"Come on, Ned," said Tom, after a moment or two of silent contemplation of Eradicate. "I don't know what this cheerful camouflager of mine is talking about, but we'll have to go to see, I suppose. You say you have shut some one up in Boomerang's stable, Rad?"

"Yes, sah, Massa Tom, dat's whut I's gone an done."

"And you say he's a German?"

"I don't know as to dat, Massa Tom, but he suah done eat sauerkraut 'mostest ebery meal. Dat's whut I call him--a Sauerkrauter! An' he suah was spyin'."

"How do you know that, Rad?"

"'Cause he done went from his own shop on annuder man's ticket into de secret shop, dat's whut he went an' done!"

"Do you mean to tell me, Rad," went on Tom, "that one of the workmen from another shop entered Number Thirteen on the pass issued in the name of one of the men regularly employed in my new shop?"

"Dat's whut he done, Massa Tom."

"How do you know?"

"'Cause I detected him doin' it. Yo'-all done made me a deteckertiff, an' I detected."

"Go on, Rad."

"Well, sah, Massa Tom, I seen dish yeah Dutchman git a ticket-pass offen one ob de reg'lar men. Den he went in de unlucky place an' stayed fo' a long time. When he come out I jest natchully nabbed him, dat's whut I done, an' I took him to Boomerang's stable."

"How'd you get him to go with you?" asked Ned, for the old colored man was feeble, and most of the men employed at Tom's plant were of a robust type.

"I done fooled him. I said as how I'd lest brought from town in mah mule cart some new sauerkraut, an' he could sample it if he liked. So he went wif me, an' when I got him to de stable I pushed him in and locked de door!"

"Come on!" cried Tom to his chum. "Rad may be right, after all, and one of my workmen may be a German spy, though I've tried to weed them all out.

"However, no matter about that, if he was employed in another shop, he had no right to go into Number Thirteen. That's a violation of rules. But if he's in Rad's ramshackle stable he can easily get out."

"No, sah, dat's whut he can't do!" insisted the colored man.

"Why not?" asked Tom.

"'Cause Boomerang's on guard, an' yo'-all knows how dat mule of mine can use his heels!"

"I know, Rad," went on Tom; "but this fellow will find a way of keeping out of their way. We must hurry."

"Oh, he's safe enough," declared the colored man. "I done tole Koku to stan' guard, too! Dat low-down white trash ob a giant is all right fo' guardin', but he ain't wuff shucks at detectin'!" said Eradicate, with pardonable pride. "By golly, maybe I's too old t' put on guard, but I kin detect, all right!"

"If this proves true, I'll begin to believe you can," replied Tom. "Hop along, Ned!"

Followed by the shuffling and chuckling negro, Tom and Ned went to the rather insecure stable where the mule Boomerang was kept. That is, the stable was insecure from the standpoint of a jail. But the sight of the giant Koku marching up and down in front of the place, armed with a big club, reassured Tom.

"Is he in there, Koku?" asked the young inventor.

"Yes, Master! He try once come out, but he approach his head very close my defense weapon and he go back again."

"I should think he would," laughed Ned, as he noted the giant's club.

"Well, Rad, let's have a look at your prisoner. Open the door, Koku," commanded Tom.

"Better look out," advised Ned. "He may be armed."

"We'll have to take a chance. Besides, I don't believe he is, or he'd have fired at Koku. There isn't much to fear with the giant ready for emergencies. Now we'll see who he is. I can't imagine one of my men turning traitor."

The door was opened and a rather miserable-looking man shuffled out. There was a bloody rag on his head, and he seemed to have made more of an effort to escape than Koku described, for he appeared to have suffered in the ensuing fight.

"Carl Schwen!" exclaimed Tom. "So it was you, was it?"

The German, for such he was, did not answer for a moment He appeared downcast, and as if suffering. Then a change came over him. He straightened up, saluted as a soldier might have done, and a sneering look came into his face. It was succeeded by one of pride as the man exclaimed:

"Yes, it is I! And I tried to do what I tried to do for the Fatherland! I have failed. Now you will have me shot as a spy, I suppose!" he added bitterly.

Tom did not answer directly. He looked keenly at the man, and at last said:

"I am sorry to see this. I knew you were a German, Schwen, but I kept you employed at work that could not, by any possibility, be considered as used against your country. You are a good machinist, and I needed you. But if what I hear about you is true, it is the end."

"It is the end," said the man simply. "I tried and failed. If it had not been for Eradicate--Well, he's smarter than I gave him credit for, that's all!"

The man spoke very good English, with hardly a trace of German accent, but there was no doubt as to his character.

"What will you do with him, Tom?" asked Ned.

"I don't know. I'll have to do a little investigating first. But he must be locked up. Schwen," went on the young inventor, "I'm sorry about this, but I shall have to give you into the custody of a United States marshal. You are not a naturalized citizen, are you?"

The man muttered something in German to the effect that he was not naturalized and was glad of it.

"Then you come under the head of an enemy alien," decided Tom, who understood what was said, "and will have to be interned. I had hoped to avoid this, but it seems it cannot be. I am sorry to lose you, but there are more important matters. Now let's get at the bottom of this."

Schwen was, after a little delay, taken in charge by the proper officer, and then a search was made of his room, for, in common with some of the other workmen, he lived in a boarding house not far from the plant

There, by a perusal of his papers, enough was revealed to show Tom the danger he had escaped.

"And yet I don't know that I have altogether escaped it," he said to Ned, as they talked it over. "There's no telling how long this spy work may have been going on. If he has discovered all the secrets of Shop Thirteen it may be a bad thing for the Allies and--"

"Look out!" warned Ned, with a laugh. "You'll be saying things you don't want to, Tom and not at all in keeping with your former silence."

"That's so," agreed the young inventor, with a sigh. "But if things go right I'll not have to keep silent much longer. I may be able to tell you everything."

"Don't tell me--tell Mary," advised his chum. "She feels your silence more than I do. I know how such things are."

"Well, I'll be able to tell her, too," decided Tom. "That is, if Schwen hasn't spoiled everything. Look here, Ned, these papers show he's been in correspondence with Blakeson and Grinder."

"What about, Tom?"

"I can't tell. The letters are evidently written in code, and I can't translate it offhand. But I'll make another attempt at it. And here's one from a person who signs himself Walter Simpson, but the writing is in German."

"Walter Simpson!" cried Ned. "That's my friend of the tree!"

"It is?" cried Tom. "Then things begin to fit themselves together. Simpson is a spy, and he was probably trying to communicate with Schwen. But the latter didn't get the information he wanted, or, if he did get it, he wasn't able to pass it on to the man in the tree. Eradicate nipped him just in time."

And, so it seemed, the colored man had done. by accident he had discovered that Schwen had prevailed on one of the workmen in Shop 13 to change passes with him. This enabled the German spy to gain admittance to the secret place, which Tom thought was so well guarded. The man who let Schwen take the pass was in the game, too, it appeared, and he was also placed under arrest. But he was a mere tool in the pay of the others, and had no chance to gain valuable information.

A hasty search of Shop 13 did not reveal anything missing, and it was surmised (for Schwen would not talk) that he had not found time to go about and get all that he was after.

Soon after Schwen's arrest the "Spy Tree," as Tom called it, was cut down.

"Eradicate certainly did better than I ever expected he would," declared Tom. "Well, if all goes well, there won't be so much need for secrecy after a day or so. We're going to give her a test, and then--"

"Give who a test?" asked Ned, with a smile.

"You'll soon see," answered Tom, with an answering grin. "I hereby invite you and Mr. Damon to come over to Shop Thirteen day after to-morrow night and then-- Well, you'll see what you'll see."

With this Ned had to be content, and he waited anxiously for the appointed time to come.

"I surely will be glad when Tom is more like himself," he mused, as he left his chum. "And i guess Mary will be, too. I wonder if he's going to ask her to the exhibition?"

It developed that Tom had done so, a fact which Ned learned on the morning of the day set for the test.

"Come over about nine o'clock," Tom said to his chum. "I guess it will be dark enough then."

Meanwhile Schwen aud Otto Kuhn, the other man involved, had been locked up, and all their papers given into the charge of the United States authorities. A closer guard than ever was kept over No. 13 shop, and some of the workmen, against whom there was a slight suspicion, were transferred.

"Well, we'll see what we shall see," mused Ned on the appointed evening, when a telephone message from Mr. Damon informed the young bank clerk that the eccentric man was coming to call for him before going on to the Swift place.