"What happened?" cried Jackson to Tom, as he leaned forward in his seat which was in the rear of the young inventor's.

"Don't know, exactly," was the answer, as Tom quickly shifted the rudders to correct the slanting fall of his craft. "Sounded as though there was a tremendous back-fire, or else the muffler blew up. The engine is dead."

"Can you take her down safely?"

"Oh, yes, I guess so. She's a bit out of control, but the stabilizer will keep her on a level keel. Good thing we installed it."

"You're right!" said Jackson.

Now they were falling earthward with great rapidity, but, thanks to the gyroscope stabilizer, the "side-slipping," than which there is no motion more dreaded by an aviator, had nearly ceased. The craft was volplaning down as it ought, and Tom had it under as perfect control as was possible under the circumstances.

"We'll get down all right if something else doesn't happen," he said to Jackson, with grim humor.

"Well, let's hope that it won't," said the mechanic. "We're a good distance up yet."

They were, as a matter of fact, for the explosion, or whatever had happened to the craft, had occurred at a height of over two miles, and they at once began falling. As yet Tom Swift was unaware of the exact nature of the accident or its cause. All he knew was that there had been a big noise and that the engine had stopped working. He could not see the silencer from where he sat, as it was constructed on the underside of the motor, but he had an idea that the same sort of mishap had occurred as on the occasion when the test machine had sailed through the roof of his workshop.

"But, luckily, this wasn't as bad," mused Tom. "Anyhow the motor is out of business."

And this was very evident. The young inventor had tried to start the apparatus after its stoppage by the explosion, but it had not responded to his efforts, and then he had desisted, fearing to cause some further damage, or, perhaps, endanger his own life and that of Jackson.

Down, down swept Silent Sam--doubly silent now, and Tom began looking about for a good place to make a landing. This was nothing new for either him or his mechanician, and they accepted the outcome as a matter of course.

"Not a very lively place down there," remarked Jackson, as he looked over the side of the cockpit.

"If we have to depend for help on any one down there, I guess we'll be a long time waiting," agreed Tom. They were about to land in a very lonely spot. It was one he had never before visited, though he knew it could not be much more than twenty miles from his own home, as they had not flown much farther than that distance.

But, somehow or other, Tom had not visited this particular section, and knew nothing of it. He saw below him, as Jackson had seen, a lonely stretch of country--a big field, once a wood-lot, evidently, as scattered about were some stumps and some second growth trees. There were also a number of evergreens--Christmas trees Jackson called them. And this was the only open place for miles, the surrounding country being a densely wooded one. There did not appear to be a house or other building in sight where they might seek help.

"But maybe we can make the repairs ourselves and keep on," the lad thought.

With practiced eye he picked out a smooth, grassy, level spot, in the midst of scattered evergreen trees, and there Tom Swift skillfully brought his Air Scout to rest. With a gentle thud the rubber-tired wheels struck the Earth, rolled along a little distance, and then called to a stop.

Hardly had the aeroplane ceased moving when Tom and his companion jumped out and began eagerly to examine the machinery to see the extent of damage.

"I thought so!" Tom exclaimed. "The silencer cracked under the strain. Those exhaust gases have more pressure that I believed possible. I increased the margin of safety on this muffler, too. But she's cracked, and I can't use the machine until I put on a new one. Good thing I didn't ask for a government inspection until after this trial flight."

"That's so," agreed Jackson. "But can't you patch it up, or go on without a muffler, so we can get back home?"

"I'm afraid not," Tom answered. "You see I removed all the old exhaust pipe fittings when I put on my new silencer. Now if I took off my attachment there wouldn't be anything to carry off the discharged gases, and they'd form a regular cloud about us. We couldn't stand it without gas masks, such as they use in the trenches, and we haven't any of those with us."

"That's right," agreed Jackson. "Well, what do you want to do? Have me stay here and guard the machine while you go for help? Or shall I go?"

"I don't know why we both can't go," said Tom. "There is no use trying to patch up this machine here. I'll have to send a truck after it, and dismantle it before I can get it home.

"As for either of us staying here on guard, I don't quite see the need of that. This looks like the jumping-off place to me. I don't believe there's a native within miles. I didn't see any houses as we came down, and I think Silent Sam will be perfectly safe here. No one can run off with him, anyhow. He'd be as hard to start as an automobile with all four wheels gone. Let's leave it here and both walk back."

"All right," agreed Jackson. "That suits me. Might as well leave our togs here, too. It will be easier walking without them," and he began taking off the fur-lined suit, his cap, and his goggles, such as he and Tom wore against the piercing cold of the upper regions.

"We can stuff them in the cockpit and leave them," went on the mechanician, as he divested himself of his garments. As he stowed them away in his seat he gave one more look at the broken muffler. As Tom Swift said, his new silencer had literally blown up, a large piece having been torn from the gas chamber.

Something that Jackson saw caused him to utter an exclamation that brought Tom Swift to his side.

"What is it?" asked the young inventor.

"Look!" was the answer. "See! Just at the edge of that break! It's been filed to make the metal thinner there than anywhere else. You didn't do that, did you?"

"I should say not!" cried Tom. "Why, to file there would mean to weaken the whole structure."

"And that's exactly what's happened!" declared Jackson, as he gave another look. "Some one has filed this nearly throughÄleaving only a thin metal skin, and when the gas pressure became too much it blew out. That's what happened!"

Tom Swift made a quick but thorough examination.

"You're right, Jackson!" he exclaimed. "That was filed deliberately to cause the accident. And it must have been done lately, for I carefully inspected the silencer when I put it on, and it was in perfect order. There's been spy work here. Some one got into the hangar and filed that casing. Then the accumulated pressure of the gases did the rest."

"As sure as you're alive!" agreed Jackson. "Maybe that's what Gale did when he called."

"No," returned Tom, shaking his head, "he didn't get a chance to do anything like that. I watched him all the while. But perhaps this is what he referred to when he said he and his company would repudiate any act of that spy with the gold tooth--Lydane, so Gale said his name was. Maybe that's what Lydane did."

"He was capable of it," agreed the mechanic, "but he couldn't have done it that time you tripped him into the mud puddle. This silencer wasn't built then."

"No, you're right," assented Tom. "Then he must have been around since, doing some of his tricky work!"

"I don't see how that could have been," said Jackson slowly. "We've kept a very careful watch, and your shop has been specially guarded."

"I know it has," said Tom. "There couldn't much get past Koku; but some one seems to have done it, or else how could that filing have been done?"

Jackson shook his head. The problem was too much for him. He looked carefully at the exploded and broken silencer, and Tom, too, gave it a critical eye. There was no doubt but that it had been filed in several places to weaken the structure of the metal.

"When did you last see that it was in perfect condition?" asked Jackson.

Tom named a certain date.

"That was just before Gale called," observed the mechanician. "He might have known of it."

"I wish I'd known of it at the time," said Tom savagely. "He wouldn't have gotten away as easily as he did. Well, there's no use standing here talking about it. Let's get back to civilization and we'll send back one of the trucks. Luckily I have another silencer I can put on for the government test. This one will never be of any more use, though I may be able to save some of the valves and baffle plates."

Slowly they turned from the disabled aeroplane and started to look for a path that would lead them out of the lonely place. Tom as the first to strike what seemed to be a cow path, or perhaps what had been a road into the wood lot in the early days.

As he tramped along it, followed by Jackson, the young inventor suddenly stopped, as he came to a sandy place, and, stooping over, looked intently at some queer marks in the soil.

"What is it?" asked the mechanician.

"Looks like the marks of an automobile," said Tom slowly. "And I was just trying to remember where I'd seen marks like these before."