It was ten o'clock in the morning, and Whiteside and Tarling were sitting on a sofa in their shirt-sleeves, sipping their coffee. Tarling was haggard and weary, in contrast to the dapper inspector of police. Though the latter had been aroused from his bed in the early hours of the morning, he at least had enjoyed a good night's sleep.

They sat in the room in which Mrs. Rider had been murdered, and the rusty brown stains on the floor where Tarling had found her were eloquent of the tragedy.

They sat sipping their coffee, neither man talking, and they maintained this silence for several minutes, each man following his own train of thought. Tarling for reasons of his own had not revealed his own adventure and he had told the other nothing of the mysterious individual (who he was, he pretty well guessed) whom he had chased through the grounds.

Presently Whiteside lit a cigarette and threw the match in the grate, and Tarling roused himself from his reverie with a jerk.

"What do you make of it?" he asked.

Whiteside shook his head.

"If there had been property taken, it would have had a simple explanation. But nothing has gone. Poor girl!"

Tarling nodded.

"Terrible!" he said. "The doctor had to drug her before he could get her to go."

"Where is she?" asked Whiteside

"I sent her on an ambulance to a nursing-home in London," said Tarling shortly. "This is awful, Whiteside."

"It's pretty bad," said the detective-inspector, scratching his chin. "The young lady could supply no information?"

"Nothing, absolutely nothing. She had gone up to see her mother and had left the door ajar, intending to return by the same way after she had interviewed Mrs. Rider. As a matter of fact, she was let out by the front door. Somebody was watching and apparently thought that she was coming out by the way she went in, waited for a time, and then as she did not reappear, followed her into the building."

"And that somebody was Milburgh?" said Whiteside.

Tarling made no reply. He had his own views and for the moment was not prepared to argue.

"It was obviously Milburgh," said Whiteside. "He comes to you in the night — we know that he is in Hertford. We know, too, that he tried to assassinate you because he thought the girl had betrayed him and you had unearthed his secret. He must have killed his wife, who probably knows much more about the murder than the daughter."

Tarling looked at his watch.

"Ling Chu should be here by now," he said.

"Oh, you sent for Ling Chu, did you?" said Whiteside in surprise. "I thought that you'd given up that idea."

"I 'phoned again a couple of hours ago," said Tarling.

"H'm!" said Whiteside. "Do you think that he knows anything about this?"

Tarling shook his head.

"I believe the story he told me. Of course, when I made the report to Scotland Yard I did not expect that you people would be as credulous as I am, but I know the man. He has never lied to me."

"Murder is a pretty serious business," said Whiteside. "If a man didn't lie to save his neck, he wouldn't lie at all."

There was the sound of a motor below, and Tarling walked to the window.

"Here is Ling Chu," he said, and a few minutes later the Chinaman came noiselessly into the room.

Tarling greeted him with a curt nod, and without any preliminary told the story of the crime. He spoke in English — he had not employed Chinese since he discovered that Ling Chu understood English quite as well as he understood Cantonese, and Whiteside was able from time to time to interject a word, or correct some little slip on Tarling's part. The Chinaman listened without comment and when Tarling had finished he made one of his queer jerky bows and went out of the room.

"Here are the letters," said Whiteside, after the man had gone.

Two neat piles of letters were arranged on Mrs. Rider's desk, and Tarling drew up a chair.

"This is the lot?" he said.

"Yes," said Whiteside. "I've been searching the house since eight o'clock and I can find no others. Those on the right are all from Milburgh. You'll find they're simply signed with an initial — a characteristic of his — but they bear his town address."

"You've looked through them?" asked Tarling

"Read 'em all," replied the other. "There's nothing at all incriminating in any of them. They're what I would call bread and butter letters, dealing with little investments which Milburgh has made in his wife's name — or rather, in the name of Mrs. Rider. It's easy to see from these how deeply the poor woman was involved without her knowing that she was mixing herself up in a great conspiracy."

Tarling assented. One by one he took the letters from their envelopes, read them and replaced them. He was half-way through the pile when he stopped and carried a letter to the window.

"Listen to this," he said:

"Forgive the smudge, but I am in an awful hurry, and I have got my fingers inky through the overturning of an ink bottle."

"Nothing startling in that," said Whiteside with a smile.

"Nothing at all," admitted Tarling. "But it happens that our friend has left a very good and useful thumb-print. At least, it looks too big for a finger-print."

"Let me see it," said Whiteside, springing up.

He went to the other's side and looked over his shoulder at the letter in his hand, and whistled. He turned a glowing face upon Tarling and gripped his chief by the shoulder.

"We've got him!" he said exultantly. "We've got him as surely as if we had him in the pen!"

"What do you mean?" asked Tarling.

"I'll swear to that thumb-print," replied Whiteside. "It's identical with the blood mark which was left on Miss Rider's bureau on the night of the murder!"

"Are you sure?"

"Absolutely," said Whiteside, speaking quickly. "Do you see that whorl? Look at those lineations! They're the same. I have the original photograph in my pocket somewhere." He searched his pocket-book and brought out a photograph of a thumb-print considerably enlarged.

"Compare them!" cried Whiteside in triumph. "Line for line, ridge for ridge, and furrow for furrow, it is Milburgh's thumb-print and Milburgh is my man!"

He took up his coat and slipped it on.

"Where are you going?"

"Back to London," said Whiteside grimly, "to secure a warrant for the arrest of George Milburgh, the man who killed Thornton Lyne, the man who murdered his wife — the blackest villain at large in the world to-day!"