We returned to the library.

“Inspector Weymouth,” said my friend, “I have a particular reason for asking that Sir Crichton’s body be removed from this room at once and the library locked.  Let no one be admitted on any pretense whatever until you hear from me.”  It spoke volumes for the mysterious credentials borne by my friend that the man from Scotland Yard accepted his orders without demur, and, after a brief chat with Mr. Burboyne, Smith passed briskly downstairs.  In the hall a man who looked like a groom out of livery was waiting.

“Are you Wills?” asked Smith.

“Yes, sir.”

“It was you who heard a cry of some kind at the rear of the house about the time of Sir Crichton’s death?”

“Yes, sir.  I was locking the garage door, and, happening to look up at the window of Sir Crichton’s study, I saw him jump out of his chair.  Where he used to sit at his writing, sir, you could see his shadow on the blind.  Next minute I heard a call out in the lane.”

“What kind of call?”

The man, whom the uncanny happening clearly had frightened, seemed puzzled for a suitable description.

“A sort of wail, sir,” he said at last.  “I never heard anything like it before, and don’t want to again.”

“Like this?” inquired Smith, and he uttered a low, wailing cry, impossible to describe.  Wills perceptibly shuddered; and, indeed, it was an eerie sound.

“The same, sir, I think,” he said, “but much louder.”

“That will do,” said Smith, and I thought I detected a note of triumph in his voice.  “But stay!  Take us through to the back of the house.”

The man bowed and led the way, so that shortly we found ourselves in a small, paved courtyard.  It was a perfect summer’s night, and the deep blue vault above was jeweled with myriads of starry points.  How impossible it seemed to reconcile that vast, eternal calm with the hideous passions and fiendish agencies which that night had loosed a soul upon the infinite.

“Up yonder are the study windows, sir.  Over that wall on your left is the back lane from which the cry came, and beyond is Regent’s Park.”

“Are the study windows visible from there?”

“Oh, yes, sir.”

“Who occupies the adjoining house?”

“Major-General Platt-Houston, sir; but the family is out of town.”

“Those iron stairs are a means of communication between the domestic offices and the servants’ quarters, I take it?”

“Yes, sir.”

“Then send someone to make my business known to the Major-General’s housekeeper; I want to examine those stairs.”

Singular though my friend’s proceedings appeared to me, I had ceased to wonder at anything.  Since Nayland Smith’s arrival at my rooms I seemed to have been moving through the fitful phases of a nightmare.  My friend’s account of how he came by the wound in his arm; the scene on our arrival at the house of Sir Crichton Davey; the secretary’s story of the dying man’s cry, “The red hand!”; the hidden perils of the study; the wail in the lane—­ all were fitter incidents of delirium than of sane reality.  So, when a white-faced butler made us known to a nervous old lady who proved to be the housekeeper of the next-door residence, I was not surprised at Smith’s saying: