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 cannotbe 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 this, and it isn't often I get a chance for a moonlight airship ride.Go a little lower, if you please, and we'll see if we attract any attention from the inhabitants of the earth.We'll see if they can possibly hear the machine, though I don't see how they can."And they did not.Tom piloted the machine over Shopton, sailing directly over the center of the town, where there was a big crowd walking about.Though the airship sailed only a few hundred feet above their heads, not a person was aware of it, since the craft's lights were put out for this test.

"That settles it," said Mr.Terrill."You have succeeded, Tom Swift!"But Tom was not yet satisfied.He wanted a longer test.Hardly knowing why he did it he sent the craft in the direction of Mary Nestor's home.As he sailed across her lawn he saw, in the moonlight, that she and her mother were walking in the garden.They did not look up as the aircraft passed over their heads, and were totally unaware of its presence,unless they caught a glimpse of it as it flitted silently along, like some great bird of the night.

"It is perfectly wonderful!" declared Mr.Terrill, and he spoke in ordinary tones, that carried perfectly to the ears of Tom and Mr.Damon.

"Wonderful!" cried the eccentric man."Bless my chimney, but it's the greatest invention in the world! Yes, it is! Don't tell me it 'isn't!"And no one did.

Passing the Nestor home, the saddened occupants of which were unaware of the passage, Tom sent the Air Scout about in a circle, intending to proceed to the hangar.And then, some whim, perhaps, caused him to guide Silent Sam out toward the lonely hut.Mr.Damon and Mr.Tenrill seemed perfectly content to sail on and on indefinitely in the moonlight.Tom thought he would take them over a lonely neighborhood, and then bring them back.

In a little while the craft was directly over the stretch of country where the aeroplane accident bad occurred, and where Tom and Jackson had found the deserted hut.

Rather idly Tom looked down, wondering if the Secret Service men were on the watch and if they had discovered anything.

Suddenly Tom was aware of an automobile moving along the field path toward the cabin.There were two men in the car, both on the front seat, and as Tom looked down the brilliant moonlight showed him the figure of another man, behind, and huddled in the tonneau of the car.The aeroplane was low enough for all these details to be seen by the moon's gleam, but the men in the car, not hearing any noise, did not look up, so they were unconscious of this aerial espionage.

"Look! Look!" exclaimed Tom in a low voice to his companions."Doesn't that seem suspicious?"