"I wonder why they're hanging around here?" asked Tom, suspiciously.

"Perhaps they want to talk to you," suggested Mr. Jackson. "They may be
fellow inventor--perhaps one of them is that Philadelphia man who had
the Whizzer."

"No," replied the lad. "He would have sent me word if he intended
calling on me. Those are strangers, I think. There they are, coming back
again."

The mysterious aeroplane was once more circling toward the watchers on
the roof. There was a movement on the steps, near which Tom was
standing, and his father came up.

"Is anything the matter?" he asked anxiously.

"Only a queer craft circling around up here," was the reply. "Come and
see, dad."

Mr. Swift ascended to the roof. The aeroplane was higher now, and those
in her could not so easily be made out. Tom felt a vague sense of fear,
as though he was being watched by the evil eyes of his enemies. More
than once he looked over to the shed where his craft was housed, as
though some danger might threaten it. But the shed of the Humming-Bird
showed no signs of invaders.

Suddenly the mysterious aeroplane increased its speed. It circled about
more quickly, and shot upward, as though to show the watchers of what it
was capable. Then, with a quick swoop it darted downward, straight for