"Bless my opera glasses, Tom!" cried Mr. Damon, "but are you seeing things?"
"No; but I'm hearing them," answered Tom with a short laugh. "Did you think you heard some one moving around near the rudders of Silent Sam, Mr. Damon?"
"No, I can't say that I did. Everything seems to me to be all right."
"Well, it doesn't to me," went on Tom grimly. "I think there is an intruder in this shed, though how any one could get in when the doors have been locked all day, is more than I can figure out. But I'm going to have a look."
"I'll help you," offered Mr. Damon, and, in the bright glare from many electric lights, the two began a search of the big hangar where the new craft was kept.
But though the young inventor and his friend went around to the rear of the aeroplane, walking in opposite directions, they saw no one, nor did any one try to escape past them.
"And yet I was sure I heard some one in here," declared Tom, when a search had revealed nothing. "It sounded as if some one were scuffling softly about in rubber-soled shoes, trying to hide."
"Bless my suspenders!" cried Mr. Damon, "who do you think it could have been, Tom?"
"Who else but some spy trying to get possession of my secrets?" was the answer. "But I guess I was too quick for them. They couldn't learn much from looking at the outside of my muffler, and it hasn't been disturbed, as far as I can see."
"Who would want to gain a knowledge of it in that unlawful way?" asked Mr. Damon.