"What?" asked the captive, wondering what strange request the mentally unbalanced man would make now. The man leaned forward and whispered something in Mark's ear, as if he was afraid the very walls would hear. "I'll not do it!" cried the youth. "I'll never aid you to deceive my friends, for that is your object. I'll never do it!" "Then I shall have to use force," was the determined response. "You may take your choice!" Poor Mark did not know what to do, yet there was little he could choose between. The man had him in his power, yet the lad was terribly afraid of the result of the daring scheme which he knew was in the mind of the lunatic, for such he believed the man to be. "Will you not give up this plan?" begged Mark. "I know Professor Henderson will pay you any sum in reason to let me go. You can become a rich man." "I don't want riches--I want revenge!" exclaimed the man. And he glared at Mark, while throughout the dismal, deserted house there sounded the rattle and bang of the flapping shutters. CHAPTER X MARK'S STRANGE ACTIONS Jack Darrow fairly burst into the big shed where the two scientists were at work over the ruined motor. They looked up at his excitable entrance, and Mr. Henderson called out: "Why, Jack, what's the matter?" "Quite a lot, I'm afraid," answered the lad, and there was that in his voice which alarmed the professors. "What do you mean?" inquired Mr. Roumann, laying aside some of the damaged motor plates. "Mark's gone!" gasped Jack. "Gone! Where?" exclaimed Mr. Henderson. "I don't know, but he went to the deserted house, where we thought the mysterious man was hiding, and since then I can't find him." Then the frightened lad proceeded to explain what he and Mark had undertaken, and the outcome of it; how his chum had failed to meet him at the rendezvous, and how Jack had searched through the old house without result. "There's but one thing to do," declared Professor Henderson, when he had listened to the story. "We must go back there and make a more thorough search." "What--to-night?" exclaimed the German. "Surely. Why not? We can't leave Mark there all alone. He may be hurt, or in trouble." "That's what I think," said Jack. "I'll tell Washington and Andy, and we'll go back and hunt for him. Poor Mark! If he had only waited for me, perhaps this would never have happened, and if I hadn't stopped at the dog-fight maybe Mark would have waited for me. Well, it's too late to worry about that now. The thing is to find him; and I guess we can." Jack would not stop longer than to snatch a hasty bite of supper before he joined the searching party. Washington and he carried lanterns, while Andy Sudds had his trusty rifle, and the two professors brought up in the rear, armed with stout clubs, for Jack's account of the affair made them think that perhaps they might have to deal with a violent man. "Hadn't you better notify the police?" suggested Andy. "A couple of constables would be some help."