all the identity of the lean figure and dark
complexion. "I believe that chap is trying to shadow me. I wonder
what in the dickens he really is up to?"

It was the second time Bob had asked that question of himself, but as
he was a poor source of information just then, he was forced to pass
into the fair-grounds and relock the gate in as mystified a state of
mind as before he put the query.

A little later, when he reached the big hangar he whirled about again,
as if half expecting to see the stranger still skulking behind him in
the grounds. To his relief he did not detect this situation exactly,
but he did see a dark face, which had been peering over the top of the
highboard fence near the gate, drop down from view on the other side.

Bob gave a grunt as he passed into the hangar and took off his coat.
"As I live, I believe he's up to some sort of mischief," growled the
boy. And when, shortly afterward, John and Paul Ross appeared he told
of his experience and repeated his suspicions.

"That is funny," asserted John; "Paul and I saw nothing of any such man
when we came along, and we passed down the same road. Perhaps he
mistook you for somebody else."

"I hope so, but I don't like his actions a little bit," declared Bob
stoutly.

With that he picked up a try-square and pencil and began laying out
some work for Paul to cut on the circular saw, while John busied
himself at the boring-machine in putting a hole through the center of
the big twelve-foot balsa-wood propeller which a little later would be
reinforced with a thin jacket of a new metal called "salinamum," which
was made chiefly from salt but whose fused components made it as light
as aluminum and stronger than tool steel.

Soon the queer actions of the stranger were quite forgotten in the deep
interest of the three young men in their work. With the prospect of a
world tour before them if the Sky-Bird turned out well, they now had
more incentive than at the beginning to build the machine with the
utmost skill and attention to every detail. Some changes, calculated
to make the craft better adapted to the peculiar conditions she would
be likely to meet in such a varied temperature were put into effect,
but on the whole they found their original plans so well laid that no
important features seemed to require modification or abandonment.

But if the man who had followed Bob dropped out of their minds the rest
of that day, he was soon to occupy a prom