Geoffrey had not long to wait. From where he was standing he could see down into the vault perfectly well. He would have been better satisfied had he understood what those people were talking about, but their words conveyed nothing to him.

On the floor of the vault the queer-looking machinery was spread out, and to the ends of the india-rubber tubes wires were attached. No sooner had this been accomplished than the woman, after giving some rapid instructions to her allies, left the vault. She was so quick that Geoffrey barely had time to conceal himself behind a pillar before she passed him.

The woman was masked and disguised beyond recognition, but Geoffrey had no need to be told who she was. He knew that he was in the presence of Mrs. May. And, despite his knowledge of her cleverness and resource, he found himself marveling to see her display so fine a knowledge of the house.

The woman passed along, dragging a number of fine light wires after her. The other ends of the wires were attached to the queer-looking apparatus in the vault.

Mrs. May went along the passages, along the corridor, and up the stairs as if she had been accustomed to the house all her life. Surely she must have been here many times before, or she would not have exhibited such fearless confidence. The idea of the black, gliding figure creeping about the house in the dead of night filled Geoffrey with loathing.

All the same, he did not neglect his opportunities. He followed swiftly and silently until he came to the main corridor on the first landing. Here, to his surprise, the woman turned into one of the bed-rooms, the room used by the head of the house. She closed the door behind her.

What to do next? But Geoffrey was not long in doubt. Ralph was standing by his side, a dark lantern in his hand.

"Where did she go?" he whispered.

"You heard her, then?" asked Geoffrey.

"Of course, I heard everything. I see with my ears. Naturally you guessed who she was. But what room did she go into?"

"My grandfather's."

"So I expected. But she means to visit all the rooms in turn. You need not be afraid, she will be there for some minutes. What do you see outside?"

Geoffrey made a close examination with the lantern.

"I see a tangle of small wires on the floor," he said. "They come up from the vaults."

"Where they are attached to a queer-looking instrument?"

"Yes, yes. I see you know all about it. One of the wires runs under the door into the room where Mrs. May is engaged."

"And where she will be engaged for some time," said Ralph. "Move that book ladder and look over the fanlight."

There were books on high shelves in the corridor, and a light librarian's ladder close at hand. Geoffrey propped this against the door and looked in through the open fanlight. All the bed-room doors had fanlights at Ravenspur.

The lantern inside was on the dressing-table and, standing on a chair by a fireplace, was Mrs. May. She had pinned the thin wire to the wall cunningly, and had turned the end of it into a plate that stood on the mantel shelf. From a flask she poured a little white powder into the plate.

This done she seemed to be satisfied. Geoffrey whipped the ladder away and the woman emerged from the room. Once more she went along the corridor with firm, resolute step, and the air of one who knows what she is doing and has a definite object in view.

From one bed-room to another she went, leaving a wire in each until every room occupied by one of the Ravenspur family had been visited. Geoffrey's room was the last. When she had finished here she took up a pair of scissors and tapped the wire. Outside the door Geoffrey and Ralph could hear the noise distinctly.

Ralph's jaws came together with a click.

"The key is outside your room door," he whispered. "Turn it."

Geoffrey wondered, but he hastened to comply. The key turned with an ease and silence that testified to the fact of its having been carefully oiled.

"What does it all mean?" Geoffrey whispered.

"She is going to test her machinery," said Ralph with a chuckle. "And she is going one step farther to her own destruction. Listen."

Again came the faint tap, and then down from far below the purring jar of electrical apparatus in motion. There was silence inside the room for a moment and then Geoffrey saw the handle turn. It was turned softly at first, then more quickly, and finally it was tugged as an angry child snatches at a toy.

Ralph chuckled. The diabolical mirth seemed to come deep from his throat.

"She is trying to get out," Geoffrey whispered.

"Of course she is," Ralph replied. "But not quite yet."

The lock was rattling loudly by this time; there was a half-angry, half-frightened muttering from within. And then there came a long, piercing, wailing scream, as of a woman in the last agony before death.

Geoffrey would have started back, but Ralph restrained him.

"No, no," he whispered violently. "It is all right; everything is turning out splendidly."

"But she is a woman and in deadly peril, uncle."

"I know it, lad. Five minutes more and that fiend will be beyond further mischief. She has been trying the effect of her infernal contrivance and will be hoist with her own petard. She is scared to death. She imagines she has fastened herself in and can't get out."

"But this is murder," Geoffrey cried.

"I dare say some people would call it so," Ralph replied coolly. "As a matter of fact, there never could be homicide more justifiable than to let that woman perish there. Still, we are not going to do anything of the kind. When those cries cease, and you hear yonder wretch fall to the ground, then open the door and drag her out."

The cries were coming wildly from behind the door; there was a hammering on the panels. The cries rang through the house, they reached the Asiatics in the vaults and the latter fled in terror into the night.

Something had happened, but what it was they did not care. They had only themselves to think of.

In spite of his strong nerves, Geoffrey shuddered. It was horrible to be alone in that grim house of tears, waiting in the darkness, opposed by grim horrors and, above all, to have that note of agony ringing in his ears.

Would it never stop? Would the time to act never come? Geoffrey would have interfered in spite of everything but for the fact that Ralph was gripping his shoulder in a grasp that at any other time would have been painful.

Suddenly the noise ceased. There was a moan and the soft, crushing fall of a body. Ralph's face blazed up instantly.

"Now," he cried, "there is no time to be lost."

Geoffrey darted forward. He had the door opened in an instant. Mrs. May lay still and white on the floor. The atmosphere of the room seemed to have vanished. It was intolerable to breathe there; air there was none.

As the door fell back the room filled as with a sudden strong draught. Geoffrey dragged the unconscious figure into the corridor.

"Will she die?" he gasped.

"No, she will not die," Ralph said coolly. "Had I intended her to die I should not have allowed you to open the door. Pick her up and throw her on one of the beds in a spare room. She will require no attention, but she will not attain consciousness for some hours. And, after that, she will be useless for a day or two. You need not worry; our scheme is working out splendidly. Pick her up."

Ralph indicated the still figure with brutal indifference. He would have shown more consideration to a sick dog. Geoffrey complied, and presently made the woman as comfortable as circumstances allowed.

Geoffrey had hardly done so before there was a light footfall in the corridor, and Tchigorsky appeared, still in disguise.

"I gather that things are well," he said. "Just now I met that she-devil's accomplices fleeing as if the Father of Lies was behind them. She was trapped, eh?"

Ralph nodded and chuckled.

"In Geoffrey's room," he explained. "When she was testing her apparatus I had the key turned on her. And she could not get out. I let her remain there as long as I considered it safe to do so, and her yells must have alarmed her confederates. Probably they have fled, leaving things intact."

"Probably," said Tchigorsky. "I will go and see."

He was back again presently, a pleased expression on his face.

"Nothing has been touched," he said. "I have removed the wires, in case of danger. We have the lady more or less under our thumb."

"What was she doing?" Geoffrey asked.

"It is an appliance for exhausting air," Tchigorsky explained. "You take a powder and place it on a hot plate. Directly it begins to burn it draws up all the air. The thing has been known in the East for thousands of years. Mrs. May applied electricity to give her greater scope. A plate of the powder was to be heated in the room of everybody in the castle when asleep.

"A few minutes and the thing is done. Then the wires are withdrawn and gradually the different rooms fill with air again. The burnt powder leaves no trace. Then you are all found dead in your beds and nobody knows how it is done. The wires are easily drawn back to the battery and the whole thing is destroyed."

Geoffrey shuddered.

"What a fiend!"