“He does know a great deal better.  It’s only that temporarily he’s knocked off his balance.  But I hardly feel as anxious as you do.  There’s Mary against May; and even if my uncle were for him, on a general, vague theory of something esoteric and outside nature, which you can’t fairly call unreasonable any more, Mannering, seeing what’s happened—­even if Sir Walter felt tempted to let him have his way, I don’t believe he’d really consent when it came to the point.”

“I hope not—­I hope not,” answered the other.  “Such a concession would take a lot of explanation if the result were another of these disasters.  There ought to be an official guard over the room.”

“After to-morrow there certainly will be,” replied Henry.  “You may be sure the police won’t leave it again till they’ve satisfied themselves.  All the same, I don’t see how a dozen of them will be any safer than one—­even if it’s some material and physical thing that happens, as we must suppose.  And for that matter, if it’s really supernatural, why should a dozen be safer than one?  Obviously they wouldn’t.  Whatever it is, it can strike as it likes and without being struck back.”

But Dr. Mannering did not answer these questions.  He was considering a little book in his pocket, which he would hand over to the police in London next morning.

“Poor chap—­if he could have begun by taking the problem by the throat, as he has written here.  But, instead, it took him by the throat!”

He took Hardcastle’s notebook from his pocket and read again the last few pages.

“He was dreaming of his theories to the last, when he should surely have been girt up in every limb to face facts,” said Lennox.  “He never realized the horrible danger.”

Perusal of the detective’s data had revealed an interesting fact.  It was known by his colleagues that he designed a book on the theory and practice of criminal investigations, and in many of his pocket-books, subsequently examined, were found memoranda and jottings, doubtless destined to be worked out at another time.  It was clear that he had, for a few moments, drifted away from the Grey Room in thought when his death overtook him.  Past events, not present problems, were apparently responsible for the reflections that occupied his mind.  He was not concentrating on the material phenomena actually under his observation when he died, but following some private meditations provoked by his experiences.

“Elimination embraces the secret of success,” he had written.  “Exercise the full force of your intelligence and spare no pains to eliminate from every case all matter not bearing directly upon the actual problem.  Nine times out of ten the issue is direct, and once permit side issues to draw their tracks across it, once admit metaphysical lines of reasoning, the result will be confusion and a problem increasing in complexity at every stage.  Only in romances, where a plot is invented and then complicated