"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 lead-pencil. He used this."
Malcolm Sage drew from his pocket the roll of twine with the thin steel instrument down the centre. It was a canvas-needle, to the eye of which the cord was attached.
"This was absolutely safe," he remarked. "Another thing I discovered was that one lock, and only one lock in the house, had recently been oiled—that of the library-door."