"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
