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:
"Hold your ship. I propose to come aboard and search her. In the name of the United States Government!"
Mr. Henderson took the radio telephone out of Mark's hand and replied:
"We wish to know who and what you really are. We will not put ourselves in your power without knowing. We are amply armed."
"Don't you dare to fire upon a United States officer in the discharge of his duty," cried the voice from below, and now the strange airship was much nearer to them. "Who do you claim to be?"
"This is the Snowbird, from Easton, Maine, She is manned by her builders, Darrow and Sampson. She carries as passengers Washington White, Andrew Sudds and Amos Henderson," declared the professor, in reply. "And she is bound for Alaska."
"Well, well!" exclaimed the voice of their pursuer. "That may all be so. But I have my suspicions. I am Ford, special agent of the Department of Justice. Stand by. Now I am coming aboard."
At a nod from the professor, Mark had already brought the Snowbird to a halt. She lay floating, with all planes extended and without motion of propellers, poised over the summit of Mt. Katahdin.
The descending moon threw its beams over the height and revealed to the vaguely anxious occupants of the Snowbird, the other machine darting up from below.
This was a craft of much different aspect from their own. It was a great deal smaller and apparently without half the power possessed by the one built by Jack and Mark.
She shot into the air above their heads at a swift pace, however, and immediately poised over them. In this attitude Ford, as he called himself, had the occupants of the Snowbird completely at his mercy. A bomb dropped upon the huge flying machine would have blown her to pieces. Or, with a gun, he could have picked off one after another of the five people below.
"Stand out of the way, there!" commanded Ford.
Instantly those upon the larger air-craft saw a figure swing down from the framework of the airship above their heads. A light rope ladder unrolled and fell upon the upper deck, or platform of the Snowbird, and the man came down this ladder, hand under hand, and in half a minute stood in their midst.
He was a small, gray man—gray suit, gray hair and close-cropped mustache, and gray face, colorless and deeply lined. His age would be hard to judge.
"The Snowbird; eh?" he observed, looking sharply from one to the other of the five passengers of the huge flying machine. "You are Amos Henderson, sir?" he pursued, nodding to the professor. "I believe I have heard your name before. Professor Henderson, whose scientific discoveries have made us all marvel of late?"
"I am Professor Henderson," said the old gentleman, quietly. "And I can vouch for my companions. These boys, my adopted sons, have built this flying machine, and we are bound for Alaska."
"Indeed! Then I fear I have caused you some slight trouble, not to say delay," said Mr. Ford. "We revenue agents are extremely anxious to overhaul and interview all aviators along the border. You understand?"
"I believe that you have cause to suspect certain flying machines operating between the Canadian towns and Maine settlements," admitted Professor Henderson. "Quite right. And if our suspicions are based on fact, innocent flying men like yourselves may well beware of the fellows we are after. To be frank with you," pursued Mr. Ford, "a band of desperate smugglers are operating by aid of one or more aeroplanes. And piracy in the air may soon became as frequent—and as grave a peril to innocent aviators—as was ever piracy on the Spanish Main."
"It seems impossible!" said the professor. "Who are these desperate criminals?"
"A man named Bainbridge is at their head. He was originally a diamond dealer and finally was caught smuggling gems into the port of New York. He had to pay a huge fine and served a term at Atlanta for that crime and since then has sworn to be revenged upon the Government that punished him.
"We learned of late that he was operating on the Mexican border—bringing into the States diamonds that had paid no duty—by aid of a flying machine. But the uprising in Chihuahua and along the border made his work exceedingly dangerous, and he was driven away from that part of the country.
"Now we believe he has joined forces hereabout with ancient enemies of the Federal officers. At least, there is a strange aeroplane reported from both sides of the border, and some fine gems have appeared in the hands of certain suspected dealers in Maine, and as far south as Boston and Providence.
"Bainbridge is known to be a desperate man. Look out for him, Professor. If you are hailed by another machine, better keep away from it," and the secret service agent laughed. "Had I been in your place I would not have halted on this occasion. You certainly can outsail any airship I have ever seen operated."
Mr. Ford seemed quite satisfied that our friends were law-abiding and he ascended to his waiting craft in a few moments; and the Snowbird started onward again through the starlight.
But the warning of the special agent had impressed the boys as well as the professor. Andy Sudds refused to lie down again, although Jack and Mark continued to operate the flying machine. The old hunter sat with a rifle in his hand for the rest of the night. But the professor went to bed.
An hour after midnight a cloud from the west completely masked the moon and the whole heavens became misty. This cloud brought both wind and rain, and low upon its edge the lightning played fitfully.
"There will be a heavy tempest about dawn," Andy promised the boys. "I have seen a thunderstorm gather like this before." "But not while you were in a flying machine," chuckled Jack.
"No, sir. But on a mountain top a tempest looks much the same."
Mark, while at the controls, had scaled the machine down the air-ways until they were not more than fifteen hundred feet from the earth. But the boys decided to let the storm gather beneath them, and so shot the Snowbird up again until the indicator registered three thousand feet.
Near the earth it must have been very warm and sultry; but up here it was down to freezing, and the party were all warmly dressed. The clouds soon hid the whole earth from them and the great flying machine traveled in space, with the star-lit heavens above and the rolling mass of vapor, streaked now and then with lightning flashes, beneath.
The deafening roll of the thunder awoke Washington White from a short nap, and the darkey was not at all sure that he was safe from the lightning bolts.
"How d'I know dem bolts won't fly disher way?" he demanded of the boys when they tried to reassure him.
"Why, the earth attracts the electric bolt, and that attraction is much stronger than any the Snowbird may have for the electricity in the clouds," Mark told him. "I don't know erbout dat," grumbled Wash. "An' if jest one o' dem crazy lightning bolts should take it into its haid ter segastuate eround disher flying merchine—biff! bang! dat would be erbout all. Dere would be a big bunch o' crape hung on Wash White's do', suah as you is bawn, boy!"
But although the roar of the thunder and whining of the wind nearly drowned other sounds in and about the flying machine, save for a freshening of the gale the Snowbird was at first but little disturbed by the tempest which raged with such fury a thousand feet below.
Suddenly Mark caught sight of something moving across the red streak in the eastern sky—the light that warned them of the approach of the sun.
"What is that—a huge bird?" he demanded of Andy Sudds, pointing this moving figure out to the hunter.
Andy's eyes were very keen, for he was used to sighting along a rifle and gazing over long distances in search of game. But he, too, thought the object must be a bird.
"I declare, I didn't know birds flew so high," said Mark. "It must be