Tom Swift, in a crisis, was not one to think first and act afterward. Usually he did his acting first, and this was one of those instances.

Like a flash of fire it ran through his mind that now was the best time to ascertain what object the foreigner could have in breaking the regulations and entering Tom’s private office.

With the end in view of settling the matter then and there, Tom dashed across the room and out of the rear door by which Barsky had left. The young inventor had a glimpse of the Russian hastening along just ahead of him. He was making good time, too. But Tom Swift, too, was a sprinter. In spite of all the machines for locomotion that Tom had invented, he could still run.

He caught up to Ivan Barsky and seized that individual by the arm.

“Hey! Wait a minute!” cried Tom, not very dramatically, perhaps, but effectively.

“Eh? What eet iss?” The man seemed to hiss the words in his peculiar way. There was a frightened look on his face and, also, one of innocence, real or assumed. “You weesh to see me, Mr. Swift?” asked Barsky.

“It might be the other way around,” announced Tom grimly as he faced the man. “It rather looks as if you wanted to see me—going into my office when I wasn’t there. Don’t you know that’s against the rules?”

“Pardon—I did not know eet.”

“Well, it is! I’m telling you now! No one from the works allowed in the private office without permission. Now, why did you go there—especially when I was out fighting the fire, which was in your department?”

“Fire? Yes, I hear the wheestle, but I see that so soon the fire he is out so I cannot think that it to a great deal amounts. I hope none of my new patterns were burned.” He seemed genuinely anxious on this score.

“Luckily the fire was only in some rubbish,” explained Tom. “But that has nothing to do with the fact that you were in my office in my absence. What did you go there for?” and the voice of the young inventor was stern.

“Oh—I my presence must explanation? Yes, of courseness. I go there to get heem!”

He held up to Tom a complicated slide rule, often used in making intricate computations that, otherwise, would entail much work at figures.

“I need these for that train stop pattern I am making for you, Mr. Swift,” the man explained.

“Yes, I thought you would need that rule, and I intended sending it out to you,” Tom said. “But you should not have gone to my office to take it while I was absent.”

“Pardon! I did not know you were not at home. Also did I not know of the prohibition against entering the office when you were not in. I saw the rule—I need heem very much to make the pattern of finished appearance. So I take the rule and hurry back to my bench.”

“You hurried all right—I’ll say that,” murmured Tom grimly.

“Yes—I am of a hurry kind. Most of us Russians are. Now that I have of explanation made, eet is permit that I take these?” and he again held up the rule.

“Oh, yes, I suppose so,” conceded Tom slowly. He could not say anything else, it seemed. There was such an air of innocence about the man that to break it down, if it were pretended, required better evidence than Tom at this moment possessed.

Certainly Barsky had taken nothing more than the slide rule, and he needed it in his work. Tom felt that the man should have had it before, for it was the most essential tool. How then, could he find fault with the foreigner for taking what, obviously, was within his right? Of course he had broken a rule, but this might be overlooked.

So Tom had to say it was all right and let the matter go at that.

“Only don’t go into my office again without permission,” concluded the young inventor.

“Naturally not—since you have told me,” said Barsky. “I shall send word in advance of my veesit next time,” and with a bow he took himself off.

“Hum,” mused Tom as he strolled back to his own quarters. “Now was that sarcasm or not?”

He thought he was perhaps rather exaggerating the importance of the incident, and was somewhat put out by the construction Barsky might put on being thus reprimanded until Tom happened to look in the room where the chest of secrets was kept.

Not only was this room open, but so, also, was the chest itself and in plain view were a number of valuable plans and drawings.

“Hang it all!” exclaimed Tom. “I wonder if that chest was open while Barsky was in here after that rule? It must have been. Some one is getting careless; but as it may have been I who opened that box and forgot to close it when the fire whistle blew, I’d better not say anything about it. Ned or Mr. Newton might think I was hitting at him. Yes, I’ve got to be more careful, with strange workmen about and valuable new plans under way.”

Tom looked carefully over his chest of business secrets, and, as far as he could ascertain, nothing had been disturbed. He knew in a general way what was in the box.

“But I think the safest way to do,” he told, himself, “would be to have Ned make a list of all the documents in there. Then we can check them over from time to time. I’ll do that.”

So, without saying anything to his chum about the visit of Barsky, Tom told the young manager to list all the documents in the chest after attending to the fire insurance matter.

“We can’t be too careful of these invention papers,” remarked the author of many of them.

“You’re dead right you can’t,” agreed Ned.

The fire was but a small incident in the day’s work at the Swift plant. Fires in such a big shop were not at all uncommon; so matters were soon running smoothly again.

During the next two or three days Tom paid several visits to the pattern department, and each time he went there he found Barsky busily at work, using the slide rule with an expertness that gave Tom a good opinion of the fellow’s ability in his own line of work.

“I’m glad I hired him—that is, so far as getting what I wanted done,” Tom said to his father. “He makes patterns better than any man we ever had.”

“Yes, I have looked at some of the models he has turned out,” Mr. Swift replied. “But I can’t get over a certain uneasy feeling when I am near that man, Tom. It’s a sort of fear, I believe.”

“Nonsense!” laughed the young fellow. “You are imagining things, Dad. Barsky is no ten-thousand-dollar beauty, I grant you that, for he has enough hair and whiskers for a dozen sofa cushions. But we aren’t running a beauty parlor, and he does do good work.”

“Yes, Tom. Well, I’ll leave it to you. It’s more in your department than mine, anyhow. By the way, is there anything new in the case of Mr. Newton? We must do all we can to help him.”

“We will, Dad. I don’t know that there are any new developments. I’ll ask Ned. But Plum has orders to look after that case to the best of his ability. Now I suppose you want to get back to your book, don’t you?”

“If you don’t mind, Son,” answered Mr. Swift, with an anxious look toward his desk which was littered with papers. The talk had taken place in the older inventor’s private office.

“All right, I’ll leave you with your pet,” laughed Tom, for the new book was Mr. Swift’s chief joy and pride. “But I’d like to have a talk with you this evening, Dad.”

“By all means, Tom. I’ll be free then. Come and chat. But just now I want to finish that chapter on hydraulics. I find it most fascinating, and I am using some of the data you evolved when you built the big submarine.”

“Yes, I fancy we discovered a new principle or two there,” answered Tom, with justifiable pride. “And you’re welcome to quote me at any length you wish, Dad,” he finished, with a laugh.

“All right!” chuckled the old inventor. “You’re somewhat of an authority, Tom, on a few subjects.”

Mr. Swift plunged into his literary work before Tom had reached the door on his way out, so eager was he to resume work on his book, and Tom was glad his father had something of this sort to interest him and give him an object in the declining years of his life.

“Tom, what say, we go to a ball game this afternoon?” suggested Ned when the young inventor returned to his office. “It’s too nice a day to work.”

Tom glanced out at the sunlight dancing over the green grass. He looked at his paper-laden desk and then at the clock.

“Ned,” he suddenly exclaimed, “I’m with you! And if you weren’t already drawing a bigger salary than you’ll ever be worth, I’d give you an advance for the valuable suggestion. How about you, Mr. Newton? Will you come to the game?” he went on, thinking to take the man’s mind from his trouble over the missing Liberty Bonds.

“Thank you, Tom, no. There are a few things I want to get straightened out on my books.”

“Better let them go for a time,” suggested Tom. “You’ll do better work afterward. That’s the way I feel about it. I’m in a sort of maze on this train-stop device. I’m up against a stone wall. So I’m going to a ball game and I order you two to come with me!”

“Oh, if it’s an order from the boss—that’s another matter,” laughed Mr. Newton. So, Tom, having seen to it that his chest of secrets was securely locked, brought around the electric runabout and all three went to the ball game.

“Is there anything new in your case, Mr. Newton?” asked Tom, as they sat in the grandstand, watching the players warm up.

“No, Tom, not a thing. I am leaving everything to Mr. Plum, as you suggested. He hasn’t reported anything new.”

“Has that fellow, Fawn, or any of the firm pestered you about the missing bonds?”

“No. But Fawn sneers every time he sees me. I fancy he is much put out that I am not in jail.”

“You needn’t worry about that,” laughed Ned, though in his heart he keenly felt the disgrace that had come to his father.

“Is this seat taken?” asked a young lady’s voice behind Tom, and he turned to look into the smiling face of Mary Nestor.

“Of course not!” he exclaimed, rising, as did his two companions. “Always room for one more. I didn’t expect to see you here, Mary,” the young inventor added.

“And you didn’t think of inviting me, I suppose,” said Mary demurely. “I think I will sit by Ned.”

“If you do I’ll discharge him, and then he won’t like you,” said Tom, as he made room beside himself for the girl to whom he was engaged. “I’m awfully glad you’re here. We came in such a hurry—on the inspiration of a spontaneous suggestion by Ned—that I never thought to ’phone over and see if you wanted to come.”

“Oh, all right—I’ll forgive you!” the girl laughed.

But there was a quick exchange of looks between Mary and Ned. In fact, Mary had suggested privately to Ned that he bring Tom to the game, as she intended to meet him there. Often it was necessary to use a ruse to get Tom away from his absorbing work of inventing things.

However, here he was, and soon he was enjoying the game with his friends. It was not much in the way of a ball contest, but it served to pass away the afternoon and change the current of all their thoughts.

That evening after supper, Tom and his father drew their chairs together in the living room for one of their old-time chats. Mr. Swift was in a pleasant mood, for he had done some work on his book that afternoon which gratified him very much and about which he was enthusiastic.

“Tom, how are things going?” he asked his son.

“Very nicely, indeed,” was the answer. “Of course, I am not making as much progress as I’d like to on that train-stop device, but it will come. And when it does, I think it will be worth good money to us.”

“Yes, that, and some of the other inventions we have under way, will be worth sixty thousand dollars, I think, when they are completed.”

“Well, that estimate may be a bit high,” returned Tom slowly.

“No!” protested his father. “I put it low. I think they’ll run over sixty thousand dollars. Why, that mill machinery idea of yours——”

“Hark!” exclaimed Tom softly, holding up a hand to caution silence.

“What is it?” asked his father.

“I heard some sort of a noise outside on the porch.”

Tom arose to go toward one of the windows, the shades of which were drawn down. But before he reached the casement Eradicate came hurrying into the room, very much excited.

“What’s the matter, Rad?” asked Tom quickly.

“Somebody on de piazzy listenin’ at de window!” exclaimed the colored man. “I jest been to see dat de henhouse was locked up, an’ I done seen somebody on piazzy! I runs up, but he runs away an’ I didn’t cotch him! But dey was somebody listenin’!”

Once again Tom Swift had occasion to act quickly, as he had when he discovered Barsky in his private office. Without waiting to question Eradicate, the young inventor made a dash for the door. A moment later he was outside, looking for a sign or trace of the mysterious intruder.

But the night was dark, there was no moon; and Tom, coming from a lighted room, was at a greater disadvantage than otherwise would have been the case. It was not until several seconds had passed that the young man could make out objects in the gloom, and none of these objects was the unknown person he sought.

Still Tom was not going to give up so easily. He shouted to Koku, and when the giant joined him and when Eradicate had come shuffling out the three began a hurried search around the house for the secret listener.

“Well, I guess he got away,” sighed Tom. He was greatly disappointed when it was evident that no one was to be caught.

“He suah has,” agreed Eradicate.

“What sort of man was it?” asked Tom. “I suppose it was a man, Rad, and not a woman?”

“Oh, yes, Massa Tom! It suah was a man aw right! An’ he was a sorta big man, but stoopin’ ober like, leanin’ down an’ tryin’ to look under de window shade.”

“Why didn’t you jump on him, Rad?”

“Dat’s jest what I was tryin’ to sneak up an’ do, Massa Tom. But I done sneezed jest when I got by de piazzy, an’ he done hears me an’ jumps away.”

“By golly! I cotch him ef I been dar!” cried the giant. “I take ’um in my two hands—so. I squoze ’urn—so!”

Truly there would have been but little chance for the trespasser had Koku got his hands on him.

“Ha! Ha!” chuckled Eradicate. “He go so fast, Giant, dat he make you look like a worm crawlin’ along!”

The giant was not very fast in his movements, and he realized this. Still he did not like to be taunted about this failing, and he now made a reach for Eradicate, who, however, easily eluded his enemy.

“Come now! None of that!” ordered Tom sharply. “I guess this fellow, whoever he was, might have been only a tramp. Take a stroll around for a while, Koku, and see if he comes back.”

“Sure, Master, I watch.”

“An’ I’ll be on the lookout, too,” offered Eradicate.

“Look around, both of you, and don’t scrap. Do you hear? Don’t scrap between yourselves. I’m getting tired of it!”

“I be good to Rad, Master,” promised the giant.

“Huh! You better, ef you all knows whut good fo’ you!” chuckled the colored man.

Tom went back to his father, finding the old gentleman somewhat nervous and upset.

“What was it, Son?” he inquired.

“Oh, just some tramp, I think,” replied the lad easily. But in his own mind he was disturbed by the incident.

“I wonder if that was Barsky or some of our old enemies?” mused Tom to himself, when he had gone to his own room after some further talk with his father concerning the various inventions and patents. “There’s always a chance of some of the old gang trying some of their tricks.

“But what object could any one have in listening to the talk between dad and me? We only mentioned generalities, and what we said would have been of no value to any one. Still, whoever was listening didn’t know that. They may have hoped to pick up some information that could be used against us.”

But though he was worried and a bit apprehensive, Tom did not dwell long on this phase of the matter. He knew it would do little, if any, good.

Still he was not so foolish as to omit all precautions, and a little later he went outside to see if Koku or Eradicate had seen any further signs of the intruder.

Both the giant and the colored man reported that they had seen no one, and Tom sent them to bed, after giving orders to see that the house was well locked for the night.

The shops, some distance away, were well guarded by watchmen, as well as by a system of electrical burglar alarms, a bell of which was set up in Tom’s room. No one could get in or over the high fence which surrounded the works without disclosing his presence.

Tom was a little apprehensive that in the night the alarm bell might ring, telling him that some one was trying to sneak into the factory. However, the hours of darkness passed uneventfully and with the coming of morning Tom’s fears passed away.

He went out to look around the porch on which the mysterious listener had crouched beneath the window, but there were no clews that he could follow.

“I’ll just have to let things take their course,” decided Tom. He reported the matter to Ned, as he generally did in such cases.

“I wouldn’t worry about it,” said Ned cheerfully.

“I’m not,” declared Tom, but his manager could see that he was and that he could not get his mind down to work. At last Tom himself became aware of his nervous condition, and shortly after noon, throwing his pencil down on his desk, he exclaimed:

“I’m going out for a ride, Ned. Come along. It will clear the cobwebs out of our brains.”

“Going in the runabout?” asked Ned.

“No, in the little biplane. She’s just been tuned up and runs like a sewing machine. We’ll take a spin up in the clouds.”

Ned considered for a moment, looked at a mass of papers on his desk, and answered:

“No, thank you, I’d better not go. Besides, Mr. Plum just telephoned that he’d like to see me about dad’s case, and I want to run over there.”

“All right,” agreed Tom. “Take all the time you want, Ned. But I don’t want to go skylarking alone. I think I’ll ’phone Mary. I haven’t had her up in the biplane for some time.”

“Yes, you and Mary go for a spin,” replied Ned. “Only take care that it isn’t a tail-spin.”

“The Hummer doesn’t indulge in such antics,” replied Tom, with a laugh. “But first I’d better see if she’ll tune up as she did the other day. I don’t want to get Mary out in her and then have something go wrong. I’ll just run her out, Ned, and give her a warming up. Then, if everything’s all right, I’ll call Mary. You can look after things here, I suppose?”

“Yes, either dad or I will. He’ll be back soon, and then I’ll take a run over to Plum’s office.”

Tom owned several planes, but of the two-winged variety the one he best liked was the Hummer, a small but speedy craft. This machine was kept in a hanger near the flying field, and it did not take the young inventor long to have her run out that he might test the engine.

“She sure sounds sweet, Mr. Swift,” remarked the mechanician who assisted him.

“Yes, Dirk, she’s throbbing like a wild duck. I guess I’ll take her up for a time.”

Going back to the office Tom reported to Ned that everything about the Hummer was in good shape and that he would probably be gone for the remainder of the day. As there was no good landing place near the home of Mary Nestor it was necessary for her to come to Tom’s flying field, and while he was getting ready he sent one of his men for the young lady in an automobile. But first Tom called her on the telephone.

“Oh, Tom!” exclaimed Mary, as soon as she found who was on the wire, “I was just going to call you up!”

“You were?”

“Yes, it’s awfully important! I want to see you very much!”

“Well, you’ll have that pleasure directly.”

“Oh, Tom, don’t joke about it! It is important.”

“What’s the trouble, Mary,” the young inventor asked, seriously enough now.

“I’ll tell you when I see you—soon!”

Tom told the girl of the proposed trip and Mary hung up the receiver while Tom wondered at the note of anxiety in his sweetheart’s message.

The young inventor was out at the flying field in his air togs going over every detail of his machine when Mary Nestor arrived in the auto driven by a messenger Tom often employed.

“Hello!” called the girl, as she leaped out of the car. She, too, wore a leather flying suit, for Tom had told her to put it on.

“Hello,” responded Tom. “All ready?”

“Yes. But I want to talk to you, and we can’t do it very well with the engine going. It makes such a noise.”

“This one doesn’t make as much racket as some do, for I’ve got a muffler on her,” Tom replied. “But still you can’t exactly carry on a whispered conversation in the Hummer.”

“This conversation is going to be—well rather private,” returned Mary in a low voice, with a glance toward the man who had brought her over in the car.

“We might start off, land in some lonely field, and talk there,” Tom suggested. “I could throttle down the engine just enough to keep her turning over, and yet not running enough to make the plane even taxi.”

“Why can’t you do that here?” asked Mary. “I want to tell you what I have to say, Tom, at once, as you might want to act on it.”

“Well, I can have him turn the propellers and get her started,” answered the youth, with a glance at his helper. “Then, with the engine idling, you can tell me the story while he goes back in the auto.”

“Do that,” suggested the girl. “I want to get it off my mind.”

Accordingly, while Tom vainly wondered what his sweetheart could have to say to him of such importance, he took his place in the forward cockpit, in charge of the control levers, while the man stationed himself at the propellers.

The Hummer was comparatively easy to start. After the engine had been turned over once or twice, with the accompanying coughs and sighs, it started with a thundering roar that made the ground throb. Tom let it run until it was well warmed up. Then, knowing it would keep going at low speed without moving the plane, he throttled the gas down, adjusted the spark, and signed for his helper to leave.

“Now, Mary, I’m ready to hear your story,” he said as he walked with his friend a short distance away from the Hummer.

“It’s a queer story,” said the girl. “And as soon as I heard it I started to call you on the ’phone. I was just going to take down the receiver when you called me.”

“When did you hear this—whatever it is?” asked Tom, who was becoming more and more mystified by Mary’s evident concern.

“Just this morning,” she answered. “I was over in Mansburg doing some early shopping with Kate Borden. Shopping always makes me terribly hungry, as it does Kate, so about noon we went into a small restaurant for lunch.”

“And I suppose you had mislaid your money and couldn’t pay, and you had to blush and ask the manager to trust you, and now you want to go there in the plane and settle your debts. Is that it?” asked Tom, with a laugh.

“No, and if you make any more fun of me I sha’n’t tell you a thing! So there, Tom Swift!” and Mary pouted bewitchingly.

“Mercy! I’ll be good!” he promised.

His sense of humor was rudely shaken a moment later as Mary went on:

“While Kate and I were eating our lunch three men were eating at the next table—eating and talking. We didn’t purposely listen—that is, not until after I heard one of them mention the Swift Construction Company. But then, as you can imagine, Tom, I was all ears. I shamelessly listened after that, and though I didn’t hear all that was said I caught enough to know that they were talking about something like a tidal engine. Is there any such thing, Tom?”

“Is there, Mary? I should say there is! It’s one of my latest and best inventions! I believe I can harness the ocean with it—at least, a part of the tide. But go on—what did they say about the tidal engine?”

“One of the men seemed angry that you hadn’t sold it to them. He spoke of Mr. Damon and said it was too bad—or words to that effect—that Mr. Damon’s negotiations had fallen through.”

“Go on,” urged Tom, as Mary hesitated a moment. “This is interesting, and it may be vitally important. Go on!”

“Then they spoke something about mill machinery,” resumed the girl. “I couldn’t get that very plainly—I don’t know much about mechanics—but they spoke of a turbine grinder. Is that right?”

“That’s right!” exclaimed Tom. “But it will be all wrong if they get on to any of my plans in that respect. I’m mighty glad you listened to this talk, Mary! Who were the men—I mean what did they look like?”

“I’ll describe them to you as well as I can. I had never seen any of them before, as far as I know. The whole trend of the conversation was to the effect that they had tried unsuccessfully, through Mr. Damon, to get you or your father to sell them some or all of the rights in these inventions. Is that the case?”

“Yes. Mr. Damon came to me some time ago—the day he landed on the roof in his little plane—and wanted me to consider negotiations. But I sent word by him to these fellows, who were represented by Mr. Blythe, not to bother, for I wasn’t in the market.”

“I didn’t hear Mr. Blythe’s name mentioned,” said Mary, knitting her forehead into a series of wrinkles as she tried to recall all the details of the affair. “But there was some one whose name began with B—let me see—I wrote it down.”

She fumbled in her pocket and brought out a slip of paper on which she had written one word—Blodgett.

“That’s the man, Tom,” she said. “Mr. Blodgett. One of the three who were talking near our table remarked: ‘Never mind. I think Blodgett will fix it.’ Those were the words he used.”

“Hum,” mused Tom. “Blodgett—and he will fix it. Fix what, I wonder?”

“That I can’t say,” answered Mary, for Tom had spoken aloud. “Right after one of the men said that, all three went out. I didn’t know what to do. I kept wishing you had been there. But I made up my mind I’d tell you about it as soon as I could.”

“Yes, Mary. Thanks! I’m glad you did. It’s all a mystery to me.”

“What do you think it means?”

“That would be hard to say. I’ll have to admit I’m a bit worried about it, in view of several things that have happened at the shop lately.”

“Oh, Tom do you think there is any danger?”

“No more than usual. There’s always danger when you have rivals. But I never heard of this Blodgett that I know of. As for the other matters: As I said, Mr. Damon opened the subject but I told him to head off any visit of the men to me, for I wouldn’t do business with them. And from the fact that they haven’t called on me, I took it that they had dropped the matter.”

“It doesn’t seem so, though, does it?” asked Mary.

“I should say not! I don’t like this at all!” Tom seemed anxious and upset over the matter. “And what I particularly don’t like is the way they said Blodgett would fix it. Is that the word they used?”

“Yes. It was ‘fix,’ I’m sure of it.”

“Smacks of desperation,” commented Tom. “I wonder if the owner of the restaurant would know those men, Mary?”

“He might.”

“Then I’m going to drop in and have a talk with him. Give me the address. Oh, I don’t mean I’m going to drop in off the Hummer and let you run the machine alone,” he went on with a laugh, as he saw Mary’s momentary gasp of surprise. “I’ll go over and see him to-morrow. Just now we’ll go for a ride. I need a little free breathing space in the upper air.”

“Yes, it’s a wonderful day for a ride, Tom. And there’s no sign at all of rain.”

“We need rain, too,” said the young inventor. “The woods and fields are as dry as tinder. If a forest fire should start now it would do a lot of damage. But as long as it hasn’t rained for some time, we’ll hope it will hold off until we get back from our spin. Come on—let’s go!”

With a roar the motor accepted the additional gas Tom turned into the cylinders, and a moment later the little plane began to move over the smooth surface of the field. Gathering speed, the Hummer slowly rose as the young inventor depressed the horizontal rudder, and a moment later up rose the machine like some creature of life—up and up toward the clouds.

“This is glorious!” cried Mary, thrilled by the sensation. Riding in a plane was not new to her, but she never failed to get a sense of exhilaration out of even a short spin in the air.

“Not so bad,” answered Tom.

By raising their voices slightly they could make themselves audible to one another, for, as the young man had said, there was a silencer, or muffler, on his engine.

“It makes one forget all their trouble,” called out Mary, as she looked over the side of the rear cockpit where she sat strapped in and glanced down at the earth rapidly dropping away below them.

“Yes, it does,” assented Tom. “That’s one reason I wanted to come out to-day—to get rid of some of the cobwebs.”

“And are they being brushed away?” asked Mary.

“Almost all gone!” he laughed, as he sent the Hummer up at a little steeper angle to gain a higher altitude more quickly.

The two young people gave themselves up to the thrill and revivifying influence of clear, pure, sunlit air. Deeply they breathed in of the life-giving particles, and the cheeks of Tom and Mary were ruddy with renewed health.

With no special object in view, they spun on through the air, now going up until they were above some low-lying clouds and again dipping down to view with pleasure the contour of some wonderful, green valley.

“Getting tired, Mary?” called Tom, after a while.

“No!” she called back to him. “I could go on like this forever.”

“Guess I’ll have to invent some new kind of machine if you want to do anything like that,” the youth countered.

“What do you mean?” challenged Mary.

“I mean perpetual motion hasn’t yet been solved, and I don’t believe it ever will be. As long as we have gasoline engines they will have to be given a drink now and then. Which reminds me that I haven’t enough in the tanks of the Hummer to go on for more than a few hours more.”

“I don’t want to ride quite that long, of course! Don’t take any chances. Go back now if you think anything is going to happen.”

“Nothing is likely to happen!” chuckled the young inventor. “But I didn’t want you to go on dreaming that dream of yours about keeping on forever.”

“As if I meant that!” laughed the girl. “Better turn back now.”

“In a little while,” promised Tom, whose eyes were just then fixed on some object or some view just ahead. Mary was quick to notice his preoccupation as he spoke to her and at once asked:

“Is anything the matter, Tom?”

“Matter? No! Why do you ask?”

“Because you weren’t thinking of what you were saying, that’s all. I can always tell. Do you see anything?”

“To be perfectly frank, Mary, I do. I see a cloud of smoke over there in the direction of Shopton, and when I see smoke I think of fire. As we recently had a little blaze at one of the shops, I am a bit anxious to see if this is another. Of course it will be as well fought with me away as with me there. But still——”

“Oh, Tom, I see it, too!” cried the girl, as a little puff of smoke made itself visible near a wooded part of the country. “Perhaps you’d better head back that way.”

“I think I will,” decided the pilot.

He moved the steering wheel slightly, banked the plane a bit, and was off in another direction, heading directly for the haze of smoke which by this time had considerably increased in volume.

At the time when Tom first saw the smoke menace he was several miles from it, though the clear air made the fire seem nearer than it was. But the Hummer was a speedy craft, and she quickly covered the distance.

As Tom Swift and Mary Nestor approached the blaze—for blaze it was, since they could now notice a redness that betokened flames—they could see it more plainly, and a sense of relief came to the young man when he noted that it was in a spot remote from his shops.

“Guess it’s a forest fire, Mary,” Tom observed. “I thought one would break out soon, it’s been so dry.”

“I’ve never seen a forest fire,” she responded. “It must be very thrilling.”

“It is—and dangerous,” replied Tom. “Well, you’re going to see one now, for we’re going right over it.”

“Do you think it will be safe, Tom?”

“Why not? We’re so high you won’t even smell the smoke. And as for the heat—well, they do get pretty warm, but you won’t feel that.”

“Oh, I didn’t mean that so much,” Mary hastened to say. “But if anything should happen to the plane——”

“Nothing is going to happen!” laughed Tom. “I’m going to give you a real view of a forest fire.”

The smoke was rolling up in great clouds now. But of course the Hummer was far above the conflagration. Nor could they hear the crackling of flames, though this sound was audible to those fighting the fire.

Men and boys were at work combating the forest fire. Tom and Mary, from their vantage point high in the air, could note figures, like ants or flies, hastening from the surrounding country toward the scene of the blaze. Some were hastening up in autos, and others in horse-drawn vehicles.

“What do they do when they fight forest fires?” asked Mary. “I don’t see any engines.”

“Engines aren’t of much use—the fire is very seldom near a water supply,” answered Tom. “The only thing to do is to take away the stuff a fire feeds on—dry leaves, sticks, wood or anything else. Sometimes they do this by digging up the ground or plowing it in broad, bare strips.

“Another way is to make a back fire. That is, they start some distance off from the blaze and set fire to a limited area. When this burns off, and of course it has to be kept under control, it leaves a black space with no fuel for the fire to feed upon, and when the original fire gets to the place it just naturally quits. They use wet bags, pieces of carpet, anything to beat out the line of flames when they actually fight the fire.”

“It must be hard work,” decided Mary.

“It is—hot, hard, dirty work. Well, we’ve got a pretty good view of this fire now, and I must say I don’t like the looks of it,” remarked Tom.

They were fairly over the burning area now. Below them were the snapping, leaping flames and the billowing clouds of smoke. More men and boys were hastening up to do what they could to combat the conflagration.

“If there should only come a rain now it would settle the fate of this fire,” remarked the young inventor.

Tom looked up and across the sky. It was blue in nearly every quarter—too blue and beautiful to suit those who wanted water to pour down from the heavens.

“There’s a little haze in the west,” remarked the lad, at length. “It might indicate the coming of a thunderstorm. That’s what is needed—a sudden, drenching thunderstorm. A gentle shower would help, but a regular cloudburst is needed. Of course, a sizzling rain would put the fire out in time, but it would take too long.”

“Oh, I do hope it rains!” exclaimed Mary.

Tom guided the Hummer over the very center of the fire, which seemed to be burning in an area of forest and brush country several miles square in extent. Of course in the very center of the blaze no attempt was being made to fight it; that would have been too dangerous. It was on the edges of the ever-increasing circle that the men and boys were making the attack.

Dropping down a bit, so Mary could see better, Tom pointed out where scores of the fire-fighters were trying to beat the flames to earth with long-handled wet bundles of rags, which from time to time they dipped in brooks and ponds.

“It looks like a losing fight,” sighed the girl. “Oh, Tom, suppose the fire reaches Shopton!”

“It won’t with the wind the way it holds now,” was the reply. “But of course we’d all feel better with the fire out.”

He swung the machine around to take in another angle of the fire.

“Isn’t there anything you can do, Tom?” asked Mary. “This is terrible!”

Tom was himself wondering if he could not be of service instead of merely looking on from his vantage point of safety in the air. He had invented an aerial fire-fighting machine, but this, with its chemical bombs, had been disposed of, and there was none on hand at Shopton.

“I might drop some army bombs down and try to blow up a big, bare area which would bring the fire to a stop,” said Tom. “But it would take some time to get ready for that, and they may have it out in a few hours. No, I’m afraid I can’t do anything just now. But we had better——”

Tom was about to say he had better start back with Mary when, with a suddenness that was startling, the motor of his plane went dead and the machine began to drop toward the heart of the forest fire.

“Oh, Tom!” cried Mary. “Don’t go down so close! It’s dangerous!”