er thing against the theory of suicide was that the second
joint of the first finger was pressing against the trigger. Mr.
Challoner was an expert shot, and would instinctively have used the
pad of the finger, not the second joint.

"The next step," continued Malcolm Sage, "was how could anyone get
into the room and approach Challoner without being heard or
'sensed.'"

"He must have been very much absorbed in what he was doing,"
suggested Sir James.

Malcolm Sage shook his head, and for a few seconds gazed at the
photographs before him.

"You will remember there was nothing on the table in front of him. I
shall come to that presently. It is very unlikely that a man sitting
at a table would not be conscious of someone approaching him from
behind, no matter how quietly he stepped, _unless that man's
presence in the room were quite a normal and natural thing_. That
gave me the clue to Peters. He is the only person who could be in
the library without Challoner taking any notice of him. Consequently
it was easy for him to approach his master and shoot him."

"But the locked door, sir," said Thompson.

"That is a very simple matter. An ordinary lead-pencil, with a piece
of string tied to one end, put through the ring of the key to act as
a lever, the cord being passed beneath the door, will lock any door
in existence. The pencil can then be drawn under the door. This will
show how it's done." Malcolm Sage reached across for a sheet of
paper, and drew a rough sketch.

[Illustration]

"That is why you examined the under-edge of the door?" suggested Sir
James.

Malcolm Sage nodded. "The marks of the cord were clearly defined and
reflected in the mirror. Had the key not been touched, it would have
helped."

"How?" asked Inspector Wensdale.

"By means of the string the key is turned only just to the point
where the lever falls through the hole to the floor. The fingers
would turn beyond that point, not being so delicate."

"Mr. Sage, you're a wonder," burst out the inspector.

"I then," proceeded Malcolm Sage, "examined all the other doors in
the house, and I found that of one room, which I after discovered to
be Peters', was heavily scored at the bottom. He had evidently
practised fairly extensively before putting the plan into operation.
He had also done the same thing with the library door, as there were
marks of more than one operation. Furthermore, he was wiser than to
take the risk of so clumsy a tool as a