n.o.body stirred.

"Why," resumed Darrow, "how, if Monsieur X was a hundred miles or so away, as Eldridge figured, did he know that McCarthy had not obeyed him? We must suppose, from the probable fact of that knowledge, that either Monsieur X had an accomplice who was keeping him informed, or he must be near enough to get the information himself."

"There is a third possibility," broke in Jack. "Monsieur X might have sent along his 'sign' at six o'clock, anyhow, just for general results."

Darrow nodded his approval.

"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 pa.s.s 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."

"Well, in that time I determined the exact extent of the phenomena. I found that it extended in a rough circle. And when I went outside and looked up--something every one else was apparently too busy to do--I saw that this phenomenon of darkness also extended above the building, out into open s.p.a.ce. At the moment I noted the fact merely, and tried to fix in my own mind approximately the dimensions. Then here is another point: when the city-wide phenomena took place, I again determined their extent.

To do so I did not have to leave my chair. The papers did it for me. They took pains to establish the farthest points to which these modern plagues of Manhattan reached."

Darrow selected several clippings from his bundle of papers.

"Here are reports indicating Highbridge, Corona, Flatbush, Morrisania, Fort Lee, Bay Ridge as the farthest points at which the phenomena were manifested. It occurred to n.o.body to connect these points with a pencil line. If that line is made curved, instead of straight, it will be found to const.i.tute a complete circle _whose center is the Atlas Building_!"

The audience broke into exclamations.

"Going back to my former impressions, I remembered that the pall of blackness extended this far and that far in the various directions, so that it required not much imagination to visualize it as a sphere of darkness. And strangely enough the center of that sphere seemed to be located somewhere near the floor on which were installed the United Wireless instruments. It at once became probable that what we may call the nullifying impulses radiated in all directions through the ether from their sending instrument.

"Next I called upon the janitor of the Atlas Building, representing myself as looking for a suitable office from which to conduct my investigations.

In this manner I gained admission to all unrented offices. All were empty.

I then asked after the one next door, but was told it was rented as a storeroom by an eccentric gentleman now away on his travels. That was enough. I now knew that we had to do with a man next door, and not miles distant, as purely scientific reasoning would seem to prove."

"But Professor Eldridge's experiments--" began Jack.

"I am coming to that," interrupted Darrow. "When Eldridge began to call up Monsieur X, that gentleman answered without a thought of suspicion; nor was he even aware of the very ingenious successive weakenings of the current. In fact, as merely the thickness of a roof separated his receiving instrument from the wires from which the messages were sent, it is probable that Eldridge might have weakened his current down practically to nihil, and still Monsieur X would have continued to get his message."

"Wouldn't he have noticed the sending getting weaker?" asked Jack shrewdly.

"Not until the very last. Our sending must have made a tremendous crash, anyway, and he probably read it by sound through the wall."

"But at about the fifty-mile limit of sending we lost him," objected Jack.

"You mean at about two o'clock in the morning," amended Darrow.

"Eh? Yes, it was about two. But how did he get on to what Eldridge was doing?"

"He read it in the paper," replied Darrow. "At twelve the reporters left.

At a little before two our enterprising friend, the _Despatch_, issued an extra in its usual praiseworthy effort to enlighten the late Broadway jag.

Monsieur X read it, and knew exactly what was up."

"How do you know?"

"Because I read the extra myself."

"But even then?"

"Then he began to pay more attention. It was easy enough to fake when he knew what was doing. For all I know, he could hear Eldridge giving his directions."

The company present ruminated over the disclosures thus far made.

"About the City Hall affair?" asked Helen finally.

"I used to sit where I could command the hall," said Darrow, "and, therefore, I was aware that Monsieur X never left his room. To make the matter certain, I powdered the sill of the door with talc.u.m, which I renewed every day after the cleaners. You remember we got to talking very earnestly in the hall, so earnestly that I, for one, forgot to watch. When I realized my remissness, I saw that the powder on the sill had been disturbed, that Monsieur X had gone out.

"My first thought then was to warn the people. To that end I was on my way to the _Despatch_ office when sheer chance switched me into the City Hall tragedy. I possessed myself of the apparatus--"

"That was the square black bag!" cried Jack.

"Of course--and hustled back to the Atlas Building. You can bet I was relieved when I found that Monsieur X had returned to his lair."

"Talc.u.m disturbed again?" asked Jack.

"Precisely."

"And the black bag?"

"Contained merely a model wireless apparatus with a clockwork arrangement set to close the circuit at a certain time. That is why Monsieur X was not involved in his own catastrophe."

"I see!"

"Then all I had to do was to sit still and wait for him to become dangerous."