"I had the fact in mind. The flying machine is being put already to uses that are not commendable, to say the least. The Maine and Canadian border has for years been used by bands of smugglers, and if one of these gangs have purchased and can use a flying craft, they may make the revenue men a deal of trouble." "You're right, sir. And I read likewise that the government officers proposed using an aeroplane themselves to track the smugglers. Perhaps the villains, if that is their ship below us, may take us for secret service men." As he spoke the lamp so far below them darted up at a sudden and sharp 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 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. "What do you know about that?" demanded Jack. "'In the name of the law', no less!"