"I'm afraid it's not quite the thing to enter people's houses in this fashion," whispered Juve, as the two men found themselves in the hall of Doctor Chaleck's little house in the Frochot district.

It was about midnight, and through the fan-light of the outer door a dim twilight enabled the detective and the journalist to get an idea of the place in which they stood.

It was a fairly large hall with double doors on either hand, leading into the drawing-and dining-rooms. At the far end rose a winding staircase, and under it a door to the cellar. A hanging lamp, unlit, was suspended from the ceiling and the walls were covered with dark tapestries.

Juve and Fandor remained silent and motionless for some moments. They might well be perturbed, for they had just entered the house in the most unwarrantable manner, and they knew the doctor to be at home. The lodge-keeper of the CitÉ had seen him return about two hours ago. For one moment Juve had asked himself whether he should not ring in the most natural manner in the world, and afterwards contrive some explanation; but the silence, the peace which prevailed and the conviction that Doctor Chaleck, quite off his guard, must be enjoying deep slumber, prompted him to try and get into the house unannounced. If the door was only bolted, if it was not secured from within by a latch, the officer might reckon on finding among his pass keys one that would allow him to open it. Juve was, indeed, equipped like the prince of burglars.

Well, the attempt had succeeded. Without trouble or noise, journalist and officer had made their way into the place.

Before imparting to Fandor his plan of operations, Juve handed him a pair of rubbers, and then at a signal they both ascended to the first floor.

The detective's plan was to make a sudden incursion into Chaleck's bedroom, and in the surprise of a sudden awakening, question him and inspect the fingers of his right hand, which, presumably, had left on the register a tell-tale trace of blood.

Juve had scarcely entered the room when Fandor switched on the lights; the two men started back in disgust; the room was empty!

Without pause, Juve cried: "To the study!"

A moment later they found themselves in the room they knew so well from having spent a whole night there, behind the window curtains.

Chaleck was not there either. Fandor searched the bathroom near by, careless of the noise he made, then hurried after Juve to the floor below in the fear that the doctor might already have made his escape.

Juve quickly reassured him the windows and shutters of the rooms were hermetically closed; the hall door had not been touched.

Suddenly slight sounds became audible from the floor above. A crackling of the boards, the muffled sounds of hasty footsteps, faint rustlings.

"Chaleck knows we are here," whispered Juve. "We must play with our cards on the table."

The two men cocked their pistols and made a rush upstairs. They had left the electric light burning on the floor above, and at first their eyes were dazzled by the sudden brightness, multiplied by the reflection from the glass which lined the octagonal-shaped landing.

Again the noises were heard. Chaleck or some one else was in the study.

Juve disappeared. In half a minute he returned and bumped into Fandor.

"Where are you coming from?" he cried. "I thought you were behind me."

"So I was," replied Fandor, "but I left you to take a look in the study."

"But it was I who was in the study!"

Fandor stared in amazement. "Are you losing your senses?"

"I've just come from there myself!"

"Well, we weren't there together, that's certain. Let's try again."

The two proceeded in the dark to the head of the staircase. With their heels they verified the last step; then Juve said in a low voice:

"I will go forward four paces. I am now in the middle of the landing; I lift the curtain, turn and go in."

The steady tick of the little Empire clock on the mantelpiece assured Juve that he was indeed in the study.

"Well, here I am," and mechanically he flung his hat on the sofa. But scarcely had he uttered these words when Fandor's voice, very clear, but some way off answered

"I am in the study, too."

Juve now switched on the light. Fandor was not there. Rushing back to the landing he ran full tilt into his friend and the two gripped each other in amazement.

"Look here," exclaimed Fandor, "if I'm not mistaken, you turned to the right past the curtain while I went to the left; there may be two separate entrances to the study."

"Let us keep together this time," replied Juve; "I propose to get to the bottom of this mystery."

As they came out of the darkness of the passage and plunged into the full light of the room, Juve stopped short. His hat was no longer on the sofa.

Fandor went to the mantelpiece, turned and confronted the detective.

"I stopped the clock some moments ago, and here it is going and keeping exact time! How do you account for it?"

Juve was about to reply, when suddenly with a dry click the light went out.

Fandor, at the same moment, gave a startled cry: "Juve! the door is fastened; we are shut in!"

With one bound Juve leaped for the window; but after opening the casement he perceived that thick iron shutters, padlocked, banished all hope of escape in that quarter. Fandor was ashy pale; Juve staggered as he moved toward him.

"Walled in!" he cried. "We are walled in!"

But a new terror suddenly confronted the two men. The floor appeared to be giving way, and as the descent proceeded regularly, they realised that they were in a strange form of elevator.

The study, however, did not drop very far. With a slight shock it reached the end of the run and stopped short.

Juve cried with an air of relief, "Well, here we are, and it now remains to find out where we are."

The existence of two studies identical in every particular, one of which was housed in an elevator, explained not only the events of the evening, but also the tragedy of two days before.

"Juve! did you feel anything?"

"Yes."

"What is it?"

"I don't know."

Both had just experienced a weird sensation, impossible to define. Upon their hands and faces slight prickings irritated the skin. The air at the same time seemed heavier and more difficult to breathe. There was, besides, a soft, vague crackling. With some difficulty Juve lighted his pocket-lamp. By its faint glimmer the two men made a discovery. A fine rain of sand was falling from the ceiling.

"It's collapsed!" cried Fandor.

"We're done for!" replied Juve.

They passed through some awful moments. All around the sand gathered and rose.

Juve tried to comfort his friend:

"It would need an enormous amount of sand to fill this room and bury us alive. It will cease to fall presently."

But horrible to relate, as the level of the sand rose on the floor, they observed by the flickering gleam of the lamp, that the ceiling was now being lowered little by little.

Fandor raised his arm and touched it. They were about to be crushed.

"Juve, do not let me die this way. Kill me!"

His comrade made no reply. At first paralysed by the shock he now felt an unspeakable fury rise up in him. He began beating the walls with his fists, shaking the furniture. He seized a chair and drove it against the door. The chair struck with a ring upon metal and broke.

Uttering a loud sigh, the detective drew out his revolver; he would, at least, save his friend the torments of an awful death. Suddenly a fearful crash resounded. The moving mass of sand was falling away from them into some gaping hole below, while at the same time fresh, moist air reached them and refreshed their lungs. Evidently some communication with the outside world had been established.

Juve relit his lamp and was bending over to examine what had taken place when the floor all at once gave way under his feet and he fell, dragging Fandor with him.

They found themselves up to mid-leg in water, but unhurt.

Juve's voice rang out: "We are saved! I see now what happened! Our trap had a thin flooring, and, when down, it rested on a fragile arch. That arch gave way, and with the sand we have tumbled into the sewer of the Place Pigalle, which, if I am not mistaken, connects with the main of the ChaussÉe d'Autin. Come along, friend Fandor, we'll find means to get out of this before long."

Floundering in the mud, they made their way along the drain until Juve halted and uttered a cry of triumph. On the left wall of the vault his hand encountered iron rings one above the other. It was a ladder leading to one of the manholes in the pavement. He quickly climbed up and, with a vigorous push, raised the heavy slab. In a few moments both men emerged and fell exhausted in the roadway.

When Fandor recovered his senses he was lying in a large, ill-lighted hall. The first sound he heard was Juve's voice arguing hotly and volubly.

"Why, you're nothing but a pack of idiots! We burglars! It's utter rot. I tell you I'm Juve, Inspector of Public Safety!"