The Annihilator was certainly falling through space, and no longer shooting forward. This was evident, as the motion was slower than when the projectile was urged on by the mysterious force. "Let's go tell Mr. Roumann and Professor Henderson," suggested Mark. They started toward the pilot house, but met the two scientists rushing back toward the engine-room. "Has anything happened?" asked Jack. "Yes," answered the German. "The Etherium motor has stopped working!" "And are we falling?" asked Mark. "Yes, in a sense," answered Mr. Henderson, as the other inventor hurried on. "The gravitation of the earth no longer attracts us, but we are not heading in a straight line for Mars. We may be falling into some other planet, or the sun." Then he, too, went to the engine-room, and the boys followed. They found the place strangely quiet, since the throbbing and humming of the main motor had ceased. The dynamos that kept the light aglow and the air and other pumps were in motion, however. "What's the matter?" asked Mr. Henderson. "There's been a breakdown," was the reply of the German. "And it looks to me as if some one had been tampering with the motor." "Tampering with the motor?" "Yes. Some of the plates have been smashed. I believe there is some one concealed on board--some enemy of mine--who hopes to destroy us." "What can we do?" asked Jack. "Nothing, until the motor is repaired," replied the German scientist. "But we are falling--" "Yes, I know. But we can't fall with anything like the speed with which we were traveling, and though we may go downward, comparatively speaking, for a day or so, we can quickly regain our former place as soon as the motor is running again." "But can you fix it?" "Yes, I have some spare plates. But I wish you boys would make a search through the projectile." "What for?" asked Mark. "For the person who smashed the plates. I believe some one is concealed here who seeks to kill, us. We must find him." "And I think I know who it is!" exclaimed Jack. "Who?" asked Mr. Henderson. "The crazy machinist. I believe he sneaked here through that open port leading into the storeroom." "That's it!" cried Mr. Roumann. "He must have done this. See if you can't find him." "Come on, Mark," said Jack. "We'll look for the rascal." "And I'll help," added old Andy. "I'm pretty good on the trail. Maybe I can locate him." "Do so, then," advised the German. "The professor and I will repair the motor." CHAPTER XVIII A VAIN SEARCH The boys, with the old hunter, immediately began a search. Washington was needed to aid the two scientific men, who quickly prepared to substitute new plates for the smashed ones. The broken plates looked as if they had been struck with a sledge hammer.