l 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,