aviator's helmet, and was being put into general use by pilots. The
two boys always adjusted this whenever they were strapped upon the
pilot's seat.

Thus, although the report of the gun had sounded but faintly to the
other members of the party, to Mark it seemed as though the explosion
was within a hundred yards. The voice hailing them likewise seemed to
ring in his ears very plainly; and beyond the words somewhat
distinguished by his companions the young operator of the _Snowbird_
could make out a further phrase spoken by the person who hailed from the
other air-craft.

"Halt in the name of the law!"

Those were the sharp words Mark had caught, and for that reason he
hesitated to increase the _Snowbird's_ speed.

In a strap hung near his left hand was a transmitter. Without taking
the advice of any of his companions in the flying machine, Mark seized
it, put it to his lips, and replied to the hail:

"Ahoy! what do you want?"

Instantly the voice rose from the black abyss below them:

"Heave to! Stop in the name of the law!"

That time the professor and Jack heard the words spoken by their
pursuer.