Inspector Bristol and the officer dragged the box fully into the
light. It was some three feet long by one foot square, and solidly
constructed.

"It is perfectly evident," remarked Bristol, "that the murderer
stayed to search for--"

"The key of the safe!"

"Exactly. If the men really heard sounds here, it would appear that
the assassin was still searching at that time."

"I assure you," the officer interrupted, "that there was no living
thing in the room when we entered."

Bristol and I looked at one another in horrified wonder.

"It's incomprehensible!" he said.

"See if the key is in the place mentioned by the Professor, Mr.
Cavanagh, whilst I break the box."

I went to a great, open bookcase, which the frantic searcher seemed
to have overlooked. Removing the bulky "Assyrian Mythology," there,
behind the volume, lay an envelope, containing a key, and a short
letter. Not caring to approach more closely to the table and to
that which lay beneath it, I was peering at the small writing, in
the semi-gloom by the bookcase, when Bristol cried--