Upon this scene came Ling Chu, imperturbable, expressionless, bringing with him his own atmosphere of mystery.

"Well," said Tarling, "what have you discovered?" and even Whiteside checked his enthusiasm to listen.

"Two people came up the stairs last night," said Ling Chu, "also the master." He looked at Tarling, and the latter nodded. "Your feet are clear," he said; "also the feet of the small-piece woman; also the naked feet."

"The naked feet?" said Tarling, and Ling Chu assented.

"What was the naked foot—man or woman?" asked Whiteside.

"It may have been man or woman," replied the Chinaman, "but the feet were cut and were bleeding. There is mark of blood on the gravel outside."

"Nonsense!" said Whiteside sharply.

"Let him go on," warned Tarling.

"A woman came in and went out——" continued Ling Chu.

"That was Miss Rider," said Tarling.

"Then a woman and a man came; then the bare-footed one came, because the blood is over the first women's footmarks."

"How do you know which was the first woman and which was the second?" asked Whiteside, interested in spite of himself.

"The first woman's foot was wet," said Ling Chu.

"But there had been no rain," said the detective in triumph.

"She was standing on the grass," said Ling Chu, and Tarling nodded his head, remembering that the girl had stood on the grass in the shadow of the bushes, watching his adventure with Milburgh.

"But there is one thing I do not understand master," said Ling Chu. "There is the mark of another woman's foot which I cannot find on the stair in the hall. This woman walked all round the house; I think she walked round twice; and then she walked into the garden and through the trees."

Tarling stared at him.

"Miss Rider came straight from the house on to the road," he said, "and into Hertford after me."

"There is the mark of a woman who has walked round the house," insisted Ling Chu, "and, therefore, I think it was a woman whose feet were bare."

"Are there any marks of a man beside us three?"

"I was coming to that," said Ling Chu. "There is a very faint trace of a man who came early, because the wet footsteps are over his; also he left, but there is no sign of him on the gravel, only the mark of a wheel-track."

"That was Milburgh," said Tarling.

"If a foot has not touched the ground," explained Ling Chu, "it would leave little trace. That is why the woman's foot about the house is so hard for me, for I cannot find it on the stair. Yet I know it came from the house because I can see it leading from the door. Come, master, I will show you."

He led the way down the stairs into the garden, and then for the first time Whiteside noticed that the Chinaman was bare-footed.

"You haven't mixed your own footmarks up with somebody else's?" he asked jocularly.

Ling Chu shook his head.

"I left my shoes outside the door because it is easier for me to work so," he said calmly, slipping his feet into his small shoes.

He led the way to the side of the house, and there pointed out the footprints. They were unmistakably feminine. Where the heel was, was a deep crescent-shaped hole, which recurred at intervals all round the house. Curiously enough, they were to be found in front of almost every window, as though the mysterious visitor had walked over the garden border as if seeking to find an entrance.

"They look more like slippers than shoes to me. They're undoubtedly a woman's," said Whiteside, examining one of the impressions. "What do you think, Tarling?"

Tarling nodded and led the way back to the room.

"What is your theory, Ling Chu?" he asked.

"Somebody came into the house," said the Chinaman, "squeezed through the door below and up the stairs. First that somebody killed and then went to search the house, but could not get through the door."

"That's right," said Whiteside. "You mean the door that shuts off this little wing from the rest of the house. That was 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 during the journey, his mind being fully occupied with the latest development of a mystery, the solution of which still evaded him.

The route through London to Scotland Yard carried him through Cavendish Place, where the nursing home was situated in which Odette Rider lay. He stopped the car to make inquiries, and found that the girl had recovered from the frenzy of grief into which the terrible discovery of the morning had thrown her, and had fallen into a quiet sleep.

"That's good news, anyway," he said, rejoining his companion. "I was half beside myself with anxiety."

"You take a tremendous interest in Miss Rider, don't you?" asked Whiteside dryly.

Tarling brindled, then laughed.

"Oh, yes, I take an interest," he admitted, "but it is very natural."

"Why natural?" asked Whiteside.

"Because," replied Tarling deliberately, "Miss Rider is going to be my wife."

"Oh!" said Whiteside in blank amazement, and had nothing more to say.

The warrant for Milburgh's arrest was waiting for them, and placed in the hands of Whiteside for execution.

"We'll give him no time," said the officer. "I'm afraid he's had a little too much grace, and we shall be very lucky if we find him at home."

As he had suspected, the house in Camden Town was empty, and the woman who came daily to do the cleaning of the house was waiting patiently by the iron gate. Mr. Milburgh, she told them, usually admitted her at half-past eight. Even if he was "in the country" he was back at the house before her arrival.

Whiteside fitted a skeleton key into the lock of the gate, opened it (the charwoman protesting in the interests of her employer) and went up the flagged path. The door of the cottage was a more difficult proposition, being fitted with a patent lock. Tarling did not stand on ceremony, but smashed one of the windows, and grinned as he did so.

"Listen to that?"

The shrill tinkle of a bell came to their ears.

"Burglar alarm," said Tarling laconically, and pushed back the catch, threw up the window, and stepped into the little room where he had interviewed Mr. Milburgh.

The house was empty. They went from room to room, searching the bureaux and cupboards. In one of these Tarling made a discovery. It was no more than a few glittering specks which he swept from a shelf into the palm of his hand.

"If that isn't thermite, I'm a Dutchman," he said. "At any rate, we'll be able to convict Mr. Milburgh of arson if we can't get him for murder. We'll send this to the Government analyst right away, Whiteside. If Milburgh did not kill Thornton Lyne, he certainly burnt down the premises of Dashwood and Solomon to destroy the evidence of his theft."

It was Whiteside who made the second discovery. Mr. Milburgh slept on a large wooden four-poster.

"He's a luxurious devil," said Whiteside. "Look at the thickness of those box springs." He tapped the side of that piece of furniture and looked round with a startled expression.

"A bit solid for a box spring, isn't it?" he asked, and continued his investigation, tearing down the bed valance.

Presently he was rewarded by finding a small eyelet hole in the side of the mattress. He took out his knife, opened the pipe cleaner, and pressed the narrow blade into the aperture. There was a click and two doors, ludicrously like the doors which deaden the volume of gramophone music, flew open.

Whiteside put in his hand and pulled something out.

"Books," he said disappointedly. Then, brightening up. "They are diaries; I wonder if the beggar kept a diary?"

He piled the little volumes on the bed and Tarling took one and turned the leaves.

"Thornton Lyne's diary," he said. "This may be useful."

One of the volumes was locked. It was the newest of the books, and evidently an attempt had been made to force the lock, for the hasp was badly wrenched. Mr. Milburgh had, in fact, made such an attempt, but as he was engaged in a systematic study of the diaries from the beginning he had eventually put aside the last volume after an unsuccessful effort to break the fastening.

"Is there nothing else?" asked Tarling.

"Nothing," said the disappointed inspector, looking into the interior. "There may be other little cupboards of this kind," he added. But a long search revealed no further hiding-place.

"Nothing more is to be done here," said Tarling. "Keep one of your men in the house in case Milburgh turns up. Personally I doubt very much whether he will put in an appearance."

"Do you think the girl has frightened him?"

"I think it is extremely likely," said Tarling. "I will make an inquiry at the Stores, but I don't suppose he will be there either."

This surmise proved to be correct. Nobody at Lyne's Store had seen the manager or received word as to his whereabouts. Milburgh had disappeared as though the ground had opened and swallowed him.

No time was lost by Scotland Yard in communicating particulars of the wanted man to every police station in England. Within twenty-four hours his description and photograph were in the hands of every chief constable; and if he had not succeeded in leaving the country—which was unlikely—during the time between the issue of the warrant and his leaving Tarling's room in Hertford, his arrest was inevitable.

At five o'clock that afternoon came a new clue. A pair of ladies' shoes, mud-stained and worn, had been discovered in a ditch on the Hertford road, four miles from the house where the latest murder had been committed. This news came by telephone from the Chief of the Hertford Constabulary, with the further information that the shoes had been despatched to Scotland Yard by special messenger.

It was half-past seven when the little parcel was deposited on Tarling's table. He stripped the package of its paper, opened the lid of the cardboard box, and took out a distorted-looking slipper which had seen better days.

"A woman's, undoubtedly," he said. "Do you note the crescent-shaped heel."

"Look!" said Whiteside, pointing to some stains on the whitey-brown inner sock. "That supports Ling Chu's theory. The feet of the person who wore these were bleeding."

Tailing examined the slippers and nodded. He turned up the tongue in search of the maker's name, and the shoe dropped from his hand.

"What's on earth the matter?" asked Whiteside, and picked it up.

He looked and laughed helplessly; for on the inside of the tongue was a tiny label bearing the name of a London shoemaker, and beneath, written in ink, "Miss O. Rider."