"Jack!" cried the youth on the machine. "Mark!" ejaculated the lad who had sprung from the projectile. "What has happened? Who is the fellow who has been masquerading as you?" "A scoundrel and a villain! Let me get at him!" and, slamming on the brakes, as he shut off the power, Mark leaped from the motorcycle, stood it up against the projectile, and clasped his chum by the hand. "What's the matter?" asked Professor Henderson, as he, too, ran out of the _Annihilator_. "What does that tramp want, Jack? Give him some money, and get back in here; we ought to have started long ago." He looked at the ragged figure. "This isn't a tramp," cried Jack. "It's Mark!" "Mark! I thought----" "There have been strange doings," gasped the lad in tramp's garments. "I have just escaped from being kept a prisoner. Where is the mysterious man? Oh, I'm glad I arrived in time! Were you about to start?" "That's what we were," replied Jack. "Oh, Mark, but I'm glad to see you again! I didn't know what to think. You acted so strange--or, rather, the fellow we thought was you had me guessing!" "Good land a' massy!" exclaimed Washington White, as he stood in the doorway, with Andy Sudds behind him. "Am dere two Marks? What's up, anyhow?" "Don't let that fellow get away--the fellow who passed himself off as me!" shouted Mark. "Lock him up! There's some mystery about him that must be explained. He's a dangerous man to be at large." Professor Henderson turned back to enter the projectile. Jack advised Andy to get his gun ready, with which to threaten the scoundrel in case of necessity. At that instant there sounded a crash of glass, and the whole front of the big observation window in the side of the _Annihilator_ was smashed to atoms. A figure leaped--a figure which no longer had its head bandaged, and whose arm was no longer in a sling--the figure of a man--the mysterious man who had held Mark a prisoner! "There he goes!" shouted Jack. "Catch him, somebody! Andy, where's your gun?" "I'll have it in a jiffy!" cried the hunter, as he dashed back to get it. But the man did not linger. Scrambling to his feet after his fall, caused by his leap from the broken window, which he had smashed with a sledge hammer as soon as he understood that his game was up, he raced out of the yard. He turned long enough to shake his fist at the group assembled around the projectile, and then leaped away, calling out some words which they could not hear. "Let's take after him," proposed Mark. "Come on," seconded Jack. "No, let him go; he's a desperate man, and you came just in time to unmask him," said Professor Henderson. "He might harm you if you took after him. Let him go. He has not done much damage. We can easily replace the broken window. But I can't understand what his object was in disguising himself as Mark. He certainly looked like you, Mark, especially when he kept his face concealed. Why did he do it?"