time. He hasn't had any success yet in locating the mine; but he hopes to find it in a week or so."

"That's just like him," murmured Tom. "Well, Dad, if you lose the ten thousand dollars I guess I'll have to make it up to you, for it was on my account that you made the investment."

"Well, you're worth it, Tom," replied his father, with a smile.