There was a subdued air of activity about the Swift plant. Subdued, owing to the fact that it was mostly confined to one building--the new, large one, about which stretched a high and strong fence, made with tongue-and-groove boards so that no prying eyes might find a crack, even, through which to peer.

In and out of the other buildings the workmen went as they pleased, though there were not many of them, for Tom and his father were devoting most of their time and energies to what was taking place in the big, new structure. But here there was an entirely different procedure.

Workmen went in and out, to be sure, but each time they emerged they were scrutinized carefully, and when they went in they had to exhibit their passes to a man on guard at the single entrance; and the passes were not scrutinized perfunctorily, either.

Near the building, about which there seemed to be an air of mystery, one day, a week after the events narrated in the opening chapters, strolled the giant Koku. Not far away, raking up a pile of refuse, was Eradicate Sampson, the aged colored man of all work. Eradicate approached nearer and nearer the entrance to the building, pursuing his task of gathering up leaves, dirt and sticks with the teeth of his rake. Then Koku, who had been lounging on a bench in the shade of a tree, Called:

"No more, Eradicate!"

"No mo' whut?" asked the negro quickly. "I didn't axt yo' fo' nuffin yit!"

"No more come here!" said the giant, pointing to the building and speaking English with an evident effort. "Master say no one come too close."

"Huh! He didn't go fo' t' mean me!" exclaimed Eradicate. "I kin go anywheres; I kin!"

"Not here!" and Koku interposed his giant frame between the old man and the first step leading into the secret building. "You no come in here."

"Who say so?"

"Me--I say so! I on guard. I what you call special policeman--detectiff--no let enemies in!"

"Huh! You's a hot deteckertiff, yo' is!" snorted Eradicate. "Anyhow, dem orders don't mean me! I kin go anywhere, I kin!"

"Not here!" said Koku firmly. "Master Tom say let nobody come near but workmen who have got writing-paper. You no got!"

"No, but I kin git one, an' l's gwine t' hab it soon! I'll see Massa Tom, dat's whut I will. I guess yo' ain't de only deteckertiff on de place. I kin go on guard, too!" and Eradicate, dropping his rake, strolled away in his temper to seek the young inventor.

"Well, Rad, what is it?" asked Tom, as he met the colored man. The young inventor was on his way to the mysterious shop. "What is troubling you?"

"It's dat dar giant. He done says as how he's on guard--a deteckertiff--an' I can't go nigh dat buildin' t' sweep up de refuse."

"Well, that's right, Rad. I'd prefer that you keep away. I'm doing some special work in there and it's--"

"Am it dangerous, Massa Tom? I ain't askeered! Anybody whut kin drive mah mule Boomerang--"

"I know, Eradicate, but this isn't so dangerous. It's just secret, and I don't want too many people about. You can go anywhere else except there. Koku is on guard."

"Den can't I be, Massa Tom?" asked the colored man eagerly. "I kin guard an' detect same as dat low-down, good- fo'-nuffin white trash Koku!"

Tom hesitated.

"I suppose I could get you a sort of officer's badge," he mused, half aloud.

"Dat's whut I want!" eagerly exclaimed Eradicate. "I ain't gwine hab dat Koku--dat cocoanut--crowin' ober me! I kin guard an' detect as good's anybody!"

And the upshot of it was that Eradicate was given a badge, and put on a special post, far enough from Koku to keep the two from quarreling, and where, even if he failed in keeping a proper lookout, the old servant could do no harm by his oversight

"It'll please him, and won't hurt us," said Tom to his father. "Koku will keep out any prying persons."

"I suppose you are doing well to keep it a secret, Tom," said Mr. Swift, "but it seems as if you might announce it soon."

"Perhaps we may, Dad, if all goes well. I've given her a partial shop-tryout, and she works well. But there is still plenty to do. Did I tell you about meeting Blakeson?"

"Yes, and I can't understand why he should be in this vicinity. Do you think he has had any intimation of what you are doing?"

"It's hard to say, and yet I would not be surprised. When Uncle Sam couldn't keep secret the fact of our first soldiers sailing for France. How can I expect to keep this secret? But they won't get any details until I'm ready, I'm sure of that."

"Koku is a good discourager," said Mr. Swift, with a chuckle. "You couldn't have a better guard, Tom."

"No, and if I can keep him and Eradicate from trying to pull off rival detective stunts, or 'deteckertiff,' as Rad calls it, I'll be all right. Now let's have another go at that carburetor. There's our weak point, for it's getting harder and harder all the while to get high-grade gasolene, and we'll have to come to alcohol of low proof, or kerosene, I'm thinking."

"I wouldn't be surprised, Tom. Well, perhaps we can get up a new style of carburetor that will do the trick. Now look at this needle valve; I've given it a new turn," and father and son went into technical details connected with their latest invention.

These were busy days at the Swift plant. Men came and went--men with queerly shaped parcels frequently--and they were admitted to the big new building after first passing Eradicate and then Koku, and it would be hard to say which guard was the more careful. Only, of course, Koku had the final decision, and more than one person was turned back after Eradicate had passed him, much to the disgust of the negro.

"Pooh! Dat giant don't know a workman when he sees 'im!" snorted Eradicate. "He so lazy his own se'f dat he don't know a workman! Ef I sees a spy, Massa Tom, or a crook, I's gwine git him, suah pop!"

"I hope you do, Rad. We can't afford to let this secret get out," said the young inventor.

It was one evening, when taking a short cut to his home, that Mr. Nestor. the father of Mary Nestor, in whom Tom was more than ordinarily interested, passed not far from the big enclosure which was guarded, on the factory side, day and night. Inside, though out of sight and hidden by the high fence, were other guards.

As Mr. Nestor passed along the fence, rather vaguely wondering why it was so high, tight and strong, he felt the ground trembling beneath his feet. It rumbled and shook as though a distant train were passing, and yet there was none due now, for Mr. Nestor had just left one, and another would not arrive for an hour.

"That's queer," mused Mary's father. "If I didn't know to the contrary, I'd say that sounded like heavy guns being fired from a distance, or else blasting. It seems to come from the Swift place," he went on. "I wonder what they're up to in there."

Suddenly the rumbling became more pronounced, and mingled with it, in the dusk of the evening, were the shouts of men.

"Look out!" some one cried. "She's going for the fence!"

A second later there was a cracking and straining of boards, and the fence near Mr. Nestor bulged out as though something big, powerful and mighty were pressing it from the inner side.

But the fence held, or else the pressure was removed, for the bulge went back into place, though some of the boards were splintered.

"Have to patch that up in the morning," called another voice, and Mr. Nestor recognized it as that of Tom Swift.

"What queer doings are going on here?" mused Mary's father. "Have they got a wild bull shut up in there, and is he trying to get out? Lucky for me he didn't," and he hurried on, the rumbling noise become fainter until it died away altogether.

That night, after his supper and while reading the paper and smoking a cigar, Mr. Nestor spoke to his daughter.

"Mary, have you seen anything of Tom Swift lately?"

"Why, yes, Father. He was over for a little while the other night, but he didn't stay long. Why do you ask?"

"Oh, nothing special. I just came past his place and I heard some queer noises, that's all. He's up to some more of his tricks, I guess. Has be enlisted yet?"

"No.

"Is he going to?"

"I don't know," and Mary seemed a bit put out by this simple question. "What do you mean by his tricks?" she asked, and a close observer might have thought she was anxious to get away from the subject of Tom's enlistment.

"Oh, like that one when he sent you something in a box labeled 'dynamite,' and gave us all a scare. You can't tell what Tom Swift is going to do next. He's up to something now, I'll wager, and I don't believe any good will come of it"

"You didn't think so after he sent his wireless message, and saved us from Earthquake Island," said Mary, smiling.

"Hum! Well, that was different," snapped Mr. Nestor. "This time I'm sure he's up to some nonsense! The idea of crashing down a fence! Why doesn't he enlist like the other chaps, or sell Liberty Bonds like Ned Newton?" and Mr. Nestor looked sharply at his daughter. "Ned gave up a big salary as the Swifts flnancial man--a place he had held for a year--to go back to the bank for less, just so he could help the Government in the financial end of this war. Is Tom doing as much for his country?"

"I'm sure I don't know," answered Mary; and soon after, with averted face, she left the room.

"Hum! Queer goings on," mused Mr. Nestor. "Tom Swift may be all right, but he's got an unbalanced streak in him that will bear looking out for, that's what I think!"

And having settled this matter, at least to his own satisfaction, Mr. Nestor resumed his smoking and reading.

A little later the bell rang. There was a murmur of voices in the hall, and Mr. Nestor, half listening, heard a voice he knew.

"There's Tom Swift now!" he exclaimed. "I'm going to find out why he doesn't enlist!"