"Good boy, Jack," said he. "That is just the point I could not be sure about. But finally, at the time, you will remember, when I predicted McCarthy's disappearance, Monsieur X made a definite threat. He said," observed Darrow, consulting one of the bundle of papers he held in his hand: "'My patience is at an end. Your last warning will be sent you at nine-thirty this morning. If you do not sail on the _Celtic_ at noon, I shall strike,' and so forth. The _Celtic_ sailed at noon, without McCarthy. At twelve thirty came the first message to the people calling on them to deliver up the traitor that is among you.' How did Monsieur X know that McCarthy had not sailed on the _Celtic_? The answer is now unavoidable: either an accomplice must have sent him word to that effect, or he must have determined the fact for himself. "I eliminated the hypothesis of an accomplice on the arbitrary grounds of plain common sense. They don't grow two such crazy men at once; and one crazy man is naturally too suspicious to hire help. I took it for granted. Had to make a guess somewhere; but, contrary to our legal friends, I believe that enough coincidences indicate a certainty. But if Monsieur X himself saw the _Celtic_ sail without McCarthy, and got back to his instrument within a half-hour, it was evident he could not be quite so far away as Eldridge and the rest of them thought." "One thing," spoke up Jack, "I often wondered what you whispered to Simmons to induce him to pass those messages over to you. Mind telling?" "Not a bit. Simmons is an exceptional man. He has nerve and intelligence. I just pointed out to him the possibility that Monsieur X might have control over heat vibrations. He saw the public danger at once, and realized that McCarthy's private rights in those messages had suddenly become very small." Jack nodded. "Go ahead," said he. "I had already," proceeded Darrow, "found out where the next wireless station is located. Monsieur X must be nearer the Atlas station than to this other. It was, therefore, easy to draw a comparatively small circle within which he must be located." "So far, so good," said Helen. "How did you finally come to the conclusion that Monsieur X was in the next office?" "Do you remember," Darrow asked Jack, "how the curtain of darkness hung about ten or twelve feet inside the corridor of the Atlas Building?" "Sure," replied Jack. "And do you remember that while the rest of you, including Eldridge, were occupied rather childishly with the spectacular side of it, I had disappeared inside the blackness?" "Certainly."