"'Nless it were Buttsy," remarked Washington, _sotto voce_. The
professor was still asleep and the boys paid little attention to the
flying object for some time. It was coming up behind the _Snowbird_, and
they had no occasion to look behind.

The sun arose, angry and red, while the thunder continued to roll below
them, and the crackling of the electric flashes was like minute guns.
The _Snowbird_ was winging its way along at about seventy-five
miles per hour. Wash had gone into the covered galley to prepare
breakfast. Jack was still in the operator's seat.

Suddenly Andy Sudds uttered a loud shout. A huge shadow was thrown
athwart the flying _Snowbird_. Some object was hovering over them
and they cast their eyes upward, at Andy's cry, to see another aeroplane
swooping down directly upon them.

It was not the machine manned by Secret Service Agent Ford and his
companion, but a much heavier and more rapid vehicle. And until its
shadow fell across the _Snowbird_, the boys had had no warning of its
approach.

At first glance it was apparent that the strange aircraft intended
mischief. It was shooting down from a higher level, its sharp bow aimed
directly for the _Snowbird_. Jack pushed over the switch and raised the
bow of their own ship. She leaped forward and began to slant upward,
too.

But instantly the course of the stranger was deflected to meet this
change in the movement of the _Snowbird_. She had the advantage of the