Peering on all sides as he dashed along the gravel walk, hoping to catch a glimpse of the unknown intruder in the garden or shrubbery, Tom sprinted on at top speed. Now and then he paused to listen, but no sound came to him to tell of some one in retreat before him. There was only Silence.

"Mighty queer," mused the youth. "Whoever it was, he couldn't have had more than a minute start of me--no, not even half a minute--and yet they've disappeared as completely as though the ground had opened and let them down; and the worst of it is, that they've taken my plans with them!"

He turned about and retraced his steps, making a careful search. He saw no one, until, turning a corner, a little later, he met Eradicate Sampson.

"You haven't seen any strangers around here just now, have you, Rad?" asked Tom anxiously.

"No, indeedy, I hasn't, Massa Tom. What fo' kind ob a stranger was him?"

"That's just what I don't know. Rad. But some one sneaked into the library lust now and took some of my plans while my father dozed off. I jumped out after him as soon as I could, but he has disappeared."

"Maybe it were th' man who done stowed hisself away on yo' airship, de time yo' all went after de diamonds," suggested the colored man.

"No, it couldn't have been him. If it was anybody, it was Andy Foger, or some of his crowd. You didn't see Andy, did you, Rad?"

"No, indeedy; but if I do, I suah will turn mah mule, Boomerang, loose on him, an' he won't take any mo' plans-- not right off, Massa Tom."

"No, I guess not. Well, I must get back to dad, or he'll worry. Keep your eyes open, Rad, and if you see Andy Foger, or any one else, around here, let me know. Just sing out for all you're worth."

"Shall I call out, Massa Tom, ef I sees dat blessin' man?"

"You mean Mr. Damon?"

"Dat's de one. De gen'man what's allers a-blessin' ob hisself or his shoelaces, or suffin laik dat. Shall I sing out ef I sees him?"

"Well, no; not exactly, Rad. Just show Mr. Damon up to the house. I'd be glad to see him again, though I don't fancy he'll call. He's off on a little trip, and won't be back for a week. But watch out, Rad." And with that Tom turned toward the house, shaking his head over the puzzle of the missing plans.

"Did you find any one?" asked his father eagerly as the young inventor entered the library.

"No," was the gloomy answer. "There wasn't a sign of any one."

Tom went over to the window and looked about for clues. There was none that he could see, and a further examination of the ground under the window disclosed nothing. There was gravel beneath the casement, and this was not the best medium for retaining footprints. Nor were the gravel walks any better.

"Not a sign of any one," murmured Tom. "Are you sure you didn't hear any noise, dad, when you dozed off?"

"Not a sound, Tom. In fact, it's rather unusual for me to go to sleep like that, but I suppose it's because of my illness. But I couldn't have been asleep long--not more than two minutes."

"That's what I think. Yet in that time someone, who must have been on the watch, managed to get in here and take my plans for the new sky racer. I don't see how they got the wire screen open from the outside, though. It fastens with a strong hook."

"And was the screen open?" asked Mr. Swift

"Yes, it was unhooked. Either they pushed a wire in through the mesh, caught it under the hook, and pulled it up from the outside, or else the screen was opened from the inside."

"I don't believe they could get inside to open the screen without some of us seeing them," spoke the older inventor. "More likely, Tom, it wasn't hooked, and they found it an easy matter to simply pull it open."

"That's possible. I'll ask Mrs. Baggert if the screen was unhooked."

But the housekeeper could not be certain on that point, and so that part of the investigation amounted to nothing.

"It's too bad!" exclaimed Mr. Swift. "It's my fault, for dozing off that way."

"No, indeed, it isn't!" declared Tom stoutly.

"Is the loss a serious one?" asked his father. "Have you no copy of the plans?"

"Yes, I have a rough draft from which I made the completed drawings, and I can easily make another set. But that isn't what worries me--the mere loss of the plans."

"What is it, then, Tom?"

"The fact that whoever took them must know what they are the plans for a sky racer that is to take part in the big meet. I have worked it out on a new principle, and it is not yet patented. Whoever stole my plans can make the same kind of a sky racer that I intended to construct, and so stand as good a chance to win the prize of ten thousand dollars as I will."

"That certainly is too bad, Tom. I never thought of that. Do you suspect any one?"

"No one, unless it's Andy Foger. He's mean enough to do a thing like that, but I didn't think he'd have the nerve. However, I'll see if I can learn anything about him. He may have been sneaking around, and if he has my plans he'd ask nothing better than to make a sky racer and beat me."

"Oh, Tom, I'm so sorry!" exclaimed Mr. Swift "I--I feel very bad about it!"

"There, never mind!" spoke the lad, seeing that his father was looking ill again. "Don't think any more about it, dad. I'll get back those plans. Come, now. It's time for your medicine, and then you must lie down." For the aged inventor was looking tired and weak.

Wearily he let Tom lead him to his room, and after seeing that the invalid was comfortable Tom called up Dr. Gladby, to have him come and see Mr. Swift. The doctor said his patient had been overdoing himself a little, and must rest more if he was to completely recover.

Learning that his father was no worse, Tom set off to find Andy Foger.

"I can't rest until I know whether or not he has my plans," he said to himself. "I don't want to make a speedy aeroplane, and find out at the last minute that Andy, or some of his cronies, have duplicated it."

But Tom got little satisfaction from Andy Foger. When that bully was accused of having been around Tom's house he denied it, and though the young inventor did not actually accuse him of taking the plans, he hinted at it. Andy muttered many indignant negatives, and called on some of his cronies to witness that at the time the plans were taken he and they were some distance from the Swift home.

So Tom was baffled; and though he did not believe the red-haired lad's denial, there was no way in which he could prove to the contrary.

"If he didn't take the plans, who did?" mused Tom.

As the young inventor turned away after cross-questioning Andy, the bully called out:

"You'll never win that ten thousand dollars!"

"What do you know about that?" demanded Tom quickly.

"Oh, I know," sneered Andy. "There'll be bigger and better aeroplanes in that meet than you can make, and you'll never win the prize."

"I suppose you heard about the affair by sneaking around under our windows, and listening," said Tom.

"Never mind how I know it, but I do," retorted the bully.

"Well, I'll tell you one thing," said Tom calmly. "If you come around again it won't be healthy for you. Look out for live wires, if you try to do the listening act any more, Andy!" And with that ominous warning Tom turned away.

"What do you suppose he means, Andy?" asked Pete Bailey, one of Andy's cronies.

"It means he's got electrical wires strung around his place," declared Sam Snedecker, "and that we'll be shocked if we go up there. I'm not going!"

"Me, either," added Pete, and Andy laughed uneasily.

Tom heard what they said, and in the next few days he made himself busy by putting some heavy wires in and about the grounds where they would show best. But the wires carried no current, and were only displayed to impress a sense of fear on Andy and his cronies, which purpose they served well.

But it was like locking the stable door after the horse had been stolen, for with all the precautions he could take Tom could not get back his plans, and he spent many anxious days seeking them. They seemed to have completely disappeared, however, and the young inventor decided there was nothing else to do but to draw new ones.

He set to work on them, and in the meanwhile tried to learn whether or not Andy had the missing plans. He sought this information by stealth, and was aided by his chum, Ned Newton. But all to no purpose. Not the slightest trace or clue was discovered.