“Do you want to take a look at it?” asked Greenbaum, and from the nearness of the voice Tom knew that the man was just outside the locked door. “No, thank you, Greenbaum, not now,” the young inventor replied. “I am busy at something else. I’ll see you in the morning.” “You are working late, Mr. Swift,” went on the man. “Could I be of any service to you? I should be glad——” “No, thank you,” Tom said. “As for lateness, you are doing a bit of overtime yourself.” “Oh, yes; but I do not mind. I think I am on the right track. If you would take a look——” “No, not now,” and Tom’s voice was a bit sharp. “I am busy. Good night!” There was no response for a moment, and then came a short: “Good night!” Greenbaum, however, did not immediately move away from the door and a look of annoyance passed over Tom’s face as he bent over his secret apparatus. “What’s he hanging around for?” thought Tom. “I wonder if he can be a spy? Two or three times I’ve caught him lurking around my private laboratory. But he can’t get in since I put on the new lock, and I know he hasn’t so much as poked his nose in during the times I have been here experimenting. Still, I wonder——” He was about to call out, to tell the man to go away when footsteps were heard moving down the corridor and toward the outer door of the small shop where most of the experimental work was carried on. “Good riddance,” murmured Tom Swift. “I don’t want to be unjust to a good workman, and Greenbaum is all of that, but I must confess I don’t like the way he hangs around me. As soon as he finishes that magnetic gear shift I’ll pay him well and let him go. Now let’s see if I can think up another way of doing this. Perhaps if I hooked up the wave distributor to the vibratory selector instead of to the polarizer we’d get better vision. I’ll try that and have Ned perform again to-morrow. Now I’ll take a look to see that my wire connections are all right and then I think I’ll go to bed. I’m tired.” Tom spent perhaps another half hour in getting things in readiness for some new experiments, and, having made sure that everything connected with his secret was put out of sight of possible prying eyes, the young inventor started toward the door. He inspected the new combination lock he had had put on, noting that it was properly set, and then opened the door to step out. The experimental laboratory was only a short walk from Tom’s home, the back of the Swift Construction plant being some distance away. As Tom opened the door there was a click, followed at once by a blinding flash of blasting fire. Then a dull explosion shook the building. Tom had no chance to leap back. The force of the blast hurled him forward, across the corridor and out through a wire-screened window into the yard. He fell heavily, uttered an inarticulate cry, and then seemed to be sinking down into a pit of dense blackness. CHAPTER II NED DISAPPEARS