Hutchinson Hatch, reporter, was standing in a corner with both hands in his coat pockets. Just three inches to the left of his second waistcoat button was the point of a stiletto, and he glanced at it from time to time in frank uneasiness, then his eyes returned to the flushed, tense face of the girl who held it. She was Italian. Her eyes were splendidly black, and there was a gleam in them that was anything but reassuring. Her scarlet lips were parted slightly, disclosing small, regular, white teeth clenched tightly together. A brilliant multicolored headdress partially confined her hair and rippled down about her shoulders. Her skirt was barely to her ankles.

“I feel like the third act of an Italian comic opera,” Hatch thought grimly. Then aloud, “What is all this?”

“You must be silent, signor!” warned the young woman in excellent English.

“I am going to be,” Hatch explained; “but still I should like to —”

“You must be silent, signor!” the girl repeated. “No, don’t take your hands from your pockets!”

“But look here!”

The stiletto point was pressed in until he felt it against his flesh. He winced involuntarily, but wisely held his tongue. It was a time to stand perfectly still and wait. He had come to the tenement in the course of his professional duties, and had rapped on this door to inquire in which apartment a certain family lived. The door had been opened by the young woman — and now this! He didn’t understand it; he didn’t even make a pretense of conjecturing what it meant. He just kept on standing still.

From outside came the varied noises of a busy city. Inside the gloom grew about him, and gradually the rigid, motionless figure of the girl became a shadowy silhouette. Then an electric arc light outside, which happened to be on a level with a window, spluttered and flashed into brilliance almost blinding him. Through the murk of the room only their motionless figures were visible.

After awhile the reporter heard vaguely a stealthy shuffle of feet as if some one was passing along the hall. Then the door leading from the hall into the next room opened and closed softly. The girl prodded him with the stiletto point to remind him to be silent. It was a needless warning, because now Hatch dimly foresaw some grave and imminent danger to himself in the presence of this third person, whoever it might be. Unconsciously he was concentrating all his forces, mental and physical, for — for something he didn’t know what.

The shuffling feet were now in the next room. He heard them moving about as if coming toward the connecting door. Then a hand was laid on the knob, the lock rattled a little, and the door was softly closed. Hatch took a deep breath of relief — whoever this third person might be, he evidently had no business in the room with them just at that moment.

With straining ears and tense nerves the reporter listened, and after awhile came a muffled chatter as of some one talking rapidly and incoherently. Then he heard a man’s voice, pleasant neither in tone nor in the expletives used, and several times he heard the chatter — quick, excited, incoherent. At last the man broke out into a string of profanity, objurgations. The chatter rose angrily, and burst finally into a strangling, guttural scream of anguish.

With a chilly creepiness along his spine and nerves strained to the breaking point, Hatch started forward involuntarily. The stiletto point at his breast stopped him. He glared at the rigid figure of the girl and choked back, with an effort, an outburst of emotion. His utter helplessness overwhelmed him.

“Some one is being killed in there!” he protested desperately between gritting teeth.

“Sh-h!” warned the girl.

From the next room came the shuffling of feet again, then a soft thump thrice repeated, and a faint gurgling cry. Hatch shivered a little; the girl was rigid as marble.

“I guess that fixed you!” Hatch heard a man say.

There was silence for a minute or so. The feet moved stealthily again, and the door leading from the other room into the hall opened and closed. The footsteps moved rapidly along, then apparently precaution was forgotten, for they clattered down the steps and were gone.

Suddenly the girl straightened up. “You will remain here, signor,” she said, “until I am out of the house? You will raise no alarm for at least five minutes? Believe me, if you do, it will be worse for you; for sometime, somewhere, you will have occasion to regret it! You promise?”

Hatch would not make himself believe that he had the slightest choice in the matter. “I promise, of course,” he said.

She bowed a little, half mockingly, flung open the door, and ran out. Hatch heard the swishing of her skirts as she sped down the stairs, then he brought himself together with a huge sigh and a nervous jerking of his limbs.

He strode across the room twice to regain possession of jumping nerves, then paused and lighted a cigarette. What was in the next room? He didn’t know. He wanted to know, and yet there was an intangible fear which clung to him and held him back when he started for the door. At last he mastered this absurd weakness, and flung the door open wide, and walked in. At first he saw nothing, and he had expected to see every evidence of a brutal crime. Then in a far corner he noticed what seemed to be a bundle of rags which had been thrown there carelessly. He strode over boldly and poked it with his foot, stooping to examine it.

What he saw brought an exclamation from him; but it was rather of astonishment than of horror. The thing he had found was the body of a monkey. The rags were the tawdry clothing in which organ grinders attire their apish companions. There was a little cap, a coat, and trousers.

“Well! What in the deuce —” exclaimed the reporter. He dropped on his knees beside the tiny body. There were three stab wounds in it — one in the throat and two in the breast. The body was still warm.

“But why,” protested Hatch, “should anyone, man or woman, murder a monkey?”

Professor Augustus S. F. X. Van Dusen — The Thinking Machine — didn’t hazard a conjecture. “Are you sure it was a monkey that was murdered?” he asked instead. “I mean are you sure that only a monkey was murdered?”

“I am sure,” he responded emphatically, “that the monkey was killed while I listened, and certainly there was nothing else that I could find or that I heard to indicate anything beyond that.”

“Did you search the place?” queried the scientist.

“Yes.”

“Find anything?”

“No, nothing.”

“Did you happen to notice, Mr. Hatch, if the monkey’s clothing had pockets?”

“There were no pockets. I looked for them.”

The Thinking Machine lay back in his chair, steadily squinting upward for several minutes, without speaking. Then: “I can comprehend readily why the monkey should have been killed as it was. Any one of half a dozen hypotheses would explain that. But if the monkey didn’t have a pocket somewhere in its clothing, then I don’t see so readily why — Oh, of course — must have been bigger than I thought,” he mused.

“What?” inquired Hatch.

“Are you sure, Mr. Hatch, that there had been nothing sewn to the clothing of the monkey?” asked The Thinking Machine, without heeding the question —“that nothing had been ripped loose from the clothing?”

“I can’t say as to that,” the reporter replied.

“Where is the monkey now?”

“Still there in the room, I suppose. I came straight from there to you here. Of course, my being held up that way wasn’t of any actual consequence — it was merely incidental, I thought, to the other.”

The Thinking Machine nodded. “Yes,” he agreed. “I presume that was merely because you happened to arrive at an inopportune moment, and that method was employed to keep you out of the way until whatever was to be done was done.”

The Thinking Machine and the reporter went out together. It was a few minutes past nine o’clock when they reached the tenement. It was dirty and illy lighted, and boldly faced a street which was a center of the Italian colony. Hatch led the way in and up the stairs to the room where he had left the monkey. The little body still lay huddled up, inert, as he had left it.

By the light of an electric bulb The Thinking Machine examined it closely. Twice Hatch saw him shake his head. When The Thinking Machine arose from the floor his face was inscrutable. He led Hatch around that room and the next and through a third which connected, and then they went out.

“It is an extraordinary case, Mr. Hatch,” he explained as they went on. “There are now three explanations of the affair, either one of which would fit in with every fact that we know. But instead of helping us, these three possibilities make it necessary for us to know more. Two of them must be removed — the remainder will be correct as surely as two and two make four, not sometimes but all the time.”

Hatch waited patiently.

“The real problem here,” the scientist continued after a moment, “is the identity of the person who owned the monkey. When we get that, we get a starting point.”

“That would not seem difficult,” Hatch suggested. “It is extremely improbable that anyone knows of the affair except the persons who were responsible for it, perhaps the owner of the monkey and ourselves. An advertisement in the newspapers would bring the owner quickly enough.”

“There is always the possibility, Mr. Hatch, that the owner is the man who killed the monkey,” replied the scientist. “In that event the advertisement would do no good; and there is a question if it would be advisable to let those persons who are responsible for the animal’s death know that the matter is being investigated. This is presuming, of course, that some one besides the owner killed it. It will be just as well to let the young woman who held you prisoner believe that the affair is at an end. Any other course just now might indirectly endanger the life of some one who has not yet appeared in the case.

“I am going home now, Mr. Hatch,” concluded the scientist, “and it is possible that within two or three hours I may devise a plan by which we can find the monkey’s owner. If so, I shall communicate with you.”

“You can reach me at police headquarters until about midnight,” replied the reporter. “I am going up there on another affair.”

It was about a quarter past eleven o’clock that night when Hatch scurried away to a telephone and eagerly cried to The Thinking Machine, “I know the man who owned the monkey!”

Ten minutes later he was in the scientist’s little reception room. “The man who owned the monkey,” he said, “is named Giacomo Bardetto. He is an organ grinder. He was found unconscious in an area way at the other end of the city tonight at ten o’clock. He had been struck down from behind, his organ smashed, his pockets rifled, and no one knows how long he had been unconscious when found. He is now in a hospital, still unconscious. The police know nothing whatever about the monkey incident; but I surmise that the dead monkey was Bardetto’s. You might have noticed that a short chain was attached to the monkey’s clothing? The other end of that chain is fastened to the hand organ.”

“How was Bardetto identified?” asked The Thinking Machine.

“By his organ grinder’s license, which was fastened to the inside of a flap on the instrument.”

“His home?”

“Here is the address,” and the reporter produced a card on which he had jotted down the street and number.

The Thinking Machine studied the card for a moment, then glanced at his watch. It was five minute............