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: "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.