ane; isn't it?"

"Yes, and not moving very fast, either. They seem to be circling around
here."

The young inventor was peering through the binoculars, and, as soon as
he had the mysterious craft in focus, he cried:

"Look, Mr. Jackson, it's a new kind of monoplane. I never saw one like
it before. I wonder who could have invented that? It's something like a
Santos-Dumont and a Bleriot, with some features of Cornu's Helicopter.
That's a queer machine."

"It certainly is," agreed the engineer, who was now sighting through
the glasses. In spite of the darkness the binoculars brought out the
peculiarities of the aeroplane with considerable distinctness.

"Can you make out who are in it?" asked Tom.

"No," answered Mr. Jackson. "You try."

But Tom had no better luck. There were two persons in the odd machine,
which was slowly flying along, moving in a great circle, with the Swift
house for its center.

"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 the building where Tom's newest invention was housed.

"Look out! They'll hit something!" cried the young inventor, as though
those in the aeroplane could hear him.

Then, just as though they had heeded his warning, the pilots of the
mysterious craft shot