"Yes, we are pointing directly toward it. Perhaps you boys will go and slow down the motor, while Professor Henderson and I make some scientific notes concerning the comet. It will be great information to the astronomers on earth. Many of their theories will be changed, I fancy." Jack and Mark started for the engine-room. They passed through the living or dining-room, where Washington was setting the table for dinner. "What I done tole yo'?" he demanded triumphantly. "I wasn't skeered ob no ole comet." "That's right, Wash," admitted Mark. "You had one on us that time." Andy Sudds was in one corner of the room, oiling his gun. "Getting ready to go hunting?" asked Jack. "Well, I heard Mr. Roumann say we'd be on Mars in a few days," replied the old man, "and if there's any game there I want to get a shot at it." "That's right," said Jack. "I guess I'll take--" He got no further. From the engine-room there sounded a tremendous racket, as if some one was pounding on the machinery with a big hammer. "What's that?" cried Mark. "Something's happened to the motor!" exclaimed Jack. "Maybe it's going too fast! Come on!" They ran to the engine-room. The sight that met their eyes was a startling one. Standing with his back to them was a strange man. Over his head he was swinging a sledge hammer, which he brought down with great force upon the Etherium motor. "I'll smash it! I'll stop this machine! I'll send us all to the bottom of the universe!" the man was muttering. "Quit that!" cried Jack, springing forward. The man paused and turned. "The crazy machinist!" shouted Jack. "Hell break the engine all to pieces!" "That's what I will!" replied the infuriated man. "I'll end this voyage now!" Once more he brought his hammer down on the machine, and the motor, with a hissing of gas and a shower of sparks, stopped working. Jack and Mark were brave lads. They sprang upon the man, though he was large and strong, and his strength was added to by his insane fury. In an instant they were in the midst of a fierce fight. The maniac tossed them aside as if they were mere infants, but they returned to the attack. They sought to hold his arms to prevent him from doing any further damage with the hammer. Fortunately for the lads, the man was forced to drop the weapon, to enable him to grapple with his two assailants. "Can you hold him?" cried Mark. "Not very well," panted Jack, as his grip of the man's arms was broken and he was flung across the room. "Help! Help!" suddenly cried Mark. "The crazy machinist is here!" Washington and Andy, in the living-room, heard the yells of the boys. They rushed to the scene, and, taking in the situation at a glance, flung themselves upon the unfortunate man, aiding the boys in holding him. Even their strength was not sufficient, and it was not until Mr. Roumann, leaving Professor Henderson in charge of the pilot house, had come up, that they were able to secure the maniac.