Detective-Inspector Dunbar arrived at New Scotland Yard in a veritable
fever of excitement. Jumping out of the cab he ran into the building
and without troubling the man in charge of the lift went straight on
upstairs to his room. He found it to be in darkness and switched on
the green-shaded lamp which was suspended above the table. Its light
revealed a bare apartment having distempered walls severely decorated
by an etching of a former and unbeautiful Commissioner. The blinds
were drawn. A plain, heavy deal table (bearing a blotting-pad, a
pewter ink-pot, several pens and a telephone), together with three
uncomfortable chairs, alone broke the expanse of highly polished
floor. Dunbar glanced at the table and then stood undecided in the
middle of the bare room, tapping his small, widely separated teeth
with a pencil which he had absently drawn from his waistcoat pocket.
He rang the bell.

A constable came in almost immediately and stood waiting just inside
the door.

"When did Sergeant Sowerby leave?" asked Dunbar.

"About three hours ago, sir."

"What!" cried Dunbar. "Three hours ago! But I have been here myself
within that time--in the Commissioner's office."

"Sergeant Sowerby left before then. I saw him go."

"But, my good fellow, he has been back again. He spoke to me on the