Poor Nickel Plate had no idea what was going on behind his back; indeed he was so surprised at the Coal Man's antics that he couldn't think of anything else. For, of course, the Coal Man saw what Billy was doing, and was laughing and slapping his knees and jumping up and down with delight. "Well, come on Drone," said Nickel Plate, starting away. "We must be going--Drone--Drone--wake up, I say." And Drone got up from a bed of lava rubbing his eyes and yawning. But he was so sleepy he paid no attention to Nickel Plate's back and out they both walked without ever discovering the trick Billy had played. As soon as they had gone Billy came out of his hiding-place. "Thank you, Mr. Coal Man, Mr. Alchemist, and Ace of Spades--I shan't ever forget your kindness to me." "Run along and don't interrupt," said the Alchemist, hard at work with his scales. "You're welcome--good-by." Ace of Spades bobbed his red head and smiled to show that Billy was welcome to all he had done. "Come along," said the Coal Man, "I'll show you the way out." And he led Billy to the floor beneath the Crater where he warmly shook hands with him. "Good-by--take good care of yourself, boy--you've made me laugh more to-day than I have for years." "Good-by," cried Billy, giving a great big jump, and up he popped out of the mouth of Vociferous and away over the green fields and forests. CHAPTER XVIII. THE ELUSIVE BRIDGE. "I must be very near the end of my journey," said Billy the morning after he ruined Nickel Plate's new coat. "I have been on the air road a long time, and certainly I have had some queer adventures. What shall I say to Bogie Man when I _do_ see him, I wonder----" but here he lit on the ground and looked around him. About half a block behind him he saw a most peculiar looking bridge--a bridge that seemed to be floating in the air and yet came very near to touching the ground at each end. But what interested him was the dark and dismal looking city on the other side of the bridge. "Why, that's strange. The bridge and the city are behind me, so I must have passed over them; yet I don't remember seeing them at all." Billy had been looking at them over his shoulder and now turned to get a better view. But when he turned he saw nothing before him but a great wide stretch of land--the bridge and the city had disappeared. "I am sure I saw a bridge and city over there," and he casually turned his head to look around him again--there at his back were the objects he sought. "I must have turned all the way round without noticing it, but this time I'm going to keep an eye on them when I turn." So Billy turned very, very slowly, keeping a close watch on the bridge and city. But as he turned, they turned; it made no difference how fast or how slow the motion.