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."

CHAPTER XXX

WHO KILLED MRS. RIDER?

The matron of the nursing home received Tarling. Odette, she said, had regained her normal calm, but would require a few days' rest. She suggested she should be sent to the country.

"I hope you're not going to ask her a lot of questions, Mr. Tarling," said the matron, "because she really isn't fit to stand any further strain."