berty Bond business, and he could not be present at the test.

However, as Tom expected to have other trials when his motor was in
even better shape, he was not exactly sorry for the absence of his
friends.

"Contact!" called the young inventor, when Jackson had stepped back,
indicating it was time to throw over the switch.

"Let her go!" cried Tom, and the next moment the motor was in
operation, but so silently that his voice and that of Mr. Damon's could
easily be heard above the machinery.

"Good, Tom! That's good!" cried Mr. Swift, and Tom easily heard his
father's voice, though under other, and ordinary, circumstances this
would have been impossible.

True, the hearing of Tom and Mr. Damon was muffled to a certain extent
by the heavy leather and fur-lined caps they wore. But Tom had several
small eyelet holes set into the flaps just over the opening of the
ears, and these holes were sufficient to admit sounds, while keeping
out most of the cold that obtains in the upper regions.

The aeroplane moved swiftly along the level starting ground, and away
from the lighted hangars. Faster and faster it swung along as Tom
headed it into the wind, and then, as the speed of the motor increased,
the Air Scout suddenly left the earth and went soaring aloft as she had
done before.

But there was this difference. She moved almost as silently as a great
owl which swoops down out of the darkness--a bit of the velvety
blackness itself. Up and up, and onward and onward, went the Air Scout.
Tom Swift's improved, silent motor urged it onward, and as the young
inventor listened to catch the noise of the machinery, his heart gave a
bound of hope. For he could detect only very slight sounds.

"She's a success!" exulted Tom to himself. "She's a success, but she
isn't perfect yet," he added. "I've got to make the muffler bigger and
put in more baffle-plates. Then I think I can turn the trick."

He swung the machine out over the open country, and then, when they
were up at a height and sailing along easily, he called back to Mr.
Damon in the seat behind him:

"How do you like it?"

"Great!" exclaimed the eccentric man. "Bless my postage stamp, but it's
great! Why, there's hardly a sound, Tom, and I can hear you quite
easily."

"And I can hear you," added Tom. "I don't believe, down below there,"
and he nodded toward the earth, though Mr. Damon could not see this, as
the airship, save for a tiny light over the instrument board,