angle, there sounded the sharp crack of some weapon, and Washington
White jumped and screamed.

"Gollyation!" he bawled. "Dem fellers is suah tryin' ter punctuate us!"

Through the blackness of the night a distant voice hailed the pilot
of the _Snowbird_.

"Ahoy! ahoy! Who goes there?" was the cry, and it was repeated twice.




CHAPTER V

BETWEEN TWO PERILS

Mark Sampson, having all the mechanism of the flying machine under his
immediate control, had it in his power to increase speed and seek to
escape the second airship. And Jack wondered why his chum did not
immediately send the _Snowbird_ flying at increased speed over the top
of Mt. Katahdin and so seek to escape the menace below.

But the young fellow at the controls of the _Snowbird_ had an advantage
over his companions that Jack had forgotten. He could hear sounds at a
much greater distance than they, and much clearer.

This was because of an invention of Professor Henderson--a small
instrument similar to part of the ordinary telephone. The sensitive
disk was a form of radio receiver which could be attached to any