e were all the elements of the mystery that might have puzzled
Sherlock Holmes. The detectives began to puzzle it out. They were all
watermen, and knew, what the doctor had apparently overlooked, that a
body will often swell after prolonged immersion in water. Although the
rope was woven tightly about the body there was only one actual knot.
They came to a directly opposite conclusion to the doctor--that the rope
had somehow enwound itself round the man after he was in the water, and
that the swelling of the body had tightened it. They began to make
enquiries. Soon they discovered that a seamen named John Duncan had
vanished from the ship _Thames_, moored at Carron Wharf, near Tower
Bridge. Also a piece of "throw line" similar to that twisted round the
body was missing. Also that Duncan, the last time he was seen alive, had
declared his intention of taking a bathe. These facts made it easy for
the sailor police to reconstruct the tragedy.

Duncan was unable to swim. He attached one end of the rope round his
chest and fastened the other end to the ship. Then he had slipped
overboard among the piles of the wharf. By some means the end of the
rope in the ship became detached. Duncan struggled to save himself and
the rope became entangled about him. That was the solution of what
seemed a baffling problem.

The men of the division receive the same pay as men ashore, but they are
a class entirely apart. On land, men are transferred from division to
division as they are promoted, or as occasion demands. On the river this
system does not apply in practice. Most of the men spend their whole
police career on the water, for it takes so long to make the complete
police officer of the Thames Division, and a man once trained is too
valuable to be used for other work.

CHAPTER XIII.

THE BLACK MUSEUM.

Outside Scotland Yard they call it the "Black Museum"; within, it is
simply the "Museum"--a private museum the like of which exists nowhere
else in the world. Money cannot purchase access to it, and curious
visitors are only admitted on orders signed by senior executive
officials who know them personally. For the museum contains too many of
the secrets of crime to be a wholesome place for the general public,
although the indiscriminate publicity that it has suffered in print has
made it appear to be a kind of gratuitous show-place. If that were its
only purpose, it would not exist at Scotland Yard.

It was originally esta