it is a wonder no one thought of it before,"
said a navy aviation expert. "It is the last word in aircraft
construction--a silent motor that will not apprise the enemy of its
approach! You have done wonders, Mr. Swift!"

"I'd rather hear you say that after the air test," replied Tom, with a
laugh. "Are you ready, Mr. Terrill?"

"Whenever you are."

"How about you, Mr. Damon?"

"Oh, I'm always ready to go with you, Tom Swift. Bless my trench
helmet, but you can't sail any too soon for me!"

There was a genial laugh at his impetuosity, and the three took their
seats in the big craft. Once more the engine was started. It operated
as silently as before, and the first good impressions were confirmed.
Even as the machine moved along the ground, just previous to taking
flight into the air, there was no noise, save the slight crunch made by
the wheels. This, of course, would be obviated when Silent Sam was
aloft.

Up and up soared the great craft, with Tom at the engine and guide
controls, while Mr. Terrill and Mr. Damon sat behind him, both eagerly
watching. Mr. Terrill was there to find fault if he could, but he was
glad he did not have to.

"The machine works perfectly, Mr. Swift," he said. "My report cannot be
otherwise than favorable."

"We mustn't be in too much of a hurry," said Tom, who had learned
caution some time ago. "I want to sail around for several hours.
Sometimes a machine will work well at first, but defects will develop
when it is overheated. I'm going to do my best to make a noise with
this new motor."

But it seemed impossible. The machinery worked perfectly, and though
Silent Sam took his passengers high and low, in big circles and small
ones, there was no appreciable noise from the motor. The passengers
could converse as easily, and with as little effort, as in a balloon.

"Of course that isn't the prime requisite," said Mr. Terrill, "but it
is a good one. What we want is a machine that can sail over the enemy's
lines at night without being heard, and I think this one will do it--in
fact, I'm sure it will. Of course the ability of the passengers to
converse and not have to use the uncertain tube is a great advantage."

As Tom Swift sailed on and on, it became evident that the test was
going to be a success. The afternoon passed, and it began to grow dark,
but a glorious full moon came up.

"Shall I take you down?" the young inventor asked Mr. Terrill.

"Not quite yet. I thoroughly enjoy