"Mr. Henley finally grew to regard the strange extinguishing of the gas as an effort to kill him, and carefully locked and barred his door and windows each night. He believed that some one came into his apartments and put out the light, leaving the gas flow. This, of course, was not true. Yet the gas was put out. How? My first idea, a natural one, was that it was turned off for an instant at the meter, when the light would go out, then turned on again. This, I convinced myself, was not true. Therefore still the question--how?
"It is a fact--I don't know how widely known it is--but it is a fact that every gas light in this house might be extinguished at the same time from this room without leaving it. How? Simply by removing the gas jet tip and blowing into the gas pipe. It would not leave a jet in the building burning. It is due to the fact that the lung power is greater than the pressure of the gas in the pipes, and forces it out.
"Thus we have the method employed to extinguish the light in Mr. Henley's rooms, and all the barred and locked doors and windows would not stop it. At the same time it threatened the life of every other person in the house--that is, every other person who used gas. It was probably for this reason that the attempt was always made late at night, I should say three or four o'clock. That's when it was done, isn't it?" he asked suddenly of the valet.
Staring at The Thinking Machine in open-mouthed astonishment the valet nodded his acquiescence before he was fully aware of it.
"Yes, that's right," The Thinking Machine resumed complacently. "This was easily found out--comparatively. The next question was how was a watch kept on Mr. Henley? It would have done no good to extinguish the gas before he was asleep, or, to have turned it on when he was not in his rooms. It might have led to a speedy discovery of just how the thing was done.
"There's a spring lock on the door of Mr. Henley's apartment. Therefore it would have been impossible for anyone to peep through the keyhole. There are no cracks through which one might see. How was this watch kept? How was the plotter to satisfy himself positively of the time when Mr. Henley was asleep? How was it the gas was put out at no time of the score or more nights Mr. Henley himself kept watch? Obviously he was watched through a window.
"No one could climb out on the window ledge and look into Mr. Henley's apartments. No one could see into that apartment from the street--that is, could see whether Mr. Henley was asleep or even in bed. They could see the light. Watch was kept with the aid offered by the flagpole, supplemented with a mirror--this mirror. A screw was driven into the frame--it has been removed now--it was swung on the flagpole rope and pulled out to the end of the pole, facing the building. To a man standing in the hall window of the third floor it offered precisely the angle necessary to reflect the interior of Mr. Henley's suite, possibly even showed him in bed through a narrow opening in the curtain. There is no shade on the windows of that suite; heavy curtains instead. Is that right?"
Again the prisoner was surprised into a mute acquiescence.
"I saw the possibility of these things, and I saw, too, that at three or four o'clock in the morning it would be perfectly possible for a person to move about the upper halls of this house without being seen. If he wore a heavy bath robe, with a hood, say, no one would recognize him even if he were seen, and besides the garb would not cause suspicion. This bath robe has a hood.
"Now, in working the mirror back and forth on the flagpole at night a tiny scarlet thread was pulled out of the robe and clung to the rope. I found this thread; later Mr. Hatch found an identical thread in these apartments. Both came from that bath robe. Plain logic shows that the person who blew down the gas pipes worked the mirror trick; the person who worked the mirror trick left the thread; the thread comes back to the bath robe--that bath robe there," he pointed dramatically. "Thus the person who desired Henley's death was in these apartments, or had easy access to them."