"It's possible," remarked Professor Henderson. "We must be on our guard. I wonder if Washington----" At that moment there sounded a violent pounding on the exterior of the projectile, and the voice of the colored man could be heard calling: "Am anything de mattah? Andy Sudds an' I is out heah, an' we heard suffin goin' on in dere. Am anybody hurted?" "It's all over now, Wash," replied Jack, for the two boys, and the two professors, had shut themselves up in the projectile while they conducted the experiment. Jack opened the door of the _Annihilator_ and stepped out, being met by the colored man and the old hunter. "You haven't seen any suspicious characters around, have you, Wash?" asked Mark. "Some one has been tampering with a motor, and it exploded." "Nobody's been around since I've been here," announced Andy Sudds, with a significant glance at his gun. "Maybe it's some ob dem moon-men, what don't laik de idea ob us goin' dere arter dere diamonds," volunteered the colored man. "Perhaps," admitted Jack, with a smile. "But certainly some one has been around here who had no business to be, and we must find out who it was. Better take a look around, Wash." "I'll help him," said Andy, and, with his rifle in readiness for any intruders, the old hunter followed the colored man outside the big shed. Meanwhile Professor Roumann and Mr. Henderson were carefully examining the exploded motor. "I should have looked at the breech plug before turning on the power," said the German, "but I had no reason to suspect that anything was wrong." He went on to explain that the explosion was something like that which occurs when the breech-block of a big navy gun is not properly in place. The force of the Cardite, instead of being directed against the piston-heads of the motor, shot out backward, and almost into the face of the professor, who was operating the machine. "But what could be their object?" asked Mark. "Who would want to injure us, or damage the projectile?" "Some enemy, of course," declared Jack. "But who? The crazy machinist is out of it, and as for that man who sent the note to you, he seemed too big a coward to attempt anything like this." "Some one evidently sneaked in here and loosened the breech-plug," went on Mark, "and it was evidently done with the idea of delaying us. The enemy could not have desired to utterly disable the projectile, or else he would have tampered with the large motor, instead of the small one." "Yes, the object seems to have been to delay us," admitted Professor Henderson; "yet, I can't understand why. Whoever did it evidently knows something about machinery." "I hope they did not discover the secret of my Cardite motor," said Professor Roumann quickly. "They hardly had time," declared Mark. "We have been in or around the projectile nearly every minute of the day, and whoever it was, must have watched his chance, slipped in, stayed a few seconds, and then slipped out again."