There was a straight passage, dimly lighted, which led from the room in which I had been working, and was the only exit from it. It ended in a curving staircase, with the commissionaire's lodge in the passage at the bottom. Half way down this staircase is a small landing, with another passage running into it at right angles. This second one leads by means of a second small stair to a side door, used by servants, and also as a short cut by clerks when coming from Charles Street. Here is a rough chart of the place.”

“Thank you. I think that I quite follow you,” said Sherlock Holmes.

“It is of the utmost importance that you should notice this point. I went down the stairs and into the hall, where I found the commissionaire fast asleep in his box, with the kettle boiling furiously upon the spirit-lamp. I took off the kettle and blew out the lamp, for the water was spurting over the floor. Then I put out my hand and was about to shake the man, who was still sleeping soundly, when a bell over his head rang loudly, and he woke with a start.

“‘Mr. Phelps, sir!’ said he, looking at me in bewilderment.

“‘I came down to see if my coffee was ready.’

“‘I was boiling the kettle when I fell asleep, sir.’ He looked at me and then up at the still quivering bell with an ever-growing astonishment upon his face.

“‘If you was here, sir, then who rang the bell?’ he asked.

“‘The bell!’ I cried. ‘What bell is it?’

“‘It's the bell of the room you were working in.’

“A cold hand seemed to close round my heart. Some one, then, was in that room where my precious treaty lay upon the table. I ran frantically up the stair and along the passage. There was no one in the corridors, Mr. Holmes. There was no one in the room. All was exactly as I left it, save only that the papers which had been committed to my care had been taken from the desk on which they lay. The copy was there, and the original was gone.”

Holmes sat up in his chair and rubbed his hands. I could see that the problem was entirely to his heart. “Pray, what did you do then?” he murmured.

“I recognized in an instant that the thief must have come up the stairs from the side door. Of course I must have met him if he had come the other way.”

“You were satisfied that he could not have been concealed in the room all the time, or in the corridor which you have just described as dimly lighted?”

“It is absolutely impossible. A rat could not conceal himself either in the room or the corridor. There is no cover at all.”

“Thank you. Pray proceed.”

“The commissionaire, seeing by my pale face that something was to be feared, had followed me upstairs. Now we both rushed along the corridor and down the steep steps which led to Charles Street. The door at the bottom was closed, but unlocked. We flung it open and rushed out. I can distinctly remember that as we did so there came three chimes from a neighboring clock. It was quarter to ten.”

“That is of enormous importance,” said Holmes, making a note upon his shirt-cuff.

“The night was very dark, and a thin, warm rain was falling. There was no one in Charles Street, but a great traffic was going on, as usual, in Whitehall, at the extremity.