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.
"What do you know about that?" demanded Jack. "'In the name of the law', no less!"
Professor Henderson jumped to the same conclusion that Mark had, and that instantly. "It may be the Secret Service men themselves," he said. "Ah, Andrew! it is just as well to withhold your fire until we know for sure."
For Andy Sudds had seized his rifle and stood ready to withstand an attack, should such an act become necessary.
Up from the depths came the cry again: