Quickly Tom Swift crossed the space between the airship, that was ready for a flight, and the tree. The man behind it had apparently not seen Tom coming, being so interested in looking at the airship, which was a wonderful craft. He was taken completely by surprise as Tom, stepping up to him, asked sharply:

"Who are you and what are you doing here?"

The man started so that he nearly dropped the opera glasses, which he had held focused on the aeroplane. Then he stepped back, and eyed Tom sharply.

"What do you want?" repeated our hero. "What right have you to be spying on that airship--on these premises?" The man hesitated a moment, and then coolly returned the glasses to his pocket. He did not seem at all put out, after his first start of surprise.

"What are you doing?" Tom again asked. He looked around to see where Koku, the giant, was, and beheld the big man walking slowly toward him, for Ned had mentioned what had taken place.

"What right have you to question my actions?" asked the man, and there was in his tones a certain authority that made Tom wonder.

"Every right," retorted our hero. "That is my airship, at which you have been spying, and this is where I live."

"Oh, it is; eh?" asked the man calmly. "And that's your airship, too?"

"I invented it, and built the most of it myself. If you are interested in such things, and can assure me that you have no spying methods in view, I can show you--"

"Have you other airships?" interrupted the man quickly.

"Yes, several," answered Tom. "But I can't understand why you should be spying on me. If you don't care to accept my offer, like a gentleman, tell me who you are, and what your object is, I will have my assistant remove you. You are on private property, as this street is not a public one, being cut through by my father. I'll have Koku remove you by force, if you won't go peaceably, and I think you'll agree with me that Koku can do it. Here Koku," he called sharply, and the big man advanced quickly.

"I wouldn't do anything rash, if I were you," said the man quietly. "As for this being private property, that doesn't concern me. You're Tom Swift, aren't you; and you have several airships?"

"Yes, but what right have you to--"

"Every right!" interrupted the man, throwing back the lapel of his coat, and showing a badge. "I'm Special Agent William Whitford, of the United States Customs force, and I'd like to ask you a few questions, Tom Swift." He looked our hero full in the face.

"Customs department!" gasped Tom. "You want to ask me some questions?"

"That's it," went on the man, in a business-like voice.

"What about?"

"Smuggling by airship from Canada!"

"What!" cried Tom. "Do you mean to say you suspect me of being implicated in--"

"Now go easy," advised the man calmly. "I didn't say anything, except that I wanted to question you. If you'd like me to do it out here, why I can. But as someone might hear us--"

"Come inside," said Tom quietly, though his heart was beating in a tumult. "You may go, Koku, but stay within call," he added significantly. "Come on, Ned," and he motioned to his chum who was approaching. "This man is a custom officer and not a spy or a detective, as we thought."

"Oh, yes, I am a sort of a detective," corrected Mr. Whitford. "And I'm a spy, too, in a way, for I've been spying on you, and some other parties in town. But you may be able to explain everything," he added, as he took a seat in the library between Ned and Tom. "I only know I was sent here to do certain work, and I'm going to do it. I wanted to make some observations before you saw me, but I wasn't quite quick enough."

"Would you mind telling me what you want to know?" asked Tom, a bit impatiently. "You mentioned smuggling, and--"

"Smuggling!" interrupted Ned.

"Yes, over from Canada. Maybe you have seen something in the papers about our department thinking airships were used at night to slip the goods over the border."

"We saw it!" cried Tom eagerly. "But how does that concern me?"

"I'll come to that, presently," replied Mr. Whitford. "In the first place, we have been roundly laughed at in some papers for proposing such a theory. And yet it isn't so wild as it sounds. In fact, after seeing your airship, Tom Swift, I'm convinced--"

"That I've been smuggling?" asked Tom with a laugh.

"Not at all. As you have read, we confiscated some smuggled goods the other day, and among them was a scrap of paper with the words Shopton, New York, on it."

"Was it a letter from someone here, or to someone here?" asked Ned. "The papers intimated so."

"No. they only guessed at that part of it. It was just a scrap of paper, evidently torn from a letter, and it only had those three words on it. Naturally we agents thought we could get a clew here. We imagined, or at least I did, for I was sent to work up this end, that perhaps the airships for the smugglers were made here. I made inquiries, and found that you, Tom Swift, and one other, Andy Foger, had made, or owned, airships in Shopton."

"I came here, but I soon exhausted the possibility of Andy Foger making practical airships. Besides he isn't at home here any more, and he has no facilities for constructing the craft as you have. So I came to look at your place, and I must say that it looks a bit suspicious, Mr. Swift. Though, of course, as I said," he added with a smile, "you may be able to explain everything."

"I think I can convince you that I had no part in the smuggling," spoke Tom, laughing. "I never sell my airships. If you like you may talk with my father, the housekeeper, and others who can testify that since my return from taking moving pictures, I have not been out of town, and the smuggling has been going on only a little while."

"That is true," assented the custom officer. "I shall be glad to listen to any evidence you may offer. This is a very baffling case. The government is losing thousands of dollars every month, and we can't seem to stop the smugglers, or get much of a clew to them. This one is the best we have had so far."

It did not take Tom many hours to prove to the satisfaction of Mr. Whitford that none of our hero's airships had taken any part in cheating Uncle Sam out of custom duties.

"Well, I don't know what to make of it," said the government agent, with a disappointed air, as he left the office of the Shopton chief of police, who, with others, at Tom's request, had testified in his favor. "This looked like a good clew, and now it's knocked into a cocked hat. There's no use bothering that Foger fellow," he went on, "for he has but one airship, I understand."

"And that's not much good." put in Ned. "I guess it's partly wrecked, and Andy has kept it out in the barn since he moved away."

"Well, I guess I'll be leaving town then," went on the agent. "I can't get any more clews here, and there may be some new ones found on the Canadian border where my colleagues are trying to catch the rascals. I'm sorry I bothered you, Tom Swift. You certainly have a fine lot of airships," he added, for he had been taken through the shop, and shown the latest, noiseless model. "A fine lot. I don't believe the smugglers, if they use them, have any better."

"Nor as good!" exclaimed Ned. "Tom's can't be beat."

"It's too late for our noiseless trial now," remarked Tom, after the agent had gone. "Let's put her back in the shed, and then I'll take you down street, and treat you to some ice cream, Ned. It's getting quite summery now."

As the boys were coming out of the drug store, where they had eaten their ice cream in the form of sundaes, Ned uttered a cry of surprise at the sight of a man approaching them.

"It's Mr. Dillon, the carpenter whom we saw in the Foger house, Tom!" exclaimed his chum. "This is the first chance I've had to talk to him. I'm going to ask him what sort of repairs he's making inside the old mansion." Ned was soon in conversation with him.

"Yes, I'm working at the Foger house," admitted the carpenter, who had done some work for Ned's father. "Mighty queer repairs, too. Something I never did before. If Andy wasn't there to tell me what he wanted done I wouldn't know what to do."

"Is Andy there yet?" asked Tom quickly.

"Yes, he's staying in the old house. All alone too, except now and then, he has a chum stay there nights with him. They get their own meals. I bring the stuff in, as Andy says he's getting up a surprise and doesn't want any of the boys to see him, or ask questions. But they are sure queer repairs I'm doing," and the carpenter scratched his head reflectively.

"What are you doing?" asked Ned boldly.

"Fixing up Andy's old airship that was once busted," was the unexpected answer, "and after I get that done, if I ever do, he wants me to make a platform for it on the roof of the house, where he can start it swooping through the air. Mighty queer repairs, I call 'em. Well, good evening, boys," and the carpenter passed on.