Slowly the dawn broke through the mists of darkness, and made the earth light. The sun came straggling in through cracks in the shutters in the home of Mr. Nestor, the gradually increasing gleam paling the electric lights, in the glare of which Tom Swift, Mary, and her aunt sat, waiting for some word of the missing man. But none came.

"What shall we do now?" asked Mary, as she looked at Tom.

"Oh, there's lots to do," he said, trying to make his voice sound cheerful. "We'll be busy all day. I sent word to have one of my touring cars ready to hurry to any part of the country the moment we should get word from your father."

"And do you think we shall get word, Tom?" the girl went on wistfully.

"Of course we shall!" he cried. "Word may come in at any time. Now get ready, eat a good breakfast, and then you can go with me as soon as we hear anything definite. Come, we'll have breakfast!"

"I can't eat a thing!" protested Mary.

"Oh, yes you can," said her aunt, who was a cheerful sort of person. "I'll see about getting something for you and Mr. Swift, and see that your mother is all right."

She left the room to give orders to the servant about the meal, and returned to say that Mrs. Nestor was sleeping quietly. She had been given a sedative. Mary managed to eat a little, and she gave Tom the address of several friends who were called up in the vain hope that, somehow, Mr. Nestor might have gone to see them.

"Tom, what do you really think has happened?" asked Mary again, as they sat facing one another in the library, during a respite from the telephone.

Tom Swift repeated, to the girl his theory of what had happened with an assumption of confidence he did not altogether feel.

His prediction of a speedy end to the suspense did not come true that day, nor for many days. No news was heard of Mr. Nestor. After the first day, when there was no information and when no reports came of any one of his description having been hurt in an automobile accident or having been taken to any hospital, the police started an energetic search.

The authorities in all near-by cities were notified, and all thought of keeping from the public what had happened was given over. Tom's story, of how he and Mr. Damon had heard the cry for help on the lonely meadow, was printed in the papers, though the young inventor did not say that he had been out trying his new aeroplane. That was a detail not needed in the finding of Mr. Nestor.

But Mary's father was not found. The mystery regarding his disappearance deepened, and there was no trace of him after he had left Tom's house that eventful evening. Persons living along the roads he might have taken in riding his bicycle were questioned, but they had seen nothing of him, nor were they aware of any accident. Tom's testimony and that of Mr. Damon was all the clew there was.

"I don't believe he's dead!" stoutly declared the young inventor, when this dire possibility had been hinted at. "I believe the persons who were responsible for the accident are afraid to reveal his whereabouts until he recovers from possible injuries. You'll see! Mr. Nestor will come back safe!"

And, somehow, though her mother was skeptical, Mary believed what Tom said.

The search was kept up, but without result, and Tom aided all he could. But there was not much he could do. The police and other authorities were at a total loss.

In the intervals of visiting Mary and her mother, and doing what he could for them, Tom worked on his new motor. He knew that he was on the right track and that all that was needed now was to make certain refinements and adjustments in the apparatus he had already constructed, so that it would operate more quietly.

"Absorbing the vibrations from the exhaust, caused by the exploded gases in the cylinders, does the trick," Tom told his father.

"But there is enormous pressure to overcome, Tom. You must be sure your muffler will stand the strain. Otherwise she is going to blow out a gasket some day, when you least expect it. Then the sudden resumption of pressure outside the cylinders is going to cause a change in the equilibrium, and you may turn turtle in the air."

"I've thought of that," said Tom. "At worst it can't be any more than looping the loop. But I'll make the muffler doubly strong."

"Better provide an auxiliary chamber to take care of part of the exhaust in case your main apparatus breaks," advised the older inventor, and Tom said he would. He did, too, for he valued his father's expert advice.

Meanwhile he was busy fitting one of his latest aeroplanes with the new motor. The motor he and Mr. Damon had used in their flight was one patched up from an old one. But now Tom was working on a complete new one, made after his revised model, and in which the silencer was an integral part, instead of being built on.

While giving Mary and her mother all the assistance in his power, Tom still found time to work on his new, pet scheme. He had matters now where he did not fear any tampering with his plans, for he had filed away his papers in a safe place, and was making his new machine from memory.

"But if some one got in and had a look at the inside of your silencer he could see how it is constructed, couldn't he?" asked Ned Newton.

"Yes," assented Tom, "But they're not going to get in very easily. Koku sleeps in the experiment shop now, and my machine is there."

"Oh, well that explains your confidence. I feel sorry for the burglar who makes the attempt, once Koku wakes up. Heard anything more from those Universal people?"

"No, not directly. I understand they are working hard on some new type of plane for army use, but I haven't bothered my head about them. I'm too much occupied with my own affairs and trying to help Mary."

"Very strange about Mr. Nestor, isn't it?"

"Worse than strange," said Tom. "If this keeps on, and he isn't heard from, it will be tragic pretty soon."

"He must be held a prisoner somewhere," declared Ned.

"It begins to look that way," assented Tom. "Though who would have an object in that I can't understand. He had no enemies, as far as is known, and his business affairs were in excellent shape. Unless, as I said, the persons who ran him down are, through fear, keeping him hidden until he recovers, I can't imagine what has become of him."

"Well, it certainly is a puzzle," said Ned. And Tom agreed with his chum.

It was about a week after the disappearance of Mr. Nestor that Mr. Damon came over to see Tom.

"Bless my shoe laces, Tom!" exclaimed the eccentric man, "but you are as busy as ever." For he found the young inventor in the experiment shop, surrounded by a mass of papers and all sorts of mechanical devices.

"Yes, I'm working a little," said Tom. "But you are just in time. Come on out, I want to introduce you to Silent Sam."

"'Silent Sam!'" exclaimed Mr. Damon. "Have you been taking a new trip to the Land of Wonders? Have you brought back some new kind of servant?"

"Not exactly a servant," said Tom with a laugh, "though I hope Silent Sam will serve me well."

"'Silent Sam?' What does it mean? Is that a joke?" asked the puzzled Mr. Damon.

"I hope it doesn't turn out a joke," replied Tom. "But come on, I'll introduce you to him, Mr. Damon."

He led the way to one of the big hangars where his various machines of the air were housed. On the way Mr. Damon asked about news of Mr. Nestor, but was told there was none.

Tom Swift opened the big, swinging doors and pulled aside an enveloping canvas curtain. There stood revealed a big aeroplane, of somewhat new pattern, the wings gleaming like silver from the varnish that had been applied. In shape it was not unlike the machines already in use, except that the propellers were of somewhat different design.

The engine was mounted in front, and even with his slight knowledge of mechanics Mr. Damon could tell that it was exceedingly powerful. But it was certain devices attached to the engine that attracted his attention, for they were totally different from any on any other aeroplane, though they bore some resemblance to apparatus on the plane in which Tom and the eccentric man had made the night flight.

"Is this your new machine, Tom?" asked Mr. Damon.

"Yes."

"Well, I don't see anything of that fellow you spoke of--Silent Sam."

"This is Silent Sam," returned Tom, with a laugh. "I've named my new noiseless aeroplane -Ämy Air Scout--I've named that Silent Sam. Wait until you hear it, or rather, don't hear it, and I think you'll agree with me. Silent Sam for Uncle Sam!"

"Good!" cried Mr. Damon. "Bless my dictionary, but that's a good name! Does it sail silently, Tom?"

"I'll let you judge presently. Silent Sam is all ready for his first trial, and I'll be glad to have you with me. Now, I'll just--"

Tom suddenly ceased speaking and held up a hand to enjoin silence. Then, while Mr. Damon watched, the young inventor began moving noiselessly toward the rear of the big shed, inside which was his new machine.