locked, was it not,
Tarling, when you made the discovery?"

"Yes," said Tarling, "it was locked."

"When they found they could not get into the house," Ling Chu went on,
"they tried to get through one of the windows."

"They, they?" said Tarling impatiently. "Who are they? Do you mean the
woman?"

The new theory was disturbing. He had pierced the second actor in the
tragedy--a brown vitriol burn on the back of his hand reminded him of his
existence--but who was the third?

"I mean the woman," replied Ling Chu quietly.

"But who in God's name wanted to get into the house after murdering Mrs.
Rider?" asked Whiteside irritably. "Your theory is against all reason,
Ling Chu. When a person has committed a murder they want to put as much
distance between themselves and the scene of the crime as they can in the
shortest possible space of time."

Ling Chu did not reply.

"How many people are concerned in this murder?" said Tarling. "A
bare-footed man or woman came in and killed Mrs. Rider; a second
person made the round of the house, trying to get in through one
of the windows----"

"Whether it was one person or two I cannot tell," replied Ling Chu.

Tarling made a further inspection of the little wing. It was, as Ling Chu
had said and as he had explained to the Chinaman, cut off from the rest
of the house, and had evidently been arranged to give Mr. Milburgh the
necessary privacy upon his visits to Hertford. The wing consisted of
three rooms; a bedroom, leading from the sitting-room, evidently used
by Mrs. Rider, for her clothes were hanging in the wardrobe; the
sitting-room in which the murder was committed, and the spare room
through which he had passed with Odette to the gallery over the hall.

It was through the door in this room that admission was secured to the
house.

"There's nothing to be done but to leave the local police in charge and
get back to London," said Tarling when the inspection was concluded.

"And arrest Milburgh," suggested Whiteside. "Do you accept Ling Chu's
theory?"

Tarling shook his head.

"I am loath to reject it," he said, "because he is the most amazingly
clever tracker. He can trace footmarks which are absolutely invisible
to the eye, and he has a bushman's instinct which in the old days in
China led to some extraordinary results."

They returned to town by car, Ling Chu riding beside the chauffeur,
smoking an interminable chain of cigarettes. Tarling spoke very little
duri