"That may be true," assented Seaton doubtfully, "but you want to remember that he knows more about electricity than the guy that invented it, and I'm not sure that he can't talk to a detectaphone and make it say anything he wants it to. Anyway, we can soon settle it. Yesterday I made a special trip down to the Bureau, with some notes as an excuse, to set this object-compass on him," taking one of the small instruments from his pocket as he spoke. "I watched him a while last night, then fixed an alarm to wake me if the needle moved much, but it pointed steady all night. See! It's moving now. That means that he is going to work early, as usual. Now I'm morally certain that he's mixed up in this thing somewhere, and I'm not convinced that he isn't slipping one over on your men some way--he's a clever devil. I wonder if you wouldn't take this compass and watch him yourself tonight, just on general principles? Or let me do it. I'd be glad to. I say 'tonight' because if he did get the stuff here he didn't deliver it anywhere last night. It's just a chance, of course, but he may do it tonight." After the compass had been explained to the detective he gladly consented to the plan, declaring that he would willingly spend the time just to watch such an unheard-of instrument work. After another hour of fruitless discussion Prescott took his leave, saying that he would mount an impregnable guard from that time on. Late that evening Prescott joined the two men who were watching DuQuesne's house. They reported that all was perfectly quiet, as usual. The scientist was in his library, the instruments registering only the usual occasional faint sounds of a man absorbed in study. But after an hour of waiting, and while the microphones made a noise as of rustling papers, the needle of the compass moved. It dipped slowly toward the earth as though DuQuesne were descending into the cellar, but at the same time the shadow of his unmistakable profile was thrown upon the window shade as he apparently crossed the room. "Can't you hear him walk?" demanded Prescott. "No. He has heavy Turkish rugs all over the library, and he always walks very lightly, besides." * * * * *