Desperation gave Tom Swift unusual strength as he lay bound and seemingly helpless in that upper story of the lonely cabin. Though the ropes about his wrists and ankles seemed very tight, he had a feeling as he strained his arms that he was going to get free. That talk he heard, floating up from the room below where, through the crack, he could see the conspirators gathered, gave him fierce energy.

“So that’s their game, is it?” mused Tom, as he heard the plot to blow up the plant. “They want to put me out of the way so they will have a free hand to wreck my plant and get the plans and models of the airline express cars and airships. I don’t believe they ever thought of inventing that sort of combination craft themselves. They are trying to steal my ideas. Kenny and Schlump must have hatched the plot. Oh, but I’ve got to get loose and warn Dad and Ned!”

It was this very necessity for quick action that emboldened Tom and gave him the fierce energy to struggle to get free of his bonds. He was glad the gag had come loose, for with that in his mouth, hindering his breathing, he could not have worked as hard as he did—he would have suffocated.

But now he could breathe easily, and he had need of all his spare wind in the exertions that followed the overhearing of the plot of the men who had captured him.

Luckily ropes, however strong, are capable of being stretched if one pulls on them long enough and often enough. And though it hurt Tom to force his bare wrists against the hemp strands, he kept at it until he found he could move his hands more freely. He wished he might find a projecting nail or some sharp object in his prison, against which he could rub his bonds to sever them. But he dared not roll about much for fear of making a noise and so bringing his captors upstairs. Once they discovered that he was making an attempt to escape, they might chain him fast or move him to some other prison whence it would not be so easy to get away.

“Once I’m free of these ropes I know I can get out,” Tom told himself. “I’m only on the second floor, I can tell that, for I counted the stairs as they carried me up. And if I can get a window open I can jump to the ground, if I don’t find some sort of a vine or a rain-water pipe that I can climb down. But the first thing to do is to get these ropes off my hands and feet.”

Tom knew that once his hands were freed it would be a comparatively easy matter to loosen the ropes on his ankles. All his energies, then, must center on his hands. They were tied behind his back, with several coils of rope wound about the wrists. Again and again Tom strained on these until, at last, he felt that he could draw out one hand.

It was not easy, even when he had done this much, but he kept at it, and finally had one hand altogether freed. The exertion had made him sweat and had tired him greatly. He panted for breath as he lay there, while below him he could hear the murmuring voices of the plotters.

A little rest brought back Tom’s breath and gave him renewed strength for what he had yet to do, which was to free his ankles. But he had accomplished the hardest part, which was to get his hands loose. It was not easy, however, to loosen the knots in the ropes about his feet, and it was several hours before he managed to free himself completely. One reason for this was the tightness of the knots, and another was that, occasionally one or more of the men below would come upstairs and flash a gleam from an electric torch into the room where Tom was a prisoner, to make sure he was still safe.

But the young man could tell, by the movement of chairs below, when this inspection was coming, and each time he was ready for it. He assumed on the floor the same position he had held when still bound and he wound the ropes back again on his wrists so it could not be seen, from a casual inspection, that they were loose. And he also placed the gag in his mouth.

Because of the necessity for stopping work every now and then to assume the position of a bound prisoner, Tom was longer engaged in freeing his feet than otherwise would have been the case.

Consequently it was almost morning when he was able to stand up and move about freely. Cautiously he crossed the room, pausing at every board to make sure it would not squeak and betray him, and at last he reached a window through which the faint rosy streaks of dawn were coming—the second morning that Tom had been away from home.

“Now to see if I can get this window open,” mused Tom. He scarcely hoped to find that it would open readily, and he was not greatly disappointed when he found it fastened. By the morning light, now growing stronger, and by feeling, he ascertained that the sash was held down by nails driven over the edges of it into the frame.

A claw hammer would have taken these nails out in a second of time, but Tom had no claw hammer, and the files by which he had removed the chain from his leg had been taken from him.

“But maybe by bending the nail back and forth often enough I can either loosen it enough to pull it out or I can break it off,” he told himself.

He felt that he must now work quickly. For some time there had been silence in the rooms below him, and he guessed that his captors were sleeping, thinking him securely bound and locked in.

“But they’ll awake soon and start getting breakfast,” Tom decided. “They’ll bring me up some, and then they’ll see that I have loosened the ropes. I’ve got to get away before they come up to feed me.”

His fingers sought a wedging nail and began to bend it back and forth. At first it gave only a little, but eventually it moved more and Tom’s hopes rose.

It was now getting lighter every minute. Tom felt that each moment was precious. Unless he got the window open soon and could manage to escape through it, he would be discovered.

“There!” he exclaimed with a breath of relief as he at last broke off one nail. “Now for the other.” The second proved easier, for after working it backward and forward a number of times and twisting it about, Tom pulled it out. Now the window could be raised, and this he did cautiously.

He waited a moment after lifting the sash and listened. There was no sound from below, and he thought that the men were still sleeping. He put his head out and looked down. To his dismay the window was higher above the ground than he had hoped, and there was on that side of the house neither a vine nor a rain-water pipe that he could descend.

“I’ve got to jump for it!” he grimly decided. “But that grass below looks soft.” There was a big clump of green below the window. Tom climbed out, sat down on the sill, edged himself over and then hung by his hands a moment. This reduced the length of his drop by his own height. He hung there a moment and then let go.

Down he plunged, coming to a stop on the earth with a thud that shook him greatly. He seemed to lose his breath and a sharp pain shot through his left ankle.

“Guess I’ve sprained it,” he mused. The pain was actually sickening, and made him feel faint. Through an open window on the first floor he heard some one exclaim:

“What was that noise?”

“What noise?” asked another.

“It sounded like some one falling.”

“Guess you were dreaming! Get up and make some coffee. I’m half starved.”

“All right,” said the one who had first spoken. “But I’ll just have a look at that bird upstairs. He’s cute—maybe he’s got away. I’ll have a look at him before I get breakfast!”

“I’ve got to run for it—and right away!” thought Tom desperately. “Though how I’m going to do it with a sprained ankle is more than I know. But it will never do to let them catch me again.”

The grass was tall and rank under the window. It would afford the fugitive cover until he could get to some better shelter. He began crawling through it, deeming this safer than trying to stand up and run. His concealment would be better in this position and it would take the strain off his hurt ankle. He hoped it was only a sprain and not a break.

He had not crawled more than a hundred feet from the old house before he heard coming from it shouts that told that his escape had been discovered.

“Now I’m in for it!” he mused. Just ahead of him he saw a brook, not very deep but rather wide. “If I stand up and run they’re sure to see me,” he reasoned. “And if I crawl I’ll leave a trail in the grass like a big snake. If I can get to the brook and crawl along in that I may throw them off the trail for a while.”

It seemed the best thing to do, and while the men back at the house were running about “in circles,” so to speak, Tom crawled to the brook, and then, having no particular choice, since he did not know where he was, he began crawling upstream. He did not hope to throw his enemies off his trail long in this way, nor did he. They were soon shouting as they ran down the grass-covered and weed-grown yard, for the open window had told them which way he had gone.

The trick of going into the brook confused them for a while, but Tom knew they would separate into two parties and soon trace him. He was desperate and at his wits’ end when he saw just ahead of him on the edge of the stream an old barrel, partly embedded in the sandy shore. He could get into this without leaving the water, and as its open end was turned rather upstream he might escape observation.

It did not take him long to get into the barrel. He took care to leave no tell-tale trail, and his strategy was well carried out, for a little later, splashing their way upstream, ran two of the men—Tom could see them through a hole in the closed end of the barrel.

“But I’d better not stay here,” the lad mused. “They’re sure to come back, and the next time they might take a notion to investigate this barrel. I’ll strike across country until I get to a house. There must be people living around here.”

Tom never liked, afterward, to recall that journey. It was a painful one because of his injured ankle. He got a tree branch, which he used as a crutch and hobbled along on that. Once or twice he fainted and sank to earth in a stupor. How long these periods of unconsciousness lasted he could not tell. He dared not call out for fear of bringing the men on his trail.

Through the woods and across a swamp he pulled himself along, and at last, in the afternoon, as he could tell by the sun, he dragged himself out on a road and saw a white farmhouse a little way down it.

“I—I guess I’m all right now,” faltered the exhausted youth.

It was a much surprised farmer who a little later saw a tall young man, obviously hurt, almost crawling up the front walk. Before the farmer could ask any questions Tom shot one at him.

“I’ve got to get an important message off at once. Have you a telephone? I’ll pay for using it!” There was something businesslike in Tom’s voice, weak and weary as it was, that impelled the farmer’s respect in spite of Tom’s rather disreputable appearance.

“Come in,” the man invited. “Looks to me like you’d better telephone for a doctor while you’re at it!”

“That can wait,” gasped Tom. “Something else is more important. Show me the telephone!”

A little later he was gasping to Ned his message:

“Just escaped! Watch the plant! Get Father to safety. Look out for bombs. I’ll try——”

Then Tom Swift fell over in a faint.

David Knowlton, the farmer upon whom Tom had called so unceremoniously, was scarcely more surprised by the sudden falling over of the young man in a faint than he had been at his eager request for a telephone.

“Great bullfrogs!” cried Mr. Knowlton, as he hurried to pick Tom up and lay him on a lounge in the room. “What’s all this goings-on, anyhow? What’s it all mean?”

“Is he dead?” asked Mrs. Knowlton, who hurried into the room, having followed Tom and her husband when she saw the stranger come up to the house.

“I don’t know, Sarah,” was the answer. “But first hang that telephone back on the hook. The inspector told me never to leave it off when we weren’t using the line and I guess this fellow is through using it.”

So the telephone went back on the hook, which defeated the plans of frantic Ned Newton, on the other end, if not to hold further talk with Tom, at least to learn from what station he was telephoning his message of warning. In vain did Ned appeal to the central operator to re-establish the connection.

“Unless you know the number of the party who called I can’t connect you,” she reported, and Ned knew, from previous attempts, that it was useless to carry the effort further. He could only hope that Tom would call again to relieve their minds. All they knew now was that he was alive, but that something dire portended.

Meanwhile Mr. and Mrs. Knowlton, kindly souls that they were, ministered to Tom Swift. The farmer’s wife brought out her bottle of camphor, and a sniff of this potent spirit, with some rubbed on his forehead, soon brought Tom out of his faint. Then he was given a drink of water, which further helped in restoring his failing energies.

“If they come for me, don’t let them get me!” begged Tom, sitting up on the couch. “Help me to get back! I must travel fast!”

“You need a doctor, that’s what you need, young man!” decided Mr. Knowlton. “You aren’t fit to travel. You’ve done too much of that already, from the looks of you, and that foot of yours is in bad shape,” he added, as he saw the swollen ankle. Tom’s shoe laces were almost bursting from the pressure of the swelled flesh, and the farmer had to cut them to loosen them. This gave Tom some relief, but the hardships he had gone through, the anxiety, and being without proper food so long, had so weakened him that he went off in another faint before he could tell his story.

“Call Doctor Prouty,” advised Mrs. Knowlton. “We’ll never get to the bottom of this until this young man is in his right mind.”

Luckily the physician was in his office in the village and drove out in his car as soon as the farmer had telephoned. A hasty examination showed that Tom was suffering from exhaustion more than from anything else, and a little warm milk, followed later by more substantial food, soon gave the youth energy enough to tell the main points of his story.

“And if these men come after me—which they may do,” he said to Mr. Knowlton, “you won’t let them get me, will you?”

“I should say not!” cried Mrs. Knowlton before her husband could answer. “The idea! You poor boy!”

While the doctor was giving some directions as to what should be done for Tom, one of the hired men on the place came to the door of the room and reported:

“There’s a couple of men outside who want to see you, Mr. Knowlton.”

“All right—I’ll see them,” answered the farmer grimly. “Now don’t you worry!” he told Tom, as the youth started to say something. “Just leave ’em to me.”

Mr. Knowlton found two unprepossessing characters awaiting him on the side porch. He recognized them at once from Tom’s description.

“Have you seen a young man passing here?” asked one of the twain. “He has escaped from an insane asylum and we want to take him back before he can do any damage. He has a delusion that he is a great inventor, named Tom Swift, and he will likely tell a very plausible story. Have you seen him?”

“Tom Swift is in my house now,” said the farmer slowly.

“Is he? That’s good! We’re glad you have him safe!” cried the taller of the two men, with a quick glance at his companion. “Poor fellow—he needs care. We’ll look after him. Much obliged for having taken him in.”

“Wait a minute,” went on the farmer, as the two men endeavored to push past him into the house. “Where are you going?”

“To get the patient and take him back to the asylum.”

“Well, I’d wait a bit about that if I were you,” went on Mr. Knowlton grimly. “Now look here,” he went on, producing a shotgun from behind one of the porch pillars. “I’ll give you fellows just one minute to run down the road and make yourselves scarce in any direction you like. Just one minute, and several seconds of that have already passed!” he added significantly, as he raised the gun.

“But I say—look here!” broke out one of the men.

“Half a minute gone!” said the inexorable farmer.

“You don’t understand!” began the other plotter.

“I understand how to use a shotgun!” said Mr. Knowlton. “There’s about fifteen seconds of that minute left and——” He cocked the gun.

But the two men did not stay to argue longer. With black looks and shaking their fists at the imperturbable farmer, they ran out of the gate, and with a grim chuckle Mr. Knowlton returned to Tom to tell what had happened.

“Thank you—a whole lot,” said the young inventor. “They are desperate men. They are going to blow up my factory. I must get back at once and look after my father. He is an old man—he may not take my telephone warning seriously. Nor may Ned. I must go there myself!”

“But you aren’t fit to travel!” expostulated Mrs. Knowlton. “One of the hired men could go.”

“No, I must make the trip,” decided Tom. “I’m all right now—except for my ankle. Have you an auto?”

“Oh, yes, we have a flivver,” said the farmer.

“Then lend it to me—or sell it to me!” cried Tom. “I must make this trip at once—before night. Where is this place, anyhow?”

“You’re in Birchville,” was the answer. “It’s about thirty miles to Shopton from here.”

“I can make it!” cried Tom. “I can hobble along and make the trip in the auto. You’ll let me go, won’t you, Doctor?”

“Well, since you’re so set on it, I reckon I’ll have to. As you say, there’s nothing much the matter except a sprained ankle, and if some one will drive the car for you——”

“I’ll drive!” cried the farmer. “I want to see this thing through now. I didn’t like the looks of those fellows with their lie about an escaped crazy man. I’ll drive you home, Tom Swift!”

A little later they were on the road, and though the flivver made good time, still to Tom it seemed only to crawl. It was evening now, and rapidly getting dark.

Just before setting out he had again called Ned on the wire, telling his manager where he was, briefly relating what had happened, and again warning about the danger of bombs.

“Don’t worry, Tom,” Ned had ’phoned back. “We’re so glad you’re safe; nothing else matters. But we’ll be on our guard.”

The lights of Shopton were in view. Mr. Knowlton drove his car down the slope that led to the Swift plant, the electric gleams of which could be made out now.

“I guess everything’s all right,” Tom said, with a note of relief in his voice.

But he had hardly spoken than there came the sound of a loud explosion.

“There goes something!” cried the farmer.

“I’m afraid so!” exclaimed Tom. “I wonder if that was at my plant? Oh, I do hope Ned and my father took all precautions!”

As the echoes of the explosion died away the little car carrying Tom and the farmer lurched forward.

Immediately after receiving the mysterious message from Tom, a message that seemed to come out of the air, Ned Newton made a frantic effort to get the operator at central to trace the call. But if one has ever tried to do this he knows how difficult it is. Unless one can give the telephone number of the party to whom he has been talking, and who made the call, if one is cut off there is little chance of the communication being re-established. Ned found this out to his sorrow.

“What is his number?” asked the telephone girl in a matter-of-fact way.

“His number? Great Scott, didn’t I tell you——”

“I’ll give you the manager,” went on the bored operator.

But the manager could give no more satisfaction than his helper. He promised to trace the call and let Ned know what success he had.

“But I know what the answer is already,” Ned remarked in disgusted tones as he gave up vainly rattling the hook. “We won’t hear another word from Tom until he calls us himself. But we are sure of one thing—he’s alive.”

“Did he give you any particulars?” asked Mr. Swift.

“Bless my telephone book, who’s been treating him this way?” demanded Mr. Damon.

Ned repeated the message as it came to him:

“Just escaped! Watch the plant! Get Father to safety! Look out for bombs! I’ll try——”

Then the voice had died away.

“It’s as we suspected,” commented Ned. “He has been captured by some of his enemies and held a captive up to a little while ago. Then he got away. Good old Tom! You can depend on him for that!”

“But it seems to me we should do something,” declared Mary, very much in earnest.

“Bless my eyeglasses, that’s what I say!” cried Mr. Damon. “Come on—we’ll get in my airship—it must be repaired by this time—and we’ll rescue Tom! Don’t lose any more time!”

“But we don’t know where he is,” said Ned. “It would be worse than useless to go scouring around the country looking for Tom in an airship. He might be only five miles from here or he might be five hundred.”

“Yes,” agreed Mary Nestor. “The thing for us to do is to follow Tom’s advice—watch the plant, get Mr. Swift to a place of safety, and look out for bombs.”

“Are you actually going to hunt through the plant for hidden bombs?” demanded Mr. Damon.

“Certainly,” Ned answered. “It’s the only thing to do after Tom’s warning message. While I don’t know what the game is, I think it likely that his enemies kidnapped Tom to get him out of the way so they could have a free run of the plant to search for and take away his models and papers of the newest invention—the airline express. Well, they got Tom, but he managed to escape, and their first attempt to sneak into the plant was a failure.

“Now they may have secreted some time bombs around the place. These may go off any minute, but, it is probable, they have been set to explode after dark. They hope to throw the place into confusion, and then to rush in and get what they want. But Tom has put us on guard.”

“Yes,” agreed Mr. Damon. “Then, as I understand it, we are now going to search for bombs that may go off at any minute?”

“That’s right,” assented Ned.

“Well, I’m glad I carry a large accident insurance,” said the eccentric man, forgetting to bless anything just then.

“Oh, there may not be much danger,” Ned stated. “If the plotters hope to get Tom’s models and papers it isn’t likely they would use bombs of very great force. To do so would be to blow things so much apart that they couldn’t get anything out of the ruins.

“So I think they will use bombs with only a small charge of explosive—enough to make a lot of noise, smoke, and confusion. But if we can find them first—the bombs, I mean—and put them out of business, we’ll be all right.”

“Yes,” said Mr. Damon, “providing they don’t find us first and put us out of business. It doesn’t take much of a bomb to blow a man sky-high.”

“No,” grimly admitted Ned. “But that’s the chance we have to take.”

“Yes, it’s a chance,” said the odd man, and then he and Ned began their perilous work—for it was perilous in spite of what the young manager had said—while Mr. Swift and Mary Nestor returned to the house.

The heads of the various factory departments were called into consultation and instructions given them to search their respective quarters with minute care to discover any possible bombs. Koku and Eradicate were also called in and with Ned and Mr. Damon formed a separate searching party.

It was Koku who found the first bomb. The giant was looking in a pile of rubbish in one corner of a certain shop when he made a dive for something and cried:

“Cannon ball—like strong man throw in circus. I stronger than circus man—I toss cannon ball!”

Ned was just in time to stop him, for the giant had picked up a round iron object and was about to use it to exhibit his great strength when the manager cried:

“Hold it, Koku! That’s a bomb!”

And so it proved to be—a bomb with a time arrangement for firing it, set to go off in about two hours. Ned quickly disconnected the firing arrangement and the bomb was put in a pail of water.

Efforts were redoubled to find the dangerous “cannon balls,” as Koku called them, and in a short time three more were discovered in various parts of the plant. They were all set with time fuses which had more than an hour yet to run, so the bombs were rendered harmless with no ill effects to the searchers.

But it was when the shadows of evening were falling, and Ned and the others had about given up expectation of finding more bombs, that Ned unexpectedly came across one hidden in a refuse box outside of Tom’s private office.

It needed but an instant’s look to show that this was timed to go off almost immediately, which fact, when Ned discovered it, caused him to shout:

“Look out! This is a live one!”

He hurled it from him, toward a pile of lumber in the shop yard. There was a deafening report—a shower of planks and boards rose in the air and settled back again with a crash, while a cloud of smoke filled the air.

“Just in time!” cried Ned. “If that had gone off here it would have killed all of us.”

And as the echoes of the explosion died away a voice was heard shouting:

“Is any one hurt? Father, are you there? Ned, is any one hurt?”

Curious, indeed, was the chance, coincidence, or fate—call it what you like—which brought Tom Swift on the scene, in company with Mr. Knowlton in the runabout, just as the bomb which Ned tossed away exploded near the lumber pile. Tom and his friend felt the force of the blast, but, aside from a stunned feeling and the shock, they were unhurt, and after a momentary stopping of the car Mr. Knowlton sent it on again.

But Tom was anxious to know what had happened; hence his cry as he saw the flash and heard the blast so near his plant and his volley of questions as soon as Mr. Knowlton brought the car to a final stop. And Ned, hearing his friend’s cry, first marveled and then rejoiced.

“Tom! Tom!” he shouted. “Are you there?”

“Yes,” was the answer. “But what happened? Is my father all right?” There was so much smoke from the bomb that Tom could not see far ahead, especially as it was now dark.

“Your father is all right—he’s back home with Mary,” Ned informed Tom, as the latter got out of the car to limp toward the entrance gate near his private office. “And the plant isn’t damaged. Come in and I’ll tell you about it.”

“Are these your friends? Are you sure everything is all right?” asked the cautious farmer, as he saw Tom preparing to go in through the big gate in the high fence. One of the men had hastened to open it when it was certain that Tom was outside.

“Yes, everything is all right,” was the answer. “This is my factory—my friends are here. But my enemies have been trying some of their tricks. Luckily the tricks didn’t work. But don’t go,” begged Tom. “I want you to meet Ned Newton and my father. He’ll want to thank you for aiding me—for bringing me back here.”

“Oh, shucks! That wasn’t anything!” expostulated Mr. Knowlton. “Anybody would have done the same. I won’t stop now. I’m in a hurry to get back home—my wife will be worried. But she’ll be glad to know you got here safe and found your friends. Come out and see us some time.”

“I will,” promised Tom, and then as his benefactor drove away, Tom and Ned rushed toward each other to shake hands, while Mr. Damon brought up the rear, murmuring:

“Bless my insurance policy! Bless my rubber boots! If this isn’t most astonishing!”

“Are you all right, Tom?” demanded Ned, anxiously looking over his chum. “But you’re hurt!” he cried.

“Only a sprained ankle,” explained Tom, who was hobbling about. “I had to jump out of a window. But is Dad all right? What was that explosion?”

“One of the bombs you warned us about. We found four—this was the fifth and just about to go off. I fired it away not a second too soon. It chewed up some of your spare lumber. I guess it’s the last. But where have you been? After the first message of yours we didn’t know what to think until you telephoned again that you were on your way in. What’s it all about?”

“Trouble, I guess,” answered Tom. “Some gang is after me and the new airline ideas and patents. They’re desperate. Wait until I ’phone to the house to let Dad and Mary know I’m all right, and then I’ll explain. Why, hello, Mr. Damon!” Tom exclaimed as he saw his eccentric friend. “Did they get you over here to hunt bombs?”

“He overheard some talk which gave us an idea of the desperate men who were after you,” stated Ned. “He came over in a hurry, and——”

“Too much of a hurry, I guess,” broke in Mr. Damon, in rather crestfallen tones. “I smashed up your mooring mast, Tom.”

“That’s a small matter—easily mended. I’m glad you weren’t hurt. I’ll tell you everything in a few minutes.”

Tom limped into his office and soon was conversing with his father and then, at more length, with Mary. They were rejoiced to learn that he had escaped and was safe. Then began the telling of the two-sided story—the events leading up to the explosion of the bomb Ned had hurled away just as Tom arrived.

Tom related how he had seen the strange man disappear behind the bush, how he had followed, had gone down the secret steps, and how he awoke out of a doped stupor to find himself a prisoner. Then he told of being taken to the lonely house and how he had escaped.

Ned, in turn, related their anxiety when Tom did not come home, and told how they had searched for him before and after the arrival of Mr. Damon.

“We sure were glad to hear your voice over the wire,” Ned stated. “But somebody cut us off.”

“No, I fainted,” Tom explained, “and Mr. Knowlton or his wife hung up the receiver without trying to carry on the talk, which, if they had done, would have told you everything. But the doctor soon pulled me around and the only thing really the matter with me now is this swollen ankle. But that will soon go down and then I’ll get after these fellows and finish work on my airline express. Now tell me where you found the bombs.”

Ned did, stating that one had been found near Tom’s office.

“Well, there may be more bombs,” Tom said. “I won’t be satisfied until we have gone over all this plant again. We can’t afford to take chances. But I’ll move my airline express models and patent papers—that is, the preliminary ones—to a place of safety in my Chest of Secrets.”

This was done, and then another careful search was made of the premises. No more bombs were found and Tom announced his intention of going home to get some much-needed rest.

“But how do you suppose, with all your guards, Tom, and the electrical fence, those fellows planted the bombs?” asked Mr. Damon.

“I don’t know,” replied the young inventor. “I’m afraid there is treachery somewhere in our working force. Without the aid of confederates those plotters couldn’t have put the bombs in here. I’ll have to make an investigation. But for the present the danger is past, I think.”

They were all in need of rest and quiet after the exciting two days through which they had passed, especially Tom, and when he reached home Mrs. Baggert insisted on putting him straight to bed, in which place, to tell the truth, the young inventor was not at all averse to spending some time.

The following day things had rather quieted down at the plant. The resulting débris was swept up, and the shattered lumber pile, devastated by the bomb, was examined for remnants of the infernal machine. Several pieces of cast iron were picked up, and Tom said he would investigate them to try to discover, if possible, where the bomb was made. It appeared to have contained no missiles, being merely a hollow shell filled with explosives, set to go off at a certain time, and Ned had hurled it away not an instant too soon.

“The first thing we’ve got to do,” decided Tom, a few days later, when he was able to be about without his crutch and with much of his former energy restored, “is to investigate that secret stairway. Maybe some of the fellows are still on Barn Door Island.”

But the delay, short as it was, had given the plotters time to vanish and to destroy some of their work. The stairs were in place, but after tearing up the planks, after the soil had been swept away, there was revealed only a blind passage. The tunnel had been caved in a short distance from the secret steps and it was impossible to traverse it.

The same conditions prevailed on Barn Door Island. The place where Tom had emerged from the tunnel was found, but a short distance back in the passage dirt and rocks were piled up, preventing a further examination being made.

“Maybe they’re walled up in the tunnel under the lake,” suggested Mr. Damon.

“Not likely,” Tom said.

“They probably cleared out after their bold plan didn’t succeed,” Ned remarked.

“Yes, they’ve gone for a time,” Tom admitted. “But that doesn’t mean it’s forever. They’re still at large and they won’t give up so easily. I’m afraid for the success of my airline express plans. But I’m going to work on them.”

That Tom’s fears were well grounded was borne out a few days later when, as the young inventor sat at his desk, his private telephone rang. Tom’s own instrument had a number not in the book and was known only to a few. Unless this number were given to the central operator Tom’s ’phone bell would not ring.

But ring it did on this occasion, and over the wire came this ominous message:

“Look out for yourself, Tom Swift! We’ll get you yet!”

Like a flash Tom Swift realized that this warning had come from those daring enemies of his who were still at large—the same men, Kenny and Schlump and the two masked ones, who had kidnapped him. He could realize their rage at his escape, their anger at the foiling of their plot to blow the place up by bombs, or, if their intention was not to cause serious damage, but only confusion, during which they might rob—this, too, had been frustrated.

For a moment the sinister character of all that had taken place stunned the young inventor. The danger under which he was, came to him with a sickening realization and he sat for a moment holding the receiver in his nerveless hand.

He was brought back to a sense of realities by hearing the somewhat distant voice of the operator asking:

“What number, please?”

That stirred Tom into action.

“Look here!” he cried into the instrument. “You don’t realize how important this is! I’ve received a threat over the wire! I must trace——”

“Hold the line,” interrupted the girl in a matter-of-fact tone, and, for a moment, Tom felt hopeful that he could thus get on the trail of those who sought to injure him. But while he was even thus hoping another voice broke in on his thoughts saying:

“This is the manager, have you any complaint to make?”

“Oh—no!” exclaimed Tom in despair, realizing how useless it was to try to trace the call thus. He was going through much the same experience Ned had gone through the time Tom called him from the farmhouse and then fainted. “I’ll call and explain. This is Tom Swift speaking,” he told the manager. “I want to trace a call that came over my private wire, but I can do it best by a personal visit, I believe.”

“We will do all we can for you,” the manager said, for she knew the Swift concern was a large and important one. “It is often difficult to trace stray calls that may be made from any of a hundred pay stations. But I will help you all I can.”

“Thank you,” said Tom, and hung up the receiver. Then he fell into deep thought.

As he had feared, the danger was not over. His enemies were only biding their time. They had failed in their first efforts, but they were not going to give up. The sinister threat was enough to disclose that.

Deciding that quick action was the best way to trace the mysterious call, Tom at once summoned Ned and they visited the local telephone exchange. There the records were gone over, but aside from establishing the fact that the call was put through from the Waterfield central, nothing was established. From just what station the threatening man had spoken, Tom could not find out. That it was a man’s voice he was certain, but whether or not it was one of the four or five who had held him prisoner in the lonely house Tom could not decide.

“But there’s something in the fact that the call came from Mr. Damon’s town,” said Ned. “And he overheard men talking about you the time he was eating in the lunch wagon. It begins to look, Tom, as if the headquarters of the gang was in or near Waterfield.”

“Yes, it does,” agreed Tom. “I think we’ll take a run over there. I want to see Mr. Damon on business, anyhow. And we can take in that old house where they had me tied up. I want to see if I can get any clews there.”

However, a visit to the lonely shack, which Tom located after some difficulty, was without result. It had long been uninhabited, and the owner, when found, said he knew nothing of the men who had been in it. This Tom and Ned could well believe. A search through the premises revealed nothing of any value as a clew. The ropes which Tom had discarded when he made his escape had been taken away, or it might have been possible to trace the place where they had been bought.

“I guess Waterfield is our next and best hope,” remarked Ned, as he and Tom came away from the lonely house.

“I think so,” agreed the young inventor. “Mr. Damon may have heard something more.”

They found the odd man contemplating his new plane, which had been repaired and taken back to his own private hangar.

Mr. Damon led his visitors to his private office, and there Tom told the latest happenings. But Mr. Damon was unable to throw any light on this new development, nor was he able to trace the men he had overheard talking in the lunch wagon. He had tried to get the police to locate them, but without avail.

“Well, we’ll let that go for the present,” decided Tom. “Now for something else—my latest idea, so to speak. I heard you say, the other day, Mr. Damon, that you had some loose funds you wished you could invest in a paying undertaking.”

“That’s right, Tom, so I have, bless my bank-book!”

“Well, I’m thinking of forming a company to exploit my airline express. I find that a large part of Father’s funds and mine are tied up in such a way, in our other inventions, that I can’t get enough ready cash in a hurry, and I need considerable to start this new method of travel. I thought perhaps you might be interested.”

“I think I may be, Tom,” said Mr. Damon. “Tell me about it.”

“Well, it’s like this,” began the young inventor. “You know over in Europe and here, too, though to a much more limited extent, great interest is being manifested in travel by aeroplane—I mean travel by private parties. They have aeroplanes now that carry ten or twelve at a time over the English Channel. You can also fly from Paris to Berlin and between other European cities. In fact, they have regular routes of travel there. But here we have only a few which might be called experimental if we exclude the air mail which is a great success between New York and Chicago and western points. Now what I plan is this: An airline express from New York to San Francisco, a straight-across-the-continent flight by daylight—say from sunrise to sunset.”

“What do you mean, Tom?” cried Wakefield Damon. “Do you mean to tell me you can build an aeroplane that will cross the continent in twelve hours?”

“Not in twelve hours, perhaps,” replied Tom, with a smile. “Though I’m not ready to admit that’s impossible. But there are more than twelve hours from sunrise to sunset—or rather, from dawn until dark. I’ll set the time at sixteen hours. That ought to be easy.”

“But you spoke of making the trip continuously—without change,” said Mr. Damon, to whom Tom’s idea was not altogether new. “None of the aeroplanes we have at present can do that—it’s all of three thousand miles. The British transatlantic fliers didn’t make as long a journey as that, though of course they were in more danger, flying over the ocean.”

“Probably it wouldn’t be a non-stop flight,” said Tom. “The air mail doesn’t do that—different planes are used. It’s just the same in making a transcontinental trip in a railroad train. No one engine makes the entire trip, nor does a single train crew. But it is possible to get in a sleeping car in New York and stay in the same car until you get to San Francisco. The car is merely coupled to different engines, made up into different trains at certain designated places.”

“Is that your plan?” asked the odd man. “I thought you said you were going to run aeroplanes, not railroad cars.”

“I am, if I can make a go of it,” replied the young inventor. “But it will be a combination aeroplane and railroad coach. Here is my idea in a rough form.”

He laid before Ned and Mr. Damon a sketch of a large and powerful plane and also a sort of coach on wheels. The two were shown separately and in combination.

“You see,” went on Tom, pointing out the different features, “the passengers would take their places in this coach—a sort of glorified automobile—at the first landing field, on Long Island. There this car, which will hold half a dozen or more, will be fastened to the aeroplane by clamps. The aeroplane will take off, and make an airline for Chicago, which will be the first of two stops to be made between New York and the Pacific coast. Landing on the Chicago field, the autocar will be detached and rolled, under its own power, to the second aeroplane which will be in waiting. It will be clamped fast to the chassis, and if the passengers happen to be asleep they will not be awakened, any more than they would when a Pullman sleeper is taken off one train and put on another.

“As soon as the car is clamped to the second plane that one starts and flies to Denver. There it descends, the car is rolled to the third plane, in waiting, and that sets off, landing in San Francisco about sixteen hours from the time the start was made—a daylight trip across the continent.”

“Can it be done?” asked Mr. Damon.

“I think so,” Tom answered. “I plan now on making one trip each way every week. There will be three laps of approximately one thousand miles each. Figure five hours to a lap, that would mean a flying rate of two hundred miles an hour—not at all impossible. We’ll charge a fare of one thousand dollars each way. There’ll be money in it, Mr. Damon. Do you want to go in with us?”

Instead of answering Mr. Damon rose and tiptoed his way softly to the door, where he stood intently listening.