--certain refinements
that must be added, and when this should be done Tom was pretty certain
that he would have what would prove to be a very quiet aeroplane, if
not an absolutely silent one.

The young inventor was engaged one day with some of the last details of
the experiment. The new motor, with the silencer and the changed
cylinders, had been attached to one of Tom's speedy aeroplanes, and he
was making some intricate calculations in relation to a new cylinder
block, to be used when he started to make a completely new machine of
the improved type.

Tom had set down on paper some computations regarding the cross-section
of one of the cylinders, and was working out the amount of stress to
which he could subject a shoulder strut, when a shadow was cast across
the drawing board he had propped up in his lap.

In an instant Tom pulled a blank sheet over his mass of figures and
looked up, a sudden fear coming over him that another spy was at hand.
But a hearty voice reassured him.

"Bless my rice pudding!" cried Mr. Damon, "you shut yourself up here,
Tom, like a hermit in the mountains. Why don't you come out and enjoy
life?"

"Hello! Glad to see you!" cried Tom, joyfully. "You're just in time!"

"Time for what--dinner?" asked the eccentric man, with a chuckle. "If
so, my reference to rice pudding was very proper."

"Why, yes, I imagine there must be a dinner in prospect somewhere, Mr.
Damon," said Tom with a smile. "We'll have to see Mrs. Baggert about
that. But what I meant was that you're just in time to have a ride with
me, if you want to go."

"Go where?"

"Oh, up in cloudland. I have just finished my first sample of a silent
motor, and I'm going to try it this evening. Would you like to come
along?"

"I would!" exclaimed Mr. Damon. "Bless my onion soup, Tom, but I would!
But why fly at night? Isn't it safer by daylight?"

"Oh, that doesn't make much difference. It's safe enough at any time.
The reason I'm going to make my first flight after dark is that I don't
want any spies about."

"Oh, I see! Are they camping on your trail?"

"Not exactly. But I can't tell where they may be. If I should start out
in daylight and be forced to make a landing-- Well, you know what a
crowd always collects to see a stranded airship."

"That's right, Tom."

"That decided me to start off after dark. Then if we have to come down
because of some sort of engine trouble or because my new attachment
doesn't work right,