Jack's first act, on arising from amid a mass of tools, into which he had been tossed by the explosion, was to run to where Professor Roumann lay in a semi-conscious condition. An instant later Mark slowly arose, and made his way to where Professor Henderson was rubbing his forehead in a dazed fashion. "Are you hurt?" asked Mark, of his aged friend. "I think not," answered Mr. Henderson slowly, "but I fear Mr. Roumann is. See to him; I'm all right." "He's breathing," cried Jack, who had bent over the German. "He isn't dead, at any rate." "But he may be, unless he gets attention," said Professor Henderson. "Get my medicine chest, Mark, and we'll see what we can do for him." Jack had raised the head of the injured man on his arm, and was giving him some water from a glass. This partially revived the German, and he opened his eyes. He looked around, into the faces of his friends, as if scarcely comprehending what had happened, and then, as his gaze wandered toward the disabled Cardite motor, he exclaimed: "Some enemy has done this! The motor was tampered with. The resistance block was loosened, and that caused the force of the Cardite to shoot out at the rear. We must watch out for the work of this enemy!" "Don't distress yourself about that now," urged Mr. Henderson. "Are you badly hurt? Do you need a doctor?" The German slowly drank the rest of the water which Jack gave him, and then gradually arose to a standing position. "I am all right," he said faintly, "except that I feel a trifle dizzy. Something hit me on the head, and the fumes from the Cardite took away my breath for a moment. I think I shall be all right soon." "Here is the medicine chest!" exclaimed Mark, coming back into the engine room. Mr. Henderson poured out some aromatic spirits of ammonia into a graduated glass, added a little water, and gave it to his fellow, inventor, who, after drinking it, declared that he felt much better. There was a cut on his forehead, where a piece of the broken motor had struck him, but, otherwise, he did not seem injured externally. As for the boys, they were only stunned, nor was Mr. Henderson more than momentarily shocked. In a few minutes the German professor was almost himself again. "We must try to discover who our enemy is," he said earnestly, as he looked over the disabled motor. "He might have blown up the whole projectile by tampering as he did with the machinery. Had I been testing the large, instead of the small motor, there would have been nothing left of the _Annihilator_, or us, either. Who could have done this? If that crazy machinist is around again----" "I don't believe he could get here from Mars," interrupted Jack, with a smile. "Hardly," added Mark. "No, I guess he is still on the Red Planet, so it couldn't have been him," went on Mr. Roumann. "But it was some one." Jack and Mark at once thought of the odd man who had sent Mark the note, and then had run away. "Could it have been him?" suggested Jack.