to think of getting back into bed, when he was aware of a peculiar sound
in the air overhead.

"I wonder if that's a heavy wind starting up?" he mused. "Good luck, if
it is! We need it." The noise increased, sounding more and more like
wind, but Tom, looking out into the night, saw the leaves of the trees
barely moving.

"If that's a breeze, it's taking its own time getting here," he went on.

The sound came nearer, and then Tom knew that it was not the noise of
the wind in the trees. It was more like a roaring and rumbling,

"Can it be distant thunder?" Tom asked himself. "There is no sign of a
storm." Once more he looked from the window. The night was calm and
clear--the trees as still as if they were painted.

The sound was even more plain now, and Tom, who had sharp ears, at once
decided that it was just over the house--directly overhead. An instant
later he knew what it was.

"The motor of an aeroplane, or a dirigible balloon!" he exclaimed. "Some
one is flying overhead!"

For an instant he feared lest the shed had been broken into, and his
Humming-Bird taken, but a glance toward the place seemed to show that it
was all right.

Then Tom hastily made his way to where a flight of stairs led to a
little enclosed observatory on the roof.